Skip the auto shop - Our mechanics make house calls

Q: P0420 code comes back repeatedly, emissions readiness issue. Takes about a week for Check Engine Light to come back on.

asked by on

Repeat code P0420 for Catalyric inefficiency bank 1. However Catalytic and 02 systems show readiness on scanner, Secondary Air system shows not ready.

I have checked for vaccum leaks and fixed a couple. Replace coolant temp sensor and flange becuause of a minor leak. Replaced all spark plugs because of a bad coil. Changed the o2 sensors with OEM Bosch parts.

Also previous owner thought the air pump was bad and replaced it earlier this year.

When I changed the o2s I inspected the Cat. Looks good externally and doesn't make the rattle noise when you shake/hit it.

I have experience working on VW/Audi but this code just keeps on coming back.

I don't want to guess and just change the catalytic if it is not actually bad, but this would be my next diagnostic.

Thanks!

My car has 130000 miles.
My car has a manual transmission.

A: If this is the only code that keeps coming ...

If this is the only code that keeps coming up, and the data stream on the oxygen sensors are reacting correctly, the only real other thing left would be the convertor. If there was a wiring issue, or a sensor not responding correctly, you should be able to pick that up with the numbers on the scanner.

According to AllDataPro, the suspect would be the convertor due to the fact that P0420 is coming up by itself. If there were additional codes for the oxygen sensors, then there would be other places to look. If you still have doubts about the function of the convertor, you could always perform an exhaust gas diagnosis.

To avoid any unnecessary repairs, have one of our mobile technicians come to your home to perform these checks and properly diagnose the trouble code firsthand.

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: I had a catalytic converter installed in my vehicle in 2013, now I'm being told I need a new one. How long is it suppose to last?

Hello. Most vehicles do not have to have the catalytic converter replaced unless it becomes damaged due to road debris or it overheating severely. With that said, it is possible your car may be running a bit too rich (too...

Q: When its cold my car door won't shut

Hello. From what you describe it would appear that your door latch mechanism may be getting stuck, perhaps due to the cold weather. You may be able to apply some sort of lubricant to the mechanism that may remedy the...

Q: my check engine light came on. Took to Auto Zone they gave me these codes P0700 & P1776

According to the diagnostic trouble code listing on AllData, it looks like both of the trouble codes are associated with the transmission. The P0700 code appears to be more of a general information code to let you know other codes...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
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...
What are the Car Pool Rules in Hawaii?
Hawaii is widely regarded as a land of vacation and relaxation, and as such, its scenic roads and routes are far better known than the state’s freeways. But, as with all...