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I recently got a Check Engine light on my '88 325is -- showing the O2 sensor code. As the car has about 200k on it, I went to the dealer and got a new sensor, installed it and cleared the code. As soon as the car began to warm up, the light came on again, with the same code. I checked the voltage output of the sensor (vs Haynes manual specs) and it seems ok. What is wrong is that I get no voltage from the electrical system to the O2 sensor on the leads that should power the "sensor heater". Haynes says this is a failure of the O2 sensor heater relay, but doesn't say where it is. BTW, the car is idling roughly and too slow, whether cold or warm. Is this relay the problem? If so, where is it located? Could something else be putting out the O2 code to the check engine light?

Thanks for the advice!
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