I think I finally figured it all out. You're exactly right. Bentley fails to mention that the car must be at operating temperature when testing the O2 sensor (just mentions "idling"). I installed a new O2 sensor tonight (after all, that is what the original fault was). I discussed the problem with the owner of a local European shop and he mentioned that later versions of DME actually did not heat the sensor at startup, and would wait until conditions were right to start looking at the sensor. Thanks for your help, though.

Bill B. '95 525ia, running right again!!

Terminal 85 of sensor heater relay is grounded by the ECU and is used as "coil defective sense" for this relay also. So, if you ground it and the ECU doesn't, it detects a defective relay coil, because this terminal should be pulled up to 12V by the relay coil.

As far as I know, the O2-sensors are heated only when they are used, this means, only if the engine is completely warmed up. Was it warmed up?

Thomas
'94 530i/5 sharked
'95 530iAT not sharked yet

Guys, this is one of those weird ones. I just changed to synthetic yesterday, and installed new Bosch platinum +4's, took the car for a quick test drive, and got the check engine light. Came back, stomp test, 1-2-2-1, O2 sensor. Did a little snooping, and found that the sensor heater was not getting voltage. A little more checking, and the relay is not switching on and off with juice applied. I had an old relay from foglights (same amperage and voltage) that I disassembled so I could see what was going on, and plugged it in. Sure enough, no ground on terminal 85 of the sensor relay. Grounding the relay gave voltage at the sensor heater, but then I got code 1-2-6-4, O2 sensor heater relay fault.

Anyone ever experience this, or have any ideas? I don't want to drive it again until I resolve this, and I am afraid I'm looking at a new ECM. HELP!!!

Bill B.
'95 525ia, dead in the water (kinda)