Reading BMW Fuel
Injection Fault Codes
Wayne R. Dempsey

[click to enlarge]

Figure 1

Figure 2

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     As with any electronic device, components can fail, triggering problems with the system.  The BMW Motronic system is designed to react to these failures and indicate them to the driver, so that they can be fixed.  If one of the computer's sensors is not working properly, then the computer may not be able to successfully identify the current state of the engine and choose the appropriate fuel mixture or timing advance level.  When this happens, the fuel mileage drops, engine performance suffers, emissions increase, and the car typically illuminates the 'check engine' light.

     In order to accurately find the sensor and fix the problem, you will need to find out which error code is being triggered by the computer.  Pre-1995 BMWs were equipped with what is known as OBD I (On Board Diagnostics Level I).  Stating in 1996, they were equipped with a more advanced version called OBD II, which was mandated by the US government, in order to standardize automotive repair and diagnostics.  The OBD system is responsible for monitoring and checking all of the fuel injection sensors and systems in the vehicle, and turns on the 'check engine' lamp if it finds a problem or irregularity with one of them.  The computer lodges a Diagnostic Trouble Code (DTC) in the main computer until it is read and reset.  Disconnecting the battery does not reset the codes - they need to be manually reset.

     For BMWs equipped with OBD I, you can read the codes using a system that is built into the car.  For OBD II compliant cars (1996 and later), the codes can only be read using a scan tool (more on this later).

     To read codes for an OBD I car, you start by turning the ignition key on (do not start the engine) and pressing the gas pedal to the floor five times within a period of five seconds.   If you proceeded correctly, then the check engine lamp will light on for five seconds, then blink off, then blink on for 2.5 seconds, and then go off for another 2.5 seconds.  After this, the computer will start to show you the trouble fault codes that are stored.

     The codes are given by short flashes of the check engine light, followed by short pauses.  The codes are all four digit numbers.  For example, the trouble fault code for monitoring the battery voltage level is 1231.  This would show up flashing as "flash - pause - flash - flash - pause - flash - flash - flash - pause - flash."  I suggest having a pen and paper handy when you do this, as the codes have a tendency to flash faster than you would think.

     If there is more than one fault code stored, then each code will be separated by a 2.5 second pause.  When there are no more codes to be read, the computer will give the code 1000, which is one short flash, and then the light will remain off.  Then the check engine light will flash a half-second and then turn off.   To read the codes again, simply turn the ignition key off and on again and repeat the procedure with the gas pedal (five times within five seconds).

     Here is a table that represents all of the possibly fault codes that can be read using this method:

     When you are finished reading the codes, you can reset the computer and clear them all out.  Make sure that the last code (1000) has occurred, and then press down on the gas pedal for more than 10 seconds.  This should clear out the memory of the DME.   Repeat the fault code reading process, and the computer should generate code 1444, which means that there are no faults stored.

     On twelve-cylinder cars, the engines are controlled by two completely separate DME systems that act independently of each other (twice as many things to go wrong!).  There are two DME computers (ECUs) located within each 12-cylinder car.  To access the 2nd ECU, you press down the accelerator six times instead of five.  The codes returned will be identical to the codes listed above, except for the first digit, which will be the number two instead of the number one.

     Wire harnesses are a major cause of fault code problems.  As the cars get older, the wiring harness have a habit of becoming what is commonly known as 'work hardened.'  This causes the wiring to become brittle and often break inside of its plastic sheath.  Only by testing the continuity of the wires end-for-end will you be able to determine whether the wire is broken or not.

     For cars manufactured starting in 1996, BMW used the industry standard OBD II method of monitoring the fuel injection system.  OBD II cars cannot be checked using the 'gas pedal' method previously described, but instead must be checked using a code scanner.  The tool that I like to use is the Peak Research R5/FCX Fault Code Tool, shown in Figure 1.  This tool is compatible with all BMWs built from 1987 to 2000 (except for 1999-2000 V8s).   It displays all of the engine fault codes, and also has the option of resetting the oil service lamp.  Since this tool works with both OBD I and OBD II cars, I prefer to use it on the pre-1996 cars, as it is much easier to read the codes using this tool than it is using the gas pedal method.

     The R5 tool is used in a similar manner to the 'gas pedal method.'  Turn on the ignition key (do not start the engine), and the install the tool into the BMW diagnostic connector located under the hood (Figure 2).   This connector contains a large 20-pin connector which the tool uses to interface with the DME computer.  Once the ignition is turned on, simply plug the tool into the socket - it can only go into the socket one way.

     With the tool plugged in, it will 'spin' the LED lights for a few moments, and then prompt you with a menu.  Hit the 'Select' button on the tool until you read the letters 'Fc' which stands for Fault Codes.  Click 'Go' on the tool, and it will read the codes one by one from the computer.  One note here though - the codes that show up on the tool are unique to this tool.  You need to have a copy of the tool's operating manual in order to decode the proper codes for your car - make sure that you don't lose a copy of this manual, because it's not available on the web.   For a list of troubleshooting tips and info on the tool, please see Peake Research's website.

     When you are done reading the codes, you can reset them all using the R5 tool.  Press the 'Select' button until you see 'cE' on the display, then press 'Go'.  You can also use the tool to reset the oil service lamp, and the inspection lamps that light up on some of the earlier BMWs.

     Well, there you have it!  You can now arm yourself with the information that you need to start troubleshooting fuel injection system problems.   In the very near future, we'll be having more technical articles on diagnosing and replacing fuel system components on the Motronic systems - keep watching the site for details! 

     See also Jared Fenton's write-up on reading fuel injection codes.

     This technical article is made possible solely through the support of Pelican Parts.  If you like what you see here, then please visit our online BMW catalog and help support the collection and creating of new and informative technical articles like this one.