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


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

3 of 3 people found this helpful.

Your Answer

Mercury Grand Marquis Experts

  • #1
    Tracy Hooks
  • #2
  • #3
View All

Related Models For Sale

Used Lincoln Town Car
19 Great Deals out of 1,434 listings starting at $1,850
Used Ford Crown Victoria
925 listings starting at $1,999
Used Chevrolet Impala
371 Great Deals out of 27,339 listings starting at $1,499
Used Ford F-150
2,488 Great Deals out of 220,740 listings starting at $1,222
Used Ford Mustang
287 Great Deals out of 38,472 listings starting at $1,995
Used Chevrolet Tahoe
641 Great Deals out of 37,663 listings starting at $795
Used Honda Accord
553 Great Deals out of 91,963 listings starting at $1,495
Used Cadillac DeVille
1,247 listings starting at $2,499
Used Chevrolet Silverado 1500
2,170 Great Deals out of 150,762 listings starting at $1,499
Used Ford Taurus
147 Great Deals out of 15,533 listings starting at $1,900

Used Cars For Sale

2010 Mercury Grand Marquis LS For Sale
67 listings starting at $2,900
2009 Mercury Grand Marquis LS For Sale
4 Great Deals out of 71 listings starting at $3,400
2008 Mercury Grand Marquis LS For Sale
70 listings starting at $3,400

Content submitted by Users is not endorsed by CarGurus, does not express the opinions of CarGurus, and should not be considered reviewed, screened, or approved by CarGurus. Please refer to CarGurus Terms of Use.