My 2004 Jetta started saying that it had a cylinder one misfire in and multiple cylinder misfire. It started driving horribly like it had no power, so I replaced the coil pack and spark plugs and this seemed to fix the issue. And then shortly after it started driving again like the coil was bad so I took it back in and they replace the spark plug wires which fixed it. And now it is throwing a P0420 code saying a problem with the catalytic converter. It’s driving pretty much normal so I don’t know what’s up with it. This has been going on forever and I need to this car to work so any help is appreciated
Take the car to a shop and have them monitor the signal out of the O2 sensor after the cat.
If the O2 sensor after the cat shows the same signal as the O2 before the cat, it means the cat was damaged while the misfires were occurring.
Tester
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Let’s say it was damaged how immediately do I need to change it?
If the cat is plugged up, that will cause the engine to run badly or stall out. Cats can also fail in a way they develop internal damage which breaks their internal honey-comb matrix in pieces, and then they’ll shift around, allowing exhaust gasses to pass sometimes, and not others. The test above is an excellent place to start to help deciding if it is time to replace the cat.
It would be a little surprising to have a cat failure like this on a 2004. Unless something happened over time, like extended unrepaired misfires, rich or lean codes, cat hit by a rock, etc.