Skip the auto shop - Our mechanics make house calls

Q: Code p0421

asked by on

I am getting a p0421 code but I just replaced the catalytic converter. I got the code erased after but it popped back on. My car seems to stumble/put a lot though. What else could it be?

My car has 148000 miles.
My car has an automatic transmission.

Most mechanics will use a diagnostic scan tool to download the source of the error codes. In the case of the P-0421, we usually find that either the catalytic converter has run its course, or the O2 sensors are giving faulty data and need to be replaced at the same time. If you’d like, a certified YourMechanic technician can come to your location to help you diagnose this code or replace the O2 sensors in your vehicle. To be safe, we recommend replacing all of the O2 sensors.

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

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

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: When start Mazda Protege, there is a squeak in the engine area

The noise is coming from the serpentine belt. The belt is most likely loose or glazed over from needing to be replaced. Check the belts for tightness by twisting it between your finger and thumb and it will be hard...

Q: I have an very bad oil leak.

The crankshaft seal is in the area you are referring to. The crankshaft seal lasts maybe 100,000 miles or so. There are other gaskets and seals in that area that would have to be examined as well. If you request...

Q: Transmission problem 3rd gear

Hey there. This is something your car will do when going into "safe mode" which can happen when the ECM detects something is wrong with any of the car's systems. This is a safety feature to keep you and the...

Related articles

Rules of the Road For Iowa Drivers
Driving on the roads requires knowledge of the rules, many of which are based on common sense and courtesy. However, even though you know the rules in...
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...
P2428 OBD-II Trouble Code: Exhaust Gas Temperature Too High Bank 1
P2428 code definition A P2428 trouble code signifies that the PCM has detected a problem in the exhaust gas temperature sensor circuit in bank 1, which subsequently contains the number one...