Diagnostic Fault Codes

The engine diagnostic fault codes are stored in non-volatile memory in the computer. To read them, all you need is a homemade VAG 1115 LED test light. Original California cars need only two pieces of wire to connect A1 to B1 because their fault codes will flash on the "check engine" light. You can install a bulb in the "check engine" socket  of the instrument cluster on 49-state cars to get the same function. I just moved the bulb from the seatbelt warning light over to the "check engine" socket until I have a chance to pick up another bulb. If you have a "check engine" light, you can ignore the LED instructions, and just use the two wires in A1 and B1 to trigger the code dump.

  1. If you have a pre-March 1990 production car, connect LED test light to the connectors in the driver's footwell as shown in the diagram at right. If you have a later car (with three connectors in the footwell), just connect the negative lead of the LED to the top slot of the blue connector (C2) instead of the bottom of the second connector (B1). All other connections remain the same.
    BEWARE! If you touch the A1 and A2 wires together, you will create a short circuit. (Experience talking here.) If you're lucky, all that will happen is a little spark, a blown fuse (#21, 10A, I believe), and all your saved codes will be erased. If your not lucky, you'll fry the computer.
  2. Leave the wires between A1 and B1 DISCONNECTED.
  3. Turn on ignition, but don't start the engine.
  4. Connect A1 and B1 together for at least 4 seconds, then disconnect.
  5. The output codes will begin to flash on the diode or "check engine" light.

Each code is a sequence of 4 digits, and each digit will be either 1, 2, 3, or 4. Each code begins with a 2.5 second long flash. The first digit will be a series of 0.5 second flashes (short flash) followed by a 2.5 second pause. Count the number of flashes for the first digit. The 2.5 second pause tells you that the next set of flashes is the second digit, and so on. The light will continuously flash the same 4-digit code until you signal the computer to give you the next 4-digit code. To get the next code, just connect A1 and B1 together again for at least 4 seconds. You have reached the end of the code list when you get four 2.5 second flashes (long flash) without any short flashes. This is code "0000" (output complete).

If the first code you receive is "4444", you have no faults in the computer. You'll have to look for the source of your problems elsewhere. All other codes are listed in the table below. Always go after the first code first, then dump the codes again when you fix it. Sometimes one problem will cause several misleading fault codes, but the first code will usually point you in the right direction.

The fault codes will be saved until power is disconnected from the computer or you run the output tests. The output test procedure steps you through an activation sequence of individual electrical devices (fuel pump, injectors, ISV, etc.). The process is similar to reading the fault codes, except you connect the A1 and B1 wires BEFORE turning on the ignition. See Bentley for instructions.

Flash Code

Fault Source Symptom Possible Cause Fault Correction


Control unit Engine does not start - Fault in control unit ground wiring
- Microprocessor in MPI control unit is faulty
- Check ground connections
- Replace MPI control unit


Transmission speed sensor Engine shuts off while decelerating w/clutch engaged - Fault in sensor wiring
- Sensor faulty
- Instrument cluster circuit faulty
Check speed signal


Engine-speed sensor - Engine does not start
- Engine misfires
- Fault in sensor wiring
- Sensor faulty
- Check engine-speed sensor and wiring
- Check distance from sensor to ring gear of flywheel (< 1.2mm)
- Check for broken teeth on ring gear


Ignition reference sensor Engine does not start - Fault in sensor wiring
- Sensor faulty
- Engine speed sensor exchanged with ignition point sensor
Check ignition reference sensor and wiring


Hall sensor - Engine lacking power
- High fuel consumption
- Fault in sensor wiring
- Sensor faulty
Check hall sensor


Ignition distributor basic ------ Distributor out of adjustment Check ignition distributor


Hall sensor not on reference point ------ - Timing belt broken
- Hall sensor out of adjustment
- Flywheel reference pin for ignition timing
- Check belt
- Check hall sensor
- Check flywheel reference pin


Idle switch Idle speed too high - Fault in wiring
- Idle switch out of adjustment or faulty
- Throttle open
- Check idle switch and wiring
- Check throttle cable


First knock regulation
- 2141 = sensor 1
- 2143 = sensor 2
- Limited loss of power
- Insufficient top speed
- Engine running rough
- Engine knock/ping
- Fuel octane too low
- Re-torque knock sensor (7 ft lb / 10 Nm)
- Use recommended octane fuel
- Check compression and fuel injection system


Knock sensor
- 2142 = sensor 1
- 2144 = sensor 2
- High fuel consumption
- Impact-like power loss (like misfire)
- Fault in sensor wiring
- Knock sensor faulty
- Check wiring to knock sensors
- Re-torque knock sensor (7 ft lb / 10 Nm)
- Replace knock sensor


Throttle valve potentiometer Power loss Output voltage too low or too high in relation to air mass Check throttle valve pot and wiring


Air mass sensor - Limp home mode
- Poor fuel intake
- Engine dies
Output voltage too low or too high in relation to RPM Check air mass sensor and wiring


Air mass sensor ------ Reference voltage greater than 1 Volt Check air mass sensor wiring


MPI control unit supply voltage - Voltage below 6 V
- Engine will not crank/start
- Vehicle system voltage too low
- Open circuit to terminal 18 of MPI connector A or fuel pump relay
- Engine started w/2 batteries in series
- Check supply voltage to control unit
- Check fuel pump relay
- Check alternator/battery


CO potentiometer - Poor acceleration
- Poor fuel intake
Output voltage too high Check air mass sensor


Coolant temperature sensor - Cold starting difficulties at low temperatures
- Poor idle and acceleration during warmup
- Fault in sensor wiring
- Coolant sensor faulty
Check coolant temp sensor and wiring


Oxygen sensor
(control limit exceeded or underexceeded)
- High fuel consumption
- Rich exhaust
- Fouled spark plugs
- Empty fuel tank
- Low fuel system pressure
- Defective ignition components (cap, rotor, plug wires, spark plugs)
- Vacuum leak
- Faulty Oxygen sensor
- Check fuel sys pressure
- Check ignition components
- Check for vacuum leaks
- Check oxygen sensor


Oxygen sensor - Emissions not within specs
- oxygen sensor goes open loop
Oxygen sensor has open or short circuit Check oxygen sensor and wiring


EGR frequency valve Engine runs rough EGR frequency valve has open or short circuit Check EGR frequency valve wiring


Carbon canister solenoid valve
- 4331 = valve 2
- 4343 = valve 1
- Poor response in part of load range
-Fuel odors
Faulty wiring to carbon canister valve Check carbon canister valve and wiring


Fuel injector (1-5) - Engine runs rough
- Possible engine stall
- Injector wiring faulty
- Resistance incorrect
Check injector and wiring


Idle stabilizer valve - Engine might die when cold
- Engine surges
Fault in sensor wiring Check ISV and wiring


No faults stored in memory


Fault transmission complete

(1) Only for cars produced before March 1990 (two diagnostic connectors in footwell)
(2) Only for cars produced during or after March 1990 (three diagnostic connectors in footwell)
(3) Does not display on "check engine" light, according to Bentley. (California, pre-March 1990 production) However, it may actually display, especially on non-California cars that have the "check engine" bulb added.

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