I'm having the same exact issue as littlemoule had to a T. (Cylinder 1 was misfiring (for 2 weeks) and dumping fuel in the cylinder that wasn't getting consumed, it's possible the raw fuel being pushed into the catalyst could have degraded the efficiency/lifespan of the catalyst)
After Johnny Mullet fixed my corroded wiring harness back in February the car has been running great but the mpg's are now suffering (was getting 37-40 mpg highway prior...currently maybe 30-33 mpg?) and now the CEL (P0421) has been on since
I've tried the old Scotty Kilmer lacquer thinner in the gas tank trick to clean the cat but that did nothing.
I just purchased two O2 sensor spacer/defoulers to try and trick the computer into turning off the CEL. Before I try it has that worked for anyone else before or should I just remove the front cat and clean it out real good with soap/water?
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