Skip the auto shop - Our mechanics make house calls

Q: P0421 comes up on scan. What is the likely culprit

asked by on

What should I need to fix.

My car has 142000 miles.
My car has an automatic transmission.

A: Hello. When you get the P0421 code, it usua...

Hello. When you get the P0421 code, it usually means that you have a failing catalytic converter. It is very common for converters to fail on this vehicle over time. Most of the time I use a scan tool to check the output of the oxygen sensors first, as a bad sensor can cause this code. If the sensors are reading right, then the converter will need to be replaced. If you want to have the code diagnosed, a certified technician from YourMechanic can come to your car's location to pinpoint the proper 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

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・Save up to 30%

Get a quote

What others are asking

Q: My car has a high idle when in park and doesn't shift properly

Hi there. Cold idle can be as high as 1500 RPM, but should lower to around 750 RPM for this vehicle once warmed up. This could be an air leak into the intake area of the engine; an auditory check...

Q: P0421 and P0401 codes have appeared

Hello. The codes P0421 and P0401 refer to the oxygen sensor and EGR valve, respectively. It is possible that an issue with one is causing the fault to be seen by the other, as they will work loosely together. It...

Q: Check engine, traction off lights are showing

The Check Engine Light is likely still on due to the oxygen sensor faults. The unfortunate thing is that engine computers sometimes cannot detect another problem if they are dealing with one already. That may be why it didn't have...

Related articles

How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...
How to Renew Your Car Registration in Oklahoma(DELETED)
Having your car registered with the Oklahoma Tax Commission is an important part of driving legally in this state. You will need to do this within 30 days of moving to Oklahoma...
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...