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Stalling and Service Engine light comes on


CCCUK Member
Hello. My 89 C4 6 speed, which has been super reliable for so many years, has an issue. At the same time (coincidence) the digital speedo and all associated bits (not rev counter) reset to 0, as well as the car stalling when moving slowly. I discovered a HT lead was resting on the exhaust manifold and cracked the silicone covering. This HT has been replaced, but the symptoms remain. The only thing I have noticed is the digital dash is now brighter and both halves are on upon start up. When the car is restarted following a stall, the Engine Service light turns off only to come on again after a few minutes, indicating a stall is possible at the next junction/red light/slow traffic. Any help very gratefully received. Thank you


CCCUK Member
A non functioning IAC valve can cause stalling at slow speeds. Its located in the throttle body and not hard to access although you need to remove the TB first. Maybe a can of carb cleaner in the TB will clean out any dirt and make a difference. I would concentrate more on the stalling problem than anything else.
Try the paperclip test and see what codes you have first. Check also that you don't have a vacuum leak somewhere. Checking the idle speed via the throttle position sensor is maybe something else to try.

To recover the codes, short pins "A" and "B" together using a small section of electrical wire or paper clip.

With the ignition turned OFF, short pins "A" and "B" on the ALDL.

Turn IGN ON (but not to RUN).

The "Check Engine" light (early C4s) or "SYS" light (later C4s) will flash a Code 12 (a single flash followed by two flashes) and will repeat three times (Flash (pause) Flash Flash (long pause), Flash (pause) Flash Flash (long pause), Flash (pause) Flash Flash (long pause).

Code 12 is a delimiter or marker code to show where the error code string begins and ends.

After the three Code 12 flashes, you will either get an error code (or codes) or you will get another string of Code 12 flashes if there are no trouble codes stored.

All codes are repeated three times with a long pause between each code group (36 or Flash Flash Flash pause followed by six flashes, repeated three times, folowed by a long pause with any additional codes stored then flashed).

The "Check Engine/System" light on with engine running means the condition(s) are currently present. If the light is not on during RUN operation, the limits were exceeded at some point in time and the event was recorded in memory, but the reading has since returned to the normal operating range.

Remember to remove the shorting device from the connector after you have read the codes.

Clearing the Codes

To clear the codes from memory, remove the negative battery cable for a minimum of 10 seconds.

Disconnecting the battery will clear all stored codes and and any stored memory (radio button presets, clock, trip odometer, average gas mileage memory, power seats). Your ECM/PCM computer will also have to relearn timing/mixture/exhaust emissions.

Make absolutely certain the ignition key is turned OFF. If you connect the battery with the ignition switch ON, you can destroy the ECM/PCM module.



Well-known user
Hi you need to expand on what HT means ?
is it a sparkplug wire ?

As mentioned you really need to first see if any error codes are present either my the method mentioned above or best with a
OBD-I scanner. That would allow others here to steer you to where problem could be

Try dis-connecting one side of battery for a few seconds and connect again, that would allow the controllers to erase and reset
any issue when coils high voltage when to ground

Check for any burned out fuses

C4 has two main controllers, 1 is the CCM ( located near radio) is is the main boss,
the other is the ECM which is for the engine controls

See if functions like HVAC, radio, lights work, that will tell you if CCM is kinda working,
After doing above to reset controllers, see if the 12 pin diag port ( smog port) is talking, meaning if it does then the CCM who is boss of network to ECM is working fine.

Check if HEI or coil are working correctly, check to see if the wires to fuel injectors are all getting 12 volts and other wire to each has good ground.

Again any error codes (DTC) your getting is needed for more info as what is going on


CCCUK Member
Whereabouts are you in the UK? I have an 89 6 speed too. I've got an OBD-1 scanner as teamzr1 suggested, it can talk to my phone with bluetooth and an app or plug into a laptop. I'm in Cardiff if you want to borrow it or need some extra help troubleshooting.

Checking for codes will be the best first step