Check Engine Light: Error Code P0133
How do I know if this car was involved in the Service Bulletin that called for the extra Wire Harness on the O2 Sensor cable?
I called Dealer, and Parts person said it is not, based on VIN.

How do you clear Check Engine light? I tried disconnecting battery, and that did not work.

Didn't know there was a Bulletin on an extra wire harness.
That would explain why it was on our '01 when we got it. I took it off as it didn't seem to be doing anything. Haven't noticed any problem with it eliminated
over the past 5-6 years.

You'll need a scan tool to turn off the CEL. Don't be surprised if it is
difficult to reset. They can be sensitive to the type of scan tool that is being used. I've got a ScanGauge II that has worked with a push of a button. There
was one time though that I needed further instruction from the ScanGauge
manufacturer. But it did do the job.
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"Differences of opinions should be tolerated, but not if they're too different' - Sharon Craig

Do you still have the extra wire harness you took off? I continue to have the problem of the light going on for a while, then shutting off.

I cleaned the plug contacts and disconnected battery to reset. But upon startup, CEL came back on. Drove to work 3 days with CEL on. No noticeable difference in performance or fuel consumption.
This morning, CEL did not come on after startup. About 5 miles into my commute, it came on. Is this a significant clue? For example, does the O2 sensor fail, or does it deteriorate.

I have read many posts concerning replacing O2 sensor and CEL coming back on, so I don't want to replace it if something else is causing the issue.

O2 sensors deteriorate and should be changed out after 100K miles. You will need a scanner to turn off the code, some autoparts stores will give you one to use in the parking lot - Autozone will not clear codes.

Amazon sells O2 sensors for around $80, OEM.

Because the CEL turned off and then on again, check for vacuum leaks before ordering an O2 sensor.

No other codes? Exhaust leaks before the O2 sensor?
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My back feels better when I sit in a Volvo seat

CEL did not come on at last startup, and stayed off for entire 10 mile trip.
I would assume that O2 sensor just dies, and does not become erratic; is that true?
Person who checked code for me at O'Reilly claimed that bad gas can cause CEL to show bad O2 sensor.
In any event, my plan is to look for a vacuum leak.