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Q: Misfire and P0420

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Originally my Alero had codes: P0300 and P0420. I replaced the plugs and wires and the misfire code went away. I now just have the P0420 code for catalyst bank 1. However, my engine is misfiring according to the Torque Pro app. The waveform on the rear oxygen sensor is not steady. It is all over the place. My car runs fun and performs fine. Also, my vehicle is known for having a bad intake manifold gasket which is plastic so I know I need to replace it with the updated one. It does leak some fluid and run hot once in awhile.
My car has an automatic transmission.

A: Hello - the P0420 and the inconsistent wave...

Hello - the P0420 and the inconsistent waveform for the post-cat O2 sensor might be related to your failing intake manifold gasket issue. If tiny droplets of coolant are being sucked into the engine - one failure mode for this problem - it is passing through to the catalytic converter, and contaminating it. It may have coated the catalyst badly enough to ruin it already - you don’t mention your mileage. The hot running you mention may be indicative of a head gasket issue as well. I would recommend having a Car Is Running Hotter than Normal Inspection completed by a mobile, professional mechanic, such as one from YourMechanic, to diagnose this problem, get an accurate assessment of damage and cost estimate for repairs.

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