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Old 03-23-2008, 10:49 AM   #1 (permalink)
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Time for another question.The CEL in my 2000OBK has been coming for a few days, then off for a couple days, for about a year now. The only code that can be read by the fine folks at Auto Zone is the infamous P0420, which I assume is the rear oxygen sensor.
The car runs every bit as as it ever did, and mileage is no different, but I'm wondering: are there long term effects of not changing this expensive little doo-dad? Will it plug my cat, or somehow whack out the computer?
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Old 03-23-2008, 11:43 AM   #2 (permalink)
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No its the front sensor, not the rear. rears outlast the front by miles. Aside from wasting gas as time go buys, and taking a chance on hurting your cat, and hurting performance as time go buy, and maybe fouling plugs, its up to you.

The o2 sensor is the heart (er nose) of the FI and emissions system. Replace it.

Get it at an on line OE supplier (1st subaru etc).


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Old 03-23-2008, 05:29 PM   #3 (permalink)
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Sorry Nipper but you're wrong on this one. The PO420 regards the rear O2 sensor and the difference in temp drop across the catalyst. Basically, when you get the code it means the cats aren't doing their job, or the sensor has fouled.

If there are exhaust modifications you can get that code, or outdoor temps being too cold...

The front O2 sensor is 99.9% of the brains when it comes to running the engine.

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Old 03-23-2008, 05:42 PM   #4 (permalink)
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Quote:
Originally posted by bheinz57
Sorry Nipper but you're wrong on this one. The PO420 regards the rear O2 sensor and the difference in temp drop across the catalyst. Basically, when you get the code it means the cats aren't doing their job, or the sensor has fouled.

If there are exhaust modifications you can get that code, or outdoor temps being too cold...

The front O2 sensor is 99.9% of the brains when it comes to running the engine.

Brian
The exact code does not specify either O2 sensor, front or rear, just catalyst effeciency. It does this by comparing the data from the front o2 sensor and the rear O2 sensro. If the front O2 sensor is original and over 80,000 miles on it, odds are its the front one. The front one takes more of a beating then the rear, unless the rear gets hit by something...


which is how i killed mine


Considering you said its a fouled sensor, sort of seems that you agreed with me.



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Old 03-23-2008, 10:04 PM   #5 (permalink)
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^Well personally I have never heard of a front O2 sensor being replaced due to that code.

I did just find basic info on the PO420:


My check engine light came on and my mechanic said the Catalytic Converter was the problem. How can I be sure?

This has been a tricky problem for most mechanics and exhaust shops, When diagnosing the problem with a scan tool. The code that comes up for an inefficient Catalytic Converter is a PO420. This is an indication that the Catalytic Converter is not doing its job properly but we have found that this code could also occur if the Oxygen Sensor (O2 Sensor) or sensors become faulty or lazy, We have also found that on the Jeep Cherokee, Subaru Outback, Ford Explorer and Honda Accord, the factory tolerances that set off the check engine light are very tight and an engine that has carbon buildup in the combustion chamber will bring up the cylinder head temperature which in turn brings up the Oxides of Nitrogen (NOX) and will set the light off. In these cases we recommend a product on the market that has been working well to cut down on the carbon build up and will reduce the dreaded check engine light and PO420 code. The product is called BG44K- Total Deposit Control Fuel Additive and can be purchased from us at our product link. A good rule of thumb here is to make sure all components that we have mentioned here have been checked before the Catalytic Converter is replaced. There is nothing worse than spending hard earned money on a Catalytic Converter and find out that the check engine light problem still exists.

A decent quick read that I found.

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Old 03-25-2008, 08:58 AM   #6 (permalink)
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I have the same problem. I went to my mechanic yesterday, he pulled codes, p0304 (cyl4 misfire) and p0420 (famous exhaust code). So I am not worrying about p0304 but p0420 could get pricey if not dealt with properly. He read my o2 sensors activity, first one was reading 1.500 and the second one was 0.650. I know it's normal for the first one to read higher then the second one but does anyone know exact numbers that are acceptable?

car is the 02 impreza outback sport.

thanks!
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Old 03-26-2008, 11:24 PM   #7 (permalink)
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is the part number for the rear o2 sensor: 22690AA38A
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Old 03-27-2008, 07:52 AM   #8 (permalink)
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That's the correct part#.
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Old 03-27-2008, 11:05 AM   #9 (permalink)
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thanks. my car is almost at 167k miles, so should i replace both? or just the rear is enough?
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Old 03-27-2008, 11:16 AM   #10 (permalink)
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If your just going to replace the rear, you can get that from any autoparts store. The front one is the one that needs to be from subaru.

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