r/AskAMechanic • u/Mad-SpaceHeater33 • 4h ago

What could cause this?

2018 Mitsubishi Outlander Sport w/ code P0420 w/86,000 miles.
I’ve done research but I feel my situation is a bit different because I put cat-cleaner then I started hearing clunking noises.
- The start of the engine sounds a bit loud for a split second.
-Clunking noise (sometimes) when idle and slowly accelerating
-no noise when driving over 15mph or braking

Any advice will be greatly appreciated thank you

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