Skip the auto shop - Our mechanics make house calls

Q: P0420 code after idle relearn

asked by on

I need help diagnosing a p0420 code. Background- 20k-30k ago bad cats blew the motor. All cat replaced and have 20k-30k on them, used motor has 120k. A while ago a throttle body clean was done the wrong way I guess (not by me) and I had to re-learn the idle which took about a week of trying. The day that I got the idle back to normal is the exact same time the p0420 popped up right after messing around with the relearn procedure. Still to this day haven't figured out what could cause the code. Since the cats are fairly new I keep telling myself it can't be that. Other causes I've heard are bad ecm program (which would make a lot of sense given the circumstance) or exhaust leak (Which I have had a minor flex pipe leak for a good while now). What are the chances I destroyed my ecm and what are the chances it could be as simple as replacing the flex pipe?

My car has 140000 miles.
My car has an automatic transmission.

A: Hello. P0420 is the code for catalyst effic...

Hello. P0420 is the code for catalyst efficiency below the correct threshold for bank 1. If you do not believe the catalytic converters are the issue, then the first thing I would do is check the bank 1 oxygen sensors. It may be possible that the downstream sensor is lazy or no longer accurate and and setting off the code. If the oxygen sensors are good, then the next things I would look at are the computer and the catalytic converters, even though they maybe new. If you would like to have an expert scope out the problem, a qualified professional from YourMechanic can come to your car’s location to diagnose the Check Engine Light and pinpoint the correct repairs.

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: Battery replaced, radio stopped working

Hello. This frequently happens after a battery replacement on this car. When this car loses battery power the programming in the computer can become corrupted and this occurs. Some of the time it will come back on its own, but...

Q: Code P0420. Need to replace catalytic converter.

Hi there. The code P0420 is coming up for the Downstream Oxygen sensor has picked up an abnormal reading and is saying that the catalytic converter is below threshold. There could be a misfire on the engine, leaking injector, ignition...

Q: Car stalls intermittently, check engine light on, 1995 Jaguar XJ6

It is possible that your Exhaust Gas Recalculation system (EGR) is faulty, and is venting exhaust back into the engine at the wrong time (during idle). There is a Recall for you year and model (T493) relating to a problem...

Related articles

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...
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...
P2103 OBD-II Trouble Code: Throttle Actuator Control Motor Circuit High
P2103 means there is a fault with the throttle actuator control motor circuit, likely due to a defective electrical component or part.