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Discussion Starter #1
Had some really strange behavior with the front oxygen sensor function of my ~53K mile 2011 recently. What happened was that I discovered during smog check that the front O2 sensor readiness was not set. This was surprising, since it had been about 9 months and about 5,000 miles of driving since the last time there would have been a code clear – which was when I replaced the battery. As some other background, also about 9 months ago I replaced the ECM with a new unit sourced from Orio that was pre-loaded with calibration software and my car’s VIN. I only needed to register it to my car with Tech 2, and it has been running great ever since.

So I was really shocked to find out the front O2S diagnostic had never fully exercised since, essentially, the ECM replacement. And I had also never verified that the OBD2 monitors had run since the ECM change, since the car was running fine. But there is clearly something that has not been working correctly.

I then verified the front O2S function with Tech 2 using the real time data display, and I could see what looked like a FO2S normal voltage – around 0.4 – 0.5 -, increasing if I revved the engine. I then tried to run the standalone “service bay” test which is designed to exercise the O2S and catalyst diagnostics. This ran all the way through, and the O2S was still “not ready”. I tried again after a battery disconnect with the same result.

So here is where the real strangeness begins. With the engine off, I unplugged the front O2S connector from the engine harness, and sprayed some contact cleaner in the plugs (I did this thinking that if there was a marginal connection, there might be noise that could prevent a O2S diagnostic from completing). BUT, with the connector plugged back in, the Tech 2 now showed 0.00 volts from the sensor! Then the OBD system set a low voltage DTC. I later did some pinout tests and verified that there was about 0.4v coming from the (disconnected) O2S at idle, and there is also a 0.4v reference voltage from the ECM as measured at the harness plug. But when the plugs are connected the voltage drops to 0 on Tech 2. And after about 60 sec or so after starting, the displayed voltage on Tech 2 slowly increases from 0 to about 1.3v. This is with the low voltage O2S DTC set, I’m not sure if this voltage rise is a default action since the car is technically open loop with the DTC.

I am just baffled at how the action of unplugging and re-plugging a connector could cause this problem. As far as I can tell, the harness wiring is all good (and there is a voltage to the O2S input and heater circuit pins).

I have a new front O2S sensor coming today, I hope this at least resolves the 0 volt issue.

As for the readiness issue, this may be a software problem with the new ECM. I did not SPS the ECM when I received it, just installed it. But I suppose this may be needed. We’ll see what happens after I get the new sensor installed later this week.

Any ideas on this would be welcome!
 

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Did you let it dry before plugging in?
 

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Discussion Starter #3 (Edited)
Probably not completely, I didn't think about that actually. But afterwards I cleaned the pins of the O2S plug with a small file, and during the diagnosis process I made a patch harness to connect the sensor and harness together and also let me measure voltage. Sensor voltage at the patch line is 0.4, then drops to 0 once I connect the sensor to the harness. And I can also measure voltage signal from the harness side to the sensor, reference voltage of 0.4v (that drops to 0 when the circuit is connected).

I have used contact cleaner before and just reconnected the plugs with no ill effects. Could this cause a problem? I guess it would seem possible considering the sequence of events.
 
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