O2 sensor pre-heating - BMW Forum - BimmerWerkz.com
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#1 (permalink) Old 11-25-2003, 09:33 AM
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The oxygen sensors on my 95 540i are not being heated. I traced it back to the ECM, which is not grounding the relay like it should. What tells the ECM to activate the relay? I checked the temperature sender at 70 degrees and it was fine but have not checked it when the problem occurs. This is really only a problem first thing in the morning when it is cold. The idle goes to hell and the check engine light comes on with the O2 sensor error. The mass air sensor also comes on once in a while with the O2 error but not always. I was told by a BMW shop that the ECM's almost never fail and when they do they will give an ECM error code, so I'm not seriously considering that as the problem. HELP!
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#2 (permalink) Old 11-25-2003, 01:26 PM
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were you able to pull codes? If so, were they specifically O2 heater codes? Typically the coolant temp sensor as well as the ambient temp sensor grounds the O2 sensors. The mass air meter can also. I would certainly double check the coolant temp sensor when cold. this is most likely your problem. ECM's do fail more than any BMW enthusiast would like to admit. However, I would not suspect that it is causing your problem. My first suspicion would be the temp sensor.

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#3 (permalink) Old 11-25-2003, 01:31 PM
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#4 (permalink) Old 11-26-2003, 04:23 AM
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Yes, I could pull codes, they were the O2 #1, O2 #2, and lamda #1, and #2 and Mass air sometimes. I will check both temp sensors tomorrow. Thanks for the Help.
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#5 (permalink) Old 12-01-2003, 04:30 AM
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I checked both the coolant and air temp sensors when cold, they were both within limits and had power. I'm thinking air mass now since the same idle problem happens when I exit off the freeway and come to a stop, even though the engine is warmed up.
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