E36 OBD1 O2 Sensor Heater Circuit
 

I have a 1992 E36/M50 that just started giving me a Code 201 - Oxygen Sensor Control.

During troubleshooting I wired in a light bulb down at the O2 sensor connection and used the scan tool to have the ECM test the heater circuit. Just as it should, the light bulb cycles on and off as the ECM toggles the HO2S relay (key on engine off). While that checks out okay the ECM is not powering the circuit when the engine is running (checked from cold start and my test light bulb never came on). The ECM not heating the sensor results in an idle surge and CEL.

Running the engine at 2800 RPM for about 30 seconds is enough to heat the sensor and return the system to closed loop with a clean, steady idle and no check engine light.

What "tells" the ECM to power on the O2 sensor heater circuit? I suspect I could have a faulty ECM but would like to make sure I cover all my bases before replacing it. What am I missing?

Thanks.

On many cars, and possibly this one as well, the coolant temp is sometimes what the ECU reads. When the engine is cold, the ECU runs in a "cold-start" mode where it sends a pre-set air/fuel mixture to get the engine warmed up.

But ECMs don't fail often on these cars. It still may be worth installing a new oxygen sensor. This car only uses one of them so the cost wouldn't be too bad.

-Matt

Thank you for your reply, Matt. My apologies for the delayed acknowledgement.

Though I failed to mention it in my initial post, the very first thing I did was replace the O2 sensor (OE replacement Bosch). I figured after 298K miles it was probably time anyway.

According to the scan tool (Snap-on MT-2500) the coolant temp sensor appears to be working properly. It reads ambient when cold and gradually rises to ~190 F when fully warmed.

While I did get it through smog by jumping the HO2S relay (to heat the sensor full-time, outside of ECM control) it still isn't clear what the problem is with the electronic control system.

I've been driving the car for 16 years so I'm determined to figure this out. I'll post here when I find the solution.

That is very peculiar. On that engine you have two coolant sensors on the cylinder head. One is for the gauge and the other is what the ECM reads. I wonder which one the scan tool reads. Either way they are both burred under the intake and not super fun to access for testing.

The ECM itself might be worth replacing if you can find a junkyard with one at a reasonable price. I believe 91-92 E36 325i and 91-92 E34 525i both use the same computer and there's not really any security features so it should be a straight swap.

Other than that I feel like it comes down to a bad wire which would require some time and effort to trace.

-Matt

UPDATE: A replacement ECM (from eBay) solved this issue. FWIW, the crankshaft sensor harness was damaged near the thermostat housing and those wires may have been touching each other. The car still ran but that may be what damaged the computer (Changing the crank sensor by itself did not solve the problem). Thanks again for helping me troubleshoot this.