XJ40 ( XJ81 ) 1986 - 1994

Negative O2 Sensor Raw Voltage

Old 07-25-2018, 08:54 PM
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Default Negative O2 Sensor Raw Voltage

I got a FF44 code last week. I had this code back in April of 2016 and replaced the sensor with a Genuine Bosch 13313. Now I've got the code back... Would 2.5 years sound correct for a sensor? I'm seeing a 52K-mile replacement interval from Jaguar.

Anyway, I hook up the PDU, run the ECM datalogger and see the H02S feedback voltage pegged at 5V, and the H02S raw voltage at 0v, with no fluctuations, even with the car hot, and the 02 sensor heater working correctly. If I disconnect the 02 sensor signal wire from the harness, the H02S raw voltage goes to 0.47v and stays there as it should per the factory manual.

With the engine running, and the 02 sensor signal wire connected to a multimeter, I'm getting NEGATIVE values! At idle, voltages like -0.5v, while opening and closing the throttle, anywhere from -0.1v to -0.75v. The only time I'd get a positive value is at Key On Engine Off. Once the engine is turned off, the negative value slowly decreased to just above positive. I think the highest I saw was +0.2v after the engine had been off for a couple minutes. I restarted the engine, and within about 30 seconds, the voltage dropped again below negative...

I've heard that O2 sensor contamination can cause a downward shift in values. Has anyone experienced this before?

Old 07-27-2018, 07:12 PM
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So I got the new Bosch 13313 in the car, drove 30 minutes to work and all appears fine. Good gas mileage is back and no hesitation or CEL.

Here's the 2.5 year old sensor I pulled out this morning. Sure does look like silicone contamination...

Now the question is... Where did it come from? I've always tried to be careful about using "O2 sensor safe" RTV...

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