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I have gone back and searched out past postings for OBDII code 0402

 

I get that it may be the O2 sensor, but there seems to be some conflict on weather it's the front or rear one.

 

How would one go about checking the cat.?

 

Car seems to be running ok, but the CEL comes on, get this code, clear it out, and it can stay out for days or even months.

 

Usually seems to come on when it's raining or been through a puddle, but not always.

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It's a 1995 Legacy 165,000 mi. Exhaust is nice and quiet. I don't know if the sensors have ever been replaced. My guess is not, as I just changed the plugs and wires. Let's not go there! It seems to be running well. MPG are around 22-26. No smoke or anything else. Just the CEL coming on periodically.

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There are several OBDII readers that allow you to see the performance of each sensor. The one they use at Autozone only outputs the problem code.

 

I think the one from Harrison let's you see the indivdual sensor.

 

BTW try and reset the computer and see if it goes away. My '98 threw the code after I went through a puddle. Reset the ECU and it went away - it's been 2 years.

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  • 8 years later...

My Forester would generate that very code from time to time.  Then its first winter with me came and it started generating that code (and a related one) much more frequently.  

 

One day, when I was was peering under the hood I noticed that the cable to the negative battery terminal was very loose.  So I took that plastic-thingy out and clamped it on real tight.  So far no more problems.

 

This winter has been especially cold so the car and its components really shrunk!

 

 

--Damien

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This thread is 9 years old, so probably won't ever know what the OP was asking about.

 

Efseiler, check out the P0420 diagnostic thread on SubaruOutback.org

Tons of info there for diagnosing the 420 code. If you have any regular maintenance items that haven't been done recently (plugs,wires, PCV, filters, etc) be sure to do those before trying anything else. Lots of times just doing a good tune-up fixes this code.

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As far as I can tell the problem really was just that.

 

 

That component of the electrical system was probably sensitive to such voltage fluctuations and gave a misleading code.  Either that or there was a ghost in the machine...

 

 

 

Makes me want to watch 'Poltergeist' one more time!

 

Cheers!

 

--Damien

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