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Emission code P0421on 2001 Mitsubishi Galant

I have a question for you.I have an emission code #P4021-warm-up catalyst below threshold bank 1.I have replaced the upper O2 sensor but not the lower O2 sensor.The inspection guy @Jiffy Lube told me today that my catalic converter could be bad.I cleared the code with my code scanner and the check engine light stayed off for about 15 miles and the light popped back on and I have the same code again P0421 any ideas or anyone had this problem before?

My first idea is not to use places like Jiffy Lube. The fact that the guy could be right this time is beside the point.

If your O2 sensor replacement was an attempt to fix the code then you started with the wrong one since that code comes out of a reading from the downstream sensor.

Here’s a web page that might help:

First just make sure that all other things are in order (basic maintenance items like plugs & wires, filters). Ultimately O2 sensor reports are just about the contents of the exhaust itself. Lots of things can throw that off.

Other than that you’re probably looking at that downstream sensor or the cat. Somebody with a decent scantool (other than Jiffy Lube) can get a good read on what the sensor is doing.

Before you buy any more parts, pay a professional to diagnose this problem. You will save money that way.