Hello I have 2001 grand marquis mercury

Asked by Oct 20, 2016 at 10:30 PM about the 2001 Mercury Grand Marquis LS

Question type: Maintenance & Repair

Hello I have a 2001 grand marquis mercury
and my check engine light is on so I took it
to AutoZone and a popped up which is
P0420 indicates that the bank 1 Catalyst
system efficiency was below threshold for
a predetermined period of time can you
please help me fix the problem or any idea
of what it could be?

1 Answer

69,295

This is telling you that your catalytic converter is worn out and needs to be replaced. Try this, using a hand held scan tool clear the computer. See if the code resets and the check engine light returns. You must keep the gas tank between 3/4 and 1/4 for the computer to check the catalytic converter. So once you clear the computer don't fill up for a few days. If you have to add gas only put enough in to keep the tank within that range. If after about a week the code doesn't come back, you're ok. However, if the code returns you'll need a new catalytic converter. When shopping for a new catalytic converter you must know if your Grand Marquis has Federal Emissions or California Emissions. The emissions decal under the hood will tell you. Consider a universal catalytic converter. They're much cheaper and work just as well as a direct fit unit. I've had a universal converter on my 2000 Grand Prix since 2013 with no issues! HTH. -Jim

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