Q: P0420 code. Jeep wrangler 2011 with 111000 miles. Code disappears after appearing. Light lawnmover sound on acceleration

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Jeep is making a motorcycle/go cart type sound on cold start, when warmed up the sound subsides a HUGE amount and you can barley hear it, engine shoots P0420 code when gas is put in engine, also on occasion when driving, then it disappears about 50 miles later, and reappears. EGR valve has been replaced. I have noticed a gap between the manifold pipe and the down pipe under the jeep under the drivers side of the car. Also when the EGR valve was replaced was told manifold bolts were loose and they were tightened.

My car has 111000 miles.
My car has an automatic transmission.

The P0420 code relates to catalyst efficiency however many of the underlying causes of the code have nothing to do with a fault in the converter itself. To evaluate that code, among the data of interest are long and short term fuel trims and a mechanic would obtain that data from an automotive scanner. The reason the data are important is catalyst overheating from lean burn is one of the causes of P0420. In your circumstance it sounds like you might have an exhaust leak. For safety reasons, all exhaust leaks should be investigated and repaired before diagnosing the P0420 code or ANY trouble code. To have the exhaust leaks pinpointed, and repaired as necessary, the recommended service is an exhaust leak diagnostic. If you request that service, the responding certified mechanic will fix the leaks and then deal with the P0420 trouble code. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.

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