1999 A8, just started today without warning. sounds like a very bad misfire. VCDS says it's cylinder 4. I changed out the plug and the coil. no difference. swapped the ECM with a good spare. no difference. fault codes are P300 & P304 for the misfires, and the third is a P1690 electrical malfunction relating to the MIL (check engine) light not working. doesn't come on when the key is turned, or after starting. I know it was working before this. I scanned the car about a month ago and it was fine. the car can not be driven over about 20 mph, at which point it shakes violently. I've been through every relay I can find. pulled them and reinserted. nothing makes a difference. all fuses I've checked are ok. this happened one other time. I unhooked the battery for an hour, and it cleared up. that was last fall. it's been a-ok till now. tried the unhook battery deal too. no luck. what would make the MIL light malfunction ?

 

changed ignition control module. runs great now. still no MIL light. don't know why...anyone know why there are 2 ICMs ?

 

Not sure, with these cars age and number of connections it could be just a poor connection. Get some connectivity grease in the spots you visit. Also have watch the ECM pins,.. I had some moisture get into my PLETH box and corrode a pin off and the car wouldn't start. By-passed it and the car fired up as if nothing happened.