2002 Mazda 626 Check engine light with code P0421 goes on a

SUNNY0219

MEMBER
2002 MAZDA 626
4 CYL
FWD
AUTOMATIC
97,868 MILES

Car: 2002 Mazda 626 LX 4cyl 2.0L

The check engine light with code P0421 goes on when the car run city miles, and goes off when running highway miles or higher MPH in city. I sent my car to an auto shop, and they said that I need to replace the CAT, which runs about $1000, because of some problems with exhaust system. But the car passed the emission test over 1 month ago, and oil change is needed now. If it's not the problem of the CAT, what are the other possibilities?
Thank you!

4 Replies

KHLOW2008

MECHANIC
41,814 POSTS

Hi sunny0219,

Check for vacumn leakages around the air intake hoses and manifold.

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Wednesday, July 9th, 2008 AT 12:08 PM

SUNNY0219

MEMBER
6 POSTS

Thank you, KHLow2008.
If there is a leakage, should the air intake hose be replaced or something else should be replaced? Do you know how much to fix a problem like that?

I am a real idiot about cars.

Thank you!

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Wednesday, July 9th, 2008 AT 4:53 PM

KHLOW2008

MECHANIC
41,814 POSTS

Hi sunny0219,

If there is any leakage, you should try to stop it by any means that you deem fit.

Taping up, sealing, any way that can prevent the leak would do or replacement if repair is not possible.

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Thursday, July 10th, 2008 AT 6:15 AM

SUNNY0219

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6 POSTS

Thank you! I went to do an oil change on the car using my own oil filter and oil. The check engine light went off itself in 2 or 3 days after I did the oil change.

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Sunday, July 20th, 2008 AT 4:24 PM