Q: Misfire and P0420

asked by on

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.

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.

Was this answer helpful?
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  1. Home
  2. Questions
  3. Misfire and P0420

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Coolant seems to disappear

If this "just started", it may be that there was trapped air in the system, since you did the water pump replacement, and so all you are doing is providing make up coolant at the moment. But, if the coolant...

Q: Issue with firing - 1991 Volvo 240

Misfires can occur from fuel supply, spark, and camshaft timing problems. You have replaced most of the ignition components that supply spark to your cylinders, but you could still need spark plug wires. They may look good and not be....

Q: Car stopped three times in one day

Hi. If the code is still present after both sensors have been replaced, and the engine is stalling, further diagnostics is needed. First of all, a new sensor doesn’t always mean a good sensor, especially aftermarket ones. Check the sensor...

Related articles

How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the...
P2103 OBD-II Trouble Code: Throttle Actuator Control Motor Circuit High
P2103 means there is a fault with the throttle actuator control motor circuit, likely due to a defective electrical component or part.