Skip the auto shop - Our mechanics make house calls

Q: The check engine light came o and the code is P0420 and I can't figure what exactly to replace

asked by on

I replaced the downstream o2 sensor and I the light went out for a while but came back on should I replace the CAT or he upstream o2 sensor

My car has 150000 miles.
My car has a manual transmission.

A: As you probably know, the catalytic convert...

As you probably know, the catalytic converter reduces emissions by injecting oxygen into the exhaust stream, mixing oxygen with the emission gases and essentially incinerating the pollutants before they can exit the tailpipe.

The converter works with two oxygen sensors, one upstream near the exhaust manifold and one downstream on the other side of the converter. If the readings between the two sensors are too similar, it indicates that the converter isn’t working properly. Your guess that the upstream O2 sensor or catalytic converter could be at fault are likely correct. The good news is that the catalytic converter may be covered under warranty. Be sure to also check the wiring and connectors to the upstream O2 sensor before replacing the unit. If you need to have this serviced, a certified professional from YourMechanic can come to your location to diagnose the Check Engine Light and can move forward with the appropriate repair.

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: Bad gas mileage

Hello there. A decrease in fuel milage can be attributed to a few different faults. The most common causes are leaking fuel injectors, oxygen sensors, coolant temperature sensors, vacuum leaks or exhaust leaks. A qualified technician, such as one from...

Q: My car has only misfired twice but idles perfectly. The Check Engine Light is also on - 2012 Buick Regal

Hi there. From the description you have provided, concerning your vehicle's Check Engine Light and two misfires, it is advisable that you get a second opinion. A timing chain problem would cause your vehicle to have a consistent drivability issue....

Q: EPC doesn't turn off

Hello. The EPC (electronic power control) can cause your vehicle to go into what most technicians call a "creep mode." What it's doing is limiting the amount of throttle that can be applied to the engine in order not cause...

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 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...
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.