my check engine light is on

Asked by Aug 08, 2012 at 04:19 PM about the 2000 Mercury Mystique 4 Dr GS Sedan

Question type: Maintenance & Repair

My car is throwing codes for both oxygen sensors, so i changed both of them and drove the car for a few days but
the light won't go off and I don't think my cat is bad so what could it be?

8 Answers


What is both sensors? 2 upstream? 2 downstream? An up and a downstream sensor?

1 out of 1 people think this is helpful.

1 upstream and 1 downstream


Codes were cleared when fixed and its confirmed the same codes are still there? The only codes were the o2 sensor codes? If the cat was bad the downstream would be the only one showing problems. What were the codes exactly?

I only disconnected the battery when i installed the 2 sensors, but i'm not sure if that clears it. Before i did the work, the diagnostics codes were "upstream bank 1 and downstream bank 2


Were they saying rich or lean? Or insufficient flow? Have them checked again and see what they are. Downstream bank 2 leads me to belive there are 2 cats, one on each bank. Is this true?

No they didn't say rich or lean or insufficient flow, it said heated something bank 1 and the same for bank 2. The car has only 1 catalytic converter


Maybe check the wiring. If the engine light is on for the same 2 codes then it could be wiring. If its on for just the downstream one then the catalytic converter is probably dead. If an upstream on goes bad it can make the car run rich and poisen the catalytic converter, and when it goes bad the downstream sensor can trow a code because its not reading what it expects.

I think I found the problem, the wires for the upstream sensor were all cut. I think the car had mice living under the hood at some point, so I reconnected the wires and wrapped it all in electrical tape and i'm pretty confident that I fixed this annoying problem! Thanks for your input, I really appreciate it.

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