Skip the auto shop - Our mechanics make house calls

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.

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.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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: After heating core replacement, oil gone and engine ruined

Hello. The heater core would not have anything to do with the oil. If anything they may not have properly installed one of the heater hoses which may have causes the coolant to leak which would have caused the engine...

Q: Code P0420. Need to replace catalytic converter.

Hi there. The code P0420 is coming up for the Downstream Oxygen sensor has picked up an abnormal reading and is saying that the catalytic converter is below threshold. There could be a misfire on the engine, leaking injector, ignition...

Q: Why is it still misfiring?

Hi there. Check the Exhaust Gas Recirculation valve on the engine. It seems to be sticking on when the engine is running at low idle and causing a misfire as the old exhaust that is burnt is reentering the engine....

Related articles

What Causes Hoses to Leak?
While the largest part of your engine is mechanical, hydraulics plays a significant role. You’ll find fluids at work in a number of different areas. Your car's fluids include: Engine oil Transmission...
How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.