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Old 10-21-2010, 11:43 AM
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Default 1997 Monte Carlo p0420 code.

Hi All,

This is my first post here. I have a 1997 Monte Carlo with 124,000 miles. Check enine light came on recently. I had the light read and it came up with P0420, Cat Efficiency Threshold. The gentleman stated that I needed to replace both 02 sensors as this was the culprit of the check engine light. He stated that considering I am having no performance/mechanical issues with my car, the cat couldn't be clogged. I just wanted to make sure I was heading in the right direction before I replaced both 02 sensors. Any help is greatly appreciated.
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Old 10-22-2010, 08:27 PM
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Dont the the o2s they wont fix it. Its your converter. Its probably broken up inside so it flowing too much and not working properly

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Old 10-22-2010, 08:41 PM
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Mine throws that code intermittently, I had to put a new gas cap on, and am being told that my car had a loose wire for the gas tank pressure sensor.

Like I said, for me, its intermittent I always have been told to start with a gas cap and then move on to maybe a bad sensor wire.

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Old 11-09-2010, 03:00 AM
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does your car running normally?
i had the same code and my car was running pretty bad. trouble accelerating and engine hesitated and it turned out the cat conv was clogged so i replaced that and the light went off and car functioned normally

you can look at this link for mor suggestions

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Old 11-11-2010, 07:43 AM
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Default P0420 code

The P0420 only refers to the rear O2 sensor/cat operation. If your front O2 sensor was not working properly then your car would be running like crap. The front O2 sensor deals with your mixture and the rear O2 sensor only deals with the operation of your cat.

Have you had any work done on your car lately, like an aftermarket exhaust? Sometimes that will trigger the P0420 code. Also, if your car is burning coolant that will affect the rear O2 sensor and probably clog up your cat over time. Do you smell rotten eggs at all? Sometimes that is an indicator of a bad cat.
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Old 11-12-2010, 09:14 PM
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Most likely your cat. converter. Advance Auto tried to sell me the o2 sensors too but I got online, did some checking and sure enough it was the converter.
BTW, my car was running fine at the time. Mine was replaced by GM under a "special policy" - they put out some bad cats during '01-02.
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Last edited by dustybird3; 11-12-2010 at 09:16 PM.
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Old 11-13-2010, 04:55 AM
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i hollowed my cat out and havent gotten that code scince then.
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