Skip the auto shop - Our mechanics make house calls

Q: p4020 code is my o2 sensor bad?

asked by on

My check engine light has come on -the code is p4020 what does it refer to?

My car has 109000 miles.
My car has an automatic transmission.

A: Hi there: The OBD-II error code P0420 gene...

Hi there:

The OBD-II error code P0420 generally indicates a Catalyst System Efficiency is below threshold (Bank 1). The upstream oxygen sensor is located in front of the catalytic converter. If the upstream oxygen sensor is working properly, its readings should fluctuate when the car is warm and when in closed loop. The downstream oxygen sensor is located behind the catalytic converter. If the downstream oxygen sensor is working properly, its readings should remain steady. In general, when this error code shows up, it means one of the two O2 sensors is either faulty, the wiring is faulty or the catalytic converter itself is damaged. To be certain, you might want to have a professional mechanic complete a check engine light inspection which will allow them to pull all codes stored in the ECU to determine which of the sensors might be faulty.

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: Sunroof squeaking.

Kia Optimas are known to have issues with their sunroofs, so you can rest assured it is not just you. Your concern with the rubber lining may actually be a quick repair. Sometimes the rubber slides out too far and...

Q: Fuel smell inside my car

This could be a sign of a liquid fuel leak or a fuel vapor leak. In either case, there is a danger of an engine fire. I recommend having a trained technician such as one from YourMechanic take a look...

Q: The car won't start, it is completely unresponsive, the lights work fine

Have the system checked to see if something is loose or came loose on the accident and is only now showing up as a problem. The air bag module may be shorted causing a problem in the data buss of...

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 P0052 code means This code is seen when the Engine Control Module (ECM) tries to control the...
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...