Oxygen Sensor Question
 

This is my first post on the Forum so please bear with me.

The car is a 1993 525i sedan with an M50 engine about 150k miles. I had it give me a check engine light the other day and read the code out as a 0d (Peake tool) which is the Oxygen sensor code. The car runs fine when cold and starts easy, runs fine when warm with no stumble and smooth idle. What happened was that when the car warmed up and switched from open loop operation to closed loop operation it stumbled for an instant and then gave the code but then continued to run fine and never stumbles again.

I am assuming that when the car is cold the DME has the engine run in "open loop" meaning it does not really read the O2 sensor until it hits a certain temperature and then switches to "closed loop" and then uses the O2 sesnor to correct for mixture? Is this correct? Again the car runs fine hot and cold with only the one time stumble which seems to happen sometime between cold and then when the car first reaches proper operating temperature.

The Bentley manual suggests testing the O2 sensor and per their instructions it seems fine. It further suggests testing the O2 heater and relay which also seems fine. EXCEPT the Bentley manual says that the heater should be on when the car is on! My DME is not turning on the O2 heater when the car is cold and I have not yet tested it when fully warmed up.

My question is on this model when should the DME turn on the O2 heater? Right at start or later in the sequence or ever? Is there a way to test this output?

Thanks for any assistance!

Everything you said sounds correct and I think the O2 heater should be on when cold. It sounds like you have tested it but the test I would try is for power at the O2 sensor.

I would reset the light and make sure it returns before going further.