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.

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?
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  1. Home
  2. Questions
  3. P0420 code comes back repeatedly, emissions readiness issue. Takes about a week for Check Engine Light to come back on.

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: After battery replacement, car still won't start

Many different items can cause a vehicle not to start. Replacing the battery, especially if it was old, is a great place to start since this this is the cause in most cases. Since you have replaced the battery and...

Q: Pulsing engine.

Hello, It sounds like you may have a vacuum leak causing the engine to idle higher than normal. High idle speed, stalling and hissing sounds are common signs of a vacuum leak. When a vacuum leak is present in an...

Q: Q: Throttle Position Sensor

Anytime you go to a parts store to get a computer scan for the Check Engine Light you are not getting a diagnosis of the problem but only a code retrieved from the engine control module (ECM) memory. This is...

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...
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...
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...