Thread: O2 Sensor-upstream or downstream

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  1. #1 O2 Sensor-upstream or downstream 
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    Hey guys
    I know I've been posting on here alot but you guys are a wealth of info.Especially my fellow Jerseyite Scottydog.My 2008 pontiac grand prix has had the engine light on now since the summertime. The last time I had it scanned it pulled a P0420- Catalyst System Efficiency Below Threshold Bank 1. I had it scanned by Advanced Auto in July. They couldn't clear the code but said if the code is there than the problem is still there. Anyway, A week before my engine light went on, my car stalled at a red light. I had the local shop check it and it had another code initially as well which they said was the crankshaft sensor. They replaced that but the light was still on. So then they said "O2 Sensor" and bank 1 meant upper, so I bought the part and replaced it myself. Well that was in July. My car has more power now but the light is still on. Im not smelling any rotten eggs smell so Im guessing my catalytic converter is ok (not that this is the only way to tell if it is shot or not). I dont have the money to bring it to another shop,spend whatever they charge just for them to give me the same P0420 code. Is it possible my downstream sensor is bad? I really need it fixed because in NJ, you can have a crack across your entire windshield,which mine does, and pass inspection, but if your engine light is on, then you automatically fail. Im due at the end of May. I can change the sensor myself and will if I know that will give me more power and turn my light off. Sometimes my car feels like it has a lot of power and other times Im afraid to drive in the fast lane up hills. It just lacks the power it used to have. Is there any other cause for that code to appear?
    John in NJ
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  3. #2 Re: O2 Sensor-upstream or downstream 
    Killa Bee Scottydoggs's Avatar
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    id hit a auto zone, they do free scans, then see what codes you actually have at this point in time.

    as far as the o2 code, theres only bank one, but theres two sensors, up and down stream. down stream can cause a cat code p0420 if its not good.

    98 Buick Regal GS, F body brakes, Caddy STS wheels, tinted tails L36 bottom end, lightly ported heads, 1.95 roller rockers, headers, 2.8, FSIC, 42 lb injectors, a BrandonHall rebuilt trans, DHP tuned and AEM water/Meth injection https://goo.gl/gpV5kW
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  4. #3 Re: O2 Sensor-upstream or downstream 
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    Now I will say absolutely check the code and confirm that it's a p0420 but honestly unless you have some other codes to go with it o2 sensors are useless, they actually performed perfectly in detecting that your cat is no longer purifying your exhaust fumes correctly. So more than likely your cat is failing, you have a few options you can replace the cat, whether you weld a new one in or replace the assembly or the cheaper and equally effective (for passing nj inspection) you can install o2 extenders. What the extenders do is the place your o2 sensor away from the exhaust stream simulating cleaner air. I live in Nj and have had many friends and family pass using this technique for the same code.
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  5. #4 Re: O2 Sensor-upstream or downstream 
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    As background I am a technician for toyota
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