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2001 2.5L OBW
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45 Posts
Discussion Starter #1
My hi-mileage H-4 2001 OBW gets a CEL every winter when it turns cold and damp. Because I failed to record it last winter, I've been waiting, defoulers in hand, for it to come on again this winter so I could confirm the code. Regular as clockwork, this past week the CEL came on again. And if the title of this thread caught you're eye, you've probably already guessed the code was the (in)famous P0420.

Can I tell from that code which O2 sensor threw it? If not, is one sensor substantially more likely than the other to be the culprit?

FWIW, I've already checked for leaks (didn't detect any), and it's been <10K miles since I last ran a can of SeaFoam through it.

Thanks for your help!
 

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I simply cannot abide useless people.
2006 2.5i and 2002 3.0 wagons.
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12,306 Posts
In my experience it is the front sensor that fails more often.

Although the defouler bypass is for the rear sensor.
 

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Registered
2001 2.5L OBW
Joined
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45 Posts
Discussion Starter #3
I failed to mention I can't tell any difference in the way it runs when the CEL is on, and fuel economy doesn't change noticeably.

Thanks for that, AWDFTW. I'd overlooked that detail about the fix.

I'm less than 1K miles from the next oil change, and I have some other under-car maintenance bits to do then too, so I'll probably de-foul the rear sensor (provided I can get it out), just on general principles.

The good news is that my emissions test is in June, when there's little chance of freezing temps here.
 

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I simply cannot abide useless people.
2006 2.5i and 2002 3.0 wagons.
Joined
·
12,306 Posts
The front sensor is easy to change, all I needed was a O2 socket, some PB Blaster (ok, a lot. stupid Chicago winters) and some creative cussing for the times I'd smack my hand into exhaust and frame.

I'd guess the rear is about the same job.
 
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