On-Board Diagnostics - Pt 1

Posted by murmini Fri, 19 Jan 2007 05:45:35 GMT

The On board Diagnostics, or OBD, is yet another computer system built into all vehicles since the late 80s to assist in the monitoring of the engines performance in the interest of emissions monitoring. Well that's how it started anyway.

In the late 80's, with California pushing hard for some standardization of emission controls, the preliminary OBD standard was adopted by manufacturers to make sure that vehicles' emission were not exceeding guidelines established to meet the Clean Air Act. About this time the Society of Automotive Engineers recommended a standard connector and a number of standardized test signals.

Starting in 1996, all cars sold in California had to provide some basic engine diagnostics to make certain that they met the ever increasing demands of the California Air Resources Board or CARB standards. So between the SAE and CARB and the automobile manufacturers, a standard was adopted that provided a common connector on all vehicles and a set of standardized tests that could be run to make certain that the vehicle was meeting the manufacturer's standards.

Later, OBD-II added another layer of connectivity between the vehicle's on-board computer and peripheral test equipment for fault location and analysis. Still not completely common across all types, OBD-II has three different communications protocols. A high level view of the field is as follows: GM cars and light trucks use SAE J1850 using variable pulse width modulation or VPWM, Chrysler, all European - including MINI - and most Asian vehicles use ISO 9141 protocols and Fords use SAE J1850 pulse width modulation PWM. Each of these define the communications protocol, or the type of data signals that carry the information on the OBD connector.

Here is a diagram of all of the possible pin-outs on a generic OBD connector. On the MINI, the connection is a part of the D-Bus and has direct connections via this bus into the Engine Control Module, the Anti-Locking Brake System, the Electro-Hydraulic Steering, the Automatic Stability Control module and the Dynamic Stability Control system if fitted. If the vehicle has automatic transmission there is also connection to the Gearbox Interface Unit.

Obdii PlugMINI uses the ISO 9141-2 protocol. This protocol has a data rate of 10.4 kbaud, and is similar to a standard RS-232 connection. Signaling between the OBD-II connector and the various computer systems in the MINi are by way of a UART, pronounced u-art, and short for universal asynchronous receiver-transmitter. This is a computer component that handles asynchronous serial communication. All computers contains a UART chip to manage the serial ports, and to send and receive the data in and out over these ports. Again, its a matter of adopting standards so thing can 'speak' the same digital language.

While OBD started life as a simple diagnostic tool for checking emissions, today's OBD-II is much more sophisticated. There are many sensors within the vehicle that report back to the Engine Control Module and subsequently the status of these sensors can be viewed with the correct diagnostic equipment, by way of the OBD connector. These sensors measure a number of dynamic functions in the following primary categories: Air Management, Fuel Management, Ignition Management. Emission Management, and Performance Management. Serving both the On Board Diagnostics and the Engine Control Module, these sensors give constant updates to the health and well being of the engine.

The data from these sensors constantly modify the way the engine is running by altering air/fuel ratios, ignition timing, and torque management, based on the sensor inputs concerning throttle opening, coolant and engine temperature, vehicle speed and of course, emission values. All this is managed by the Engine Control Module and it is here where the computations are made based on a number of tuning conditions provided in look-up tables.

If at any time the vehicle emissions exceed the Federally mandated criteria, the OBD will illuminate Malfunction Indicator Lamp on the cockpit display and store the Diagnostic Trouble Code so that it can be read by a remote diagnostic device and aid in determining what is causing the problem. Any malfunction that occurs in the four categories have OBD-II standard codes that can be read by the analyzer.

The codes are arranged in a specific format with the 1st digit identifying it to be a powertrain, body or chassis problem, the 2nd code indicates if its an SAE standard code or one specific to MINI, the 3rd code determines where the problem exists either in the total system, air/fuel, ignition, emission, vehicle speed/idle control, ECM or transmission and the 4th and 5th codes specify individual circuits or faulty components.

Obd CodeSo, an error code of P0304 would indicate:

P = powertrain issue
0 = using an SAE standard code
3 = related to an ignition system misfire
04 indicating the misfire is on cylinder #4.

In addition to activating a dashboard warning light, these "P-codes take a snapshot of the vehicles performance at the moment when the fault first occurred. This snapshot minimally contains information about engine load, RPM, short and long term fuel trim, speed, coolant temps, intake manifold pressure, fuel pressure and the diagnostic trouble code. In addition to the standard SAE codes, MINI has implemented some additional BMW/MINI specific codes that can be read by their diagnostic scan tools.

Below is a list of the main sensors and how they function:

COOLANT SENSOR - used to monitor the temperature of the engine coolant. Its resistance changes in relationship to the changing coolant temperature. The sensor on the MINI is located in cylinder head and tells the computer the engine temperature.

OXYGEN (O2) SENSOR - this is a crucial sensor involved in the fuel mixture feedback control loop. There are two in the MINI engine, one located either side of the exhaust catalytic converter. They look at how much unburned oxygen is contained in the exhaust gases. Again, the voltage coming from these sensors is proportional to the amount of unburned oxygen in the exhaust. The 'post' catalytic sensor is used to determine the efficiency of the catalytic converter. When the fuel mixture is rich, most of the oxygen is consumed during combustion so there is little unburned oxygen in the exhaust.

The signal coming from the primary oxygen sensor is actually vacillating between high and low. When it reads "lean" the PCM increases the 'on-time' of the injectors to make the fuel mixture go rich. Conversely, when the sensor reads "rich" the PCM shortens the on-time of the injectors to make the fuel mixture go lean. This causes a rapid back-and-forth switching from rich to lean and back again as the engine is running. These even waves result in an "average" mixture that is almost perfectly balanced for clean combustion. When the oxygen sensor is controlling the fuel sent to the injectors, its referred to as 'closed loop' operation. when specific demands like wide open throttle, or engine braking occurs, this changes to "open loop" operation.

TEMPERATURE AND MANIFOLD (T-MAP) SENSOR - Is located in the air stream next to the electronic throttle valve. Measuring the pressure and the temperature of airflow into the engine, it is capable of determining the volume of air being used by the engine as well as the engine load and the intake temperature (outside air temperature). In the Cooper S, it determines the pressure differential across the supercharger in cooperation with the MAP sensor.

MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR This sensor is mounted on the left side of the cylinder head and measures intake vacuum from a line connected to the supercharger supply ducting. It changes voltage as intake pressure changes. The computer uses this information to measure engine load so ignition timing can be advanced and retarded as needed. It performs essentially the same job as the vacuum advance diaphragm on an old fashioned mechanical distributor that changed the ignition timing by rotating the distributer under heavy manifold vacuum. WHen the ignition is turned on, it measures atmospheric pressure. Once the engine is running, it measures the absolute manifold pressure which is the barometric air pressure minus the vacuum created by the pistons.

THROTTLE POSITION SENSOR - Mounted on the electronic throttle body, there are actually two sensors here, a primary and a backup. As the MINI uses a 'drive by wire' throttle, (see here) this approach provide a fail-over in the event of a problem. The throttle position sensor (TPS) changes resistance as the throttle opens and closes. The computer uses this information to monitor engine load, acceleration, deceleration based on throttle position. The sensor's signal is used by the to compute how much to enrich the fuel mixture during acceleration, as well as to retard and advance ignition timing.

CRANKSHAFT POSITION SENSOR - Located in front of the crankcase at the flywheel end of the engine the crankshaft position sensor serves essentially the same purpose as the ignition pickup in an electronic distributor. It uses a Hall effect circuit and "sees" the presence of a reluctor ring bolted to the end of the crankshaft and as it rotates, there are 58 targets and two missing "teeth" on the ring. It generates a signal that the ECU can use to determine the position of the crankshaft and the piston position in number one cylinder. This information is necessary to control ignition timing and for the accurate operation of the fuel injectors.

CAMSHAFT POSITION SENSOR - This is located in the front of the cylinder head just under the valve cover and again uses a Hall effect circuit with a reluctor ring to determine the exact position of the camshaft. Viewed in correlation with the crankshaft position sensor this is used to determine injector timing in both the open and closed loop and has a clever design that controls the four injectors completely individually. So that if one injector circuit fails, the other three can continue to provide power, albeit limited.

KNOCK SENSOR - The knock sensor detects engine vibrations that indicate incorrect fuel detonation is occurring. If this occurs, the computer can momentarily retard the ignition timing. It is actually an acoustic or vibration sensor like a small microphone and listens for unusual vibrations typical of partially or spontaneously ignited fuel - called pinging. It is bolted to the crankcase just below the intake manifold.

There are many other sensors at play in this complex and highly evolved system. Many feed other subsystems and their appropriate control modules which in turn feed data back to mission central - the engine control module. Pt 2 of this OBD article will look at the signals coming from the OBD connector and evaluate their contribution to a smoothly running, well tuned and happy MINI engine.



  1. Scott said 1 day later:

    Does the Mini OBD connector have access to the i-Bus or just the D-Bus?

  2. murmini said 1 day later:

    Scott: the MINI OBD connector is connected to the D bus and the DS-2 bus. It has no direct connection to the I bus. The instrument cluster (IKE) shares connections to the DS-2 bus, the K bus and the CAN bus. The I-bus is a subset or secondary K-bus and connects such devices as park distance controller, radio, CD player, digital signal processor and navigation system to the General Body Module.

  3. Scott said 3 days later:

    Got it. Am looking for a “easy” place to connect up to the i-Bus to run NavCoder on the Mini. Got an iBus-Serial adaptor on it’s way! Looking forward to getting my British voice back.


  4. murmini said 3 days later:

    Scott: One way you can certainly access the I-Bus easily in the MINI is via the CD Changer cables in the right, rear access area. They are ususally strapped to the wiring harness with masking tape but easily can be accessed as well as as a 12v supply. I have a picture of the two connectors in an article I wrote about the Dension IceLink Plus here. This might be easier to connect your serial port to than say behind the radio or at the naviagtion system.

  5. Pedro said 4 days later:

    Nice job Murray!

  6. Scott said 4 days later:

    sweet! That’s zactly what I was looking for! Thanks!

  7. Josh, said 27 days later:

    Is there any way to get the mini specific codes?

  8. murmini said 27 days later:

    Josh: there are the DTC (Diagnostic Trouble Codes) which are industry sanctioned and are available on websites such as here. Many of them have equivalent MINI Fault Codes then there are MINI specific ones. If you can’t find it here… let me know. My article specifies how the digits are arranged and what they mean. Let me know if you still can’t find one and maybe I can help.

  9. MB Parts Blog said about 1 month later:

    These computer-assisted programs are indeed a great invention. It had made driving conveniently easy for us. I also have GM oxygen sensor. This is an example of simple inventions created for safety and comfort of the driver.

  10. Derek said about 1 month later:

    Is there a reasonably priced OBD tool that works well with 2002 Mini Cooper base? For instance, I’d like ability to reset the Oil Change mileage counter.

  11. murmini said about 1 month later:

    Derek, I will cover this in OBD Part II. I have been looking at a few and will provide a couple of options… should have it published in a few days.

  12. Rodger Cook said 3 months later:

    Hello… can anyone tell me what the resistor is for on the OBDII connector door that opens for access to the connector on a 2006 mini S convertible? thanks!!!

  13. murmini said 3 months later:

    Very Interesting ! After I dislocated my back crawling under the dash to see it I see what you mean! It appears to provide a small voltage drop between the L line and +ve. It would only happen when the door was closed but as to why they do it is a mystery to me. I will have to pour over the wiring diagrams a bit more and do some more research. I learn something new every day. Thanks for pointing this out.

  14. cesar said 3 months later:

    where is the ambient temperature sensor located at

  15. murmini said 3 months later:

    The outside temperature sensor is located behind the lower front grill, slightly towards the drivers (USA) side.

  16. Dee said 8 months later:

    We hooked up the ODB to my Mini Cooper S 2002…SAE Code came up P0130 DTC Code 16514 Description 02 sensor circuit, Bank1-sensor 1 manfunction. Can anyone help me with which part this would be. From what I have read, there are 2 sensors.

    Thanks for your help.


  17. bill said 10 months later:

    What does it mean when EML light appears? The book says engine malfunction electronics but what causes this?

  18. murmini said 10 months later:

    Bill, you need to get the codes from a reader to tell you what threw the alarm light. Any service station can read the code for you and then you can find the reason that caused the error. Sorry I can’t be of more help.

  19. Bill said 12 months later:

    I just visited your site after buying recently buying a 2003 MINI. Do you have any recommendations for an OBD code reader?

  20. murmini said about 1 year later:

    Bill take a look at the follow up article from this one here

  21. amos.maghini@sympatico.ca said about 1 year later:

    How do we access the fuel injectors on the R-56 ?

    do WE HAVE TO COMPLETELY REMOVE THE VALVE COVER? I would appreciate your input. Thanks

  22. murmini said about 1 year later:

    Hmm I can’t help you on the R56, although I am sure some other readers can. Try NAM or MINI 2

  23. murmini said about 1 year later:

    I am still getting sorted on the R56, should be getting the manuals shortly so I can’t help you this one… Maybe Chad will chime in with some tips… I will ask him.

  24. Steveh said about 1 year later:

    I have an R56 Cooper S (2007) and I installed a Borla Street exhaust, K&N replacement filter (not CAI) and a unichip and I get a CEL with OBD code P115D (mfg control Fuel Air Metering). I don’t find this code in the standard lists – any hints as to what it means?

  25. murmini said about 1 year later:

    This code and sometime P0420 have been seen when an imbalance of airflow is detected. That means that exhaust measurements are detecting incorrect air/fuel ratios. I think your unichip is the issue, so I would start by talking with them. Sorry I can’t be of more help.

  26. John said about 1 year later:

    Do you know what degree of difficulty it is to change out a defective camshaft sensor? Do you think it’s a do-it-yourself fix?

  27. murmini said about 1 year later:

    John, you would need to remove the engine vibration damper bracket, the fuel line bracket mounting bolt and detach any fuel lines still attached. Then lift off the bracket. Then remove the fuel tank vent valve, via quick-fit connectors and slide valve off bracket, Remove electrical harness connectors Supporting the engine from under the oil pan, remove right-side engine bracket, remove the engine ground cable, and the 4 engine mounting bolts from the engine block. Unlock and detach the sensor mounting bolt and remove the sensor. Replace the O-ring with a new sensor.

    I don’t think you should attempt this if your not mildly experienced in auto-repair. A professional shop is going to be able to do this fairly quickly.

    Hope this helps.

  28. Drew said about 1 year later:

    Really good article. Thanks. Would failure of this component be a candidate to generate P2138 error code? My son’s mini, which I am now driving, gets this code intermittently. The dealer changed the throttle body and wiring harness, which besides being expensive did not fix the problem.

    Two other great articles were your OBD discussions. Would AutoEnginuity allow me to monitor the pedal outputs?

    Thanks again!

  29. brian bonello said over 2 years later:

    i just purchased a second hand mini cooper 2001 model petrol 1.6 and cannot locate where the eobd plug is, if there is? Can you help please by e-mailing on bribon@euroweb.net.mt


  30. murmini said over 2 years later:

    It is located under the dash on the left hand sive of the steering wheel, behind a small cover flap.

  31. LUCIO said over 2 years later:


  32. Igor said over 2 years later:

    What ISO 9141-2 request should I execute in order to pull EHPS DTC? Can you help please by e-mailing on ipynda@cardone.com

  33. jason said over 2 years later:

    I got this 05 cooper that it gaves a code “P101?”. The scaner shows the code “P101?” . What does it mean? any help, thanks,

  34. murmini said over 2 years later:

    The OBD error is “Mass Air Flow Circuit Range/Performance” Which means the sensor has exceeded its specified range. I have a feeling its the TEMPERATURE AND MANIFOLD (T-MAP) SENSOR that is located in the air stream next to the electronic throttle valve. It measures the temp. and pressure air as it flows into the engine. I am not certain, but I think it falls under an extended warranty as it pertains to emissions. Call your dealer and discuss.

  35. afa168@pldtdsl.net said over 2 years later:

    I would like to know where I can get an OBD for an 08 MCS auto/hardtop. Thanks and hope to hear from you soon.

  36. murmini said over 2 years later:

    I am not sure I follow what you mean. Are you trying to locate an OBD reader to view error codes etc? Take a look at pt. 2 of the article there is a review of two types.

  37. Sarah said over 3 years later:

    Can anyone tell me where to find the diagnostic plug connection for a mini cooper S 54 plate? been looking all over for it and now we are stumped! thank you

  38. murmini said over 3 years later:

    It is located under the dash, (on the US version above the driver’s knee) under a small plastic cover. Pull the cover open and there it is.

  39. BITTOR said over 3 years later:

    I have a cooper R50, from 09/2002, with the engine W10B16BA, and i have problems with the automatic transmission, when i shift to lower speeds the car ever selects the 2 gear. I find the fault P1786 in the engine control, but I dont know where is this CVT motor of gear ratio. can you help me please??

  40. murmini said over 3 years later:

    P1786 is a fault with the Transmission Ratio Control Circuit Range/Performance. I am not up on CVTs other than the knowledge that over the years there have been a few issues with them. I think a quick call to the service manager at a MINI dealer, with that error code might be a good plan and at least try to establish why this actuator is presenting the diagnostics with an out of range error. Sorry that I can’t be of any more help.

  41. Sam said over 3 years later:

    Is there a code or sensor for a bad catalytic converter under warranty by federally required warranty to 80,000 miles. My converter rattles under vibration indicating cracked ceramic substrate.

  42. murmini said over 3 years later:

    The OBD (On Board Diagnostics) will throw a Check Engine Light, but the very best thing to do here is to get the dealer to check it as they will have to initiate any warranty claim.

  43. chris said over 3 years later:

    just had a clutch fitted in my 52plate mnini cooper s works, all the have done is fitted clucth but now the garage can not get it to run correct, it runs very bad on 2-3 cylinders & wont tick over. could they have damaged the cps causeing this??? or any other idears? the can ran perfect before it went in the garage & the engine code that is comeing up when revved hard is P1698 ELECTRONIC THROTTLE MONITORING, LEVEL 2/3 TORQUE CALCULATION ERROR

    any info would be great


  44. murmini said over 3 years later:

    I really can’t throw any light on this for you I am sorry to say. I will ask if anyone else can add any advice and have forwarded it to a specialist I know.

  45. Jen said over 3 years later:

    My mechanic has found two codes tripped: p1698 and p1789. My understanding is that p1698 is a transmission control module control error and p1798 is a transmission ratio actuator communication error. My mechanic mentioned (gearbox interface box or gib) and an ECU internal failure message. He was leaving his shop and going back in morning to try to reset it. I am a little confused but trust him. My understanding by talking to mini on the phone today is that I may be looking at a whole new transmission ($9200 Can). Plus another 400 to get it to mini. Can these fault codes be fixed by my mechanic? He obviously has the tools to diagnose the fault codes. I really would appreciate any in put. I love your site. Thanks in advance.

  46. murmini said over 3 years later:

    Jenn: I cant really be of much help as its a remote diagnosis issue and I am sure you understand. Fault codes are generated by the system because something has exceeded the system’s preset parameters. The cause if this is very hard to say and the analysis of the fault code is not much help. I will let you know if I find any more information.

  47. Steven said over 3 years later:

    With a P0732 Trouble Code (Gear 2 Incorrect Gear Ratio) on my 2007 Mini Cooper, my car is only allowing me to use one gear… What could be happening and how can I resolve this issue? It has 90,000 miles on it.

  48. murmini said over 3 years later:

    You need to take it to a shop. It could be as simple as a bad solenoid or a speed sensor, but you need to get it to a MINI service center.

  49. faissal said over 3 years later:

    moi faissal elec treseti

  50. Chad said over 3 years later:


    you should try my business email-totalminibmw@att.net-...I check it more!

    Jen, The ratio actuator fault is a valid code and usually is a result of a faulty ratio actuator inside of the transmission that requires the replacement of the valve body. These parts are not available through the dealer and I have only found them through one source. THe other code you have, IIRC is a throttle position type fault. There ia bulletin for a procedure to check this that you mechanic can follow. Email me if you have questions


  51. Chad said over 3 years later:


    1698 on your car generally means a problem with the supercharger bypass valve, in particular, if this fault is being produced after replacing a clutch, I would see if they got the linkage hung up with the brackets around the Throttle body. It can happen, ask me how I know….!

  52. Chad said over 3 years later:


    Just a guess here, clearing the code and driving it may help to clarify. My guess would be that second gear is slipping. I would think that there would be more speed/ratio faults if there were a problem with a sensor.


  53. murmini said over 3 years later:

    Thanks for your input Chad…

  54. talen said over 3 years later:

    #42 – chris Did you ever find a solution to this? – I’ve got the exact same problem!

  55. miller said over 5 years later:

    Got a code on my 02 mini 1741. I know it is the tranny stuck in limp mode. What i dont know is, is it a sensor problem or does the tranny need changed out bought the car, 1 week ago and drove it for 4 days then it went into limp mode anyone help me on this?

  56. juliango@dmws.durban.gov.za said over 5 years later:

    coolant fan not operational after gearbox was removed want to how the fan works inconjuction with sensor.

  57. murmini said over 5 years later:

    I would check the wiring, also see if volts are getting to the fan There is a notorious resistor on the fan that burns up. Unfortunately requiring a whole fan replacement. Look see if volts are getting go the connector.

  58. Vez said over 5 years later:

    Thanks for the information. I am looking for this code: P112B It appeared (and disappeared) during very cold temperature outside (minus 20C and colder = minus 4). Thank you.

  59. murmini said over 5 years later:

    The P112B comes up air fuel ratio…I think there was a bulletin put out by MINI. Call your local dealer they may have to do a re-flash but they should be able to tell you.

  60. Vez said over 5 years later:

    Thanks. Dealer is changing the thermostat today.

  61. don@archiverde.us said over 5 years later:

    Is there a history of fuel pumps going bad on Mini’s? We have a 2006 with 39,000 miles. The fuel pump was replaced and then wouldn’t start again one day later.

  62. (Trina) itsmsrite@gmail.com said over 5 years later:

    My outside temperature and odometer reading is still displayed after engine is shut off why? how can I fix this??

  63. murmini said over 5 years later:

    You really need to get someone to run some diagnostics on this.

  64. chrisroy02@aol.com said over 10 years later:

    i’m having a obd2 code p 1786 which means transmission ratio control actuator circuit range performance. How can we fix it?

  65. davidgatenby2@btinternet.com said over 12 years later:

    Hi, I have an r56 Cooper D, I am using Bmw Rheingold software to try to access the obd2 port. The setup ( cable etc ) works on other similar aged BMW’s we have ( z4, X5 etc) but although there’s power to the port, I can’t establish any communication at all. Any ideas to point me in the right direction please? Kind regards David

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