Weird Electrical Problem - Ideas?

[bimmer525]

Hello All,
The 525i (95, 158K miles) had the CEL on 4 months ago, with a O2 sensor code. I got the sensor replaced by the dealer.
I now have the weirdest issue, the CEL is constantly on with the same code (1221) O2 sensor. The dealer says that its probably something to do with the computer and would cost around $1200 to fix. ( I had an alternator changed before and he says it could have a sent a voltage surge).

Anyway what I now notice is that CEL turns off and on, and when it is on there is no problem (I get 22 mpg, city) but when it is off the rpm meter starts oscillating and the car stalls after sometime. When i restart the CEL is on and the car runs fine.

Any clue whats happening here? Anyone had any similar problems with their car?

Thanks in Advance.

[632 Regal]

first thing you should do is reboot the computer and see how it acts...while its rebooting check the intake system very closely for leaks. The O2 code can be caused by any leaks in the system and is common.

[Javier]

deal with them issues, but, what you describes have some sense for me.

This is my wild guess:

To have the computer reporting a bad O2 sensor should mean it reject the signal and operates open loop, working fairly good, if it uses a bad O2 signal, it should run weird.

Before messing with the computer (but after taking Jeff advice, which is good indeed), check the O2 sensor signal carefully, also that it gets the proper power for the heater from the O2 sensor relay. May be the relay or connector is bad and not heating it properly.

Javier