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.

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: Went got a test at Autozone for alternator, she stated everything test normal but battery voltage is too low? What does that mean?

Hello. This could mean the alternator is not charging the battery high enough or the battery will not accept and hold a charge. You may need to replace the battery first, then recheck the alternator charging system for proper operation....

Q: Catalytic Converter Replacement - 2001 Lexus IS300.

Hi there. Before you decide to have the catalytic converter replaced in your vehicle, consider performing the following checks: Check the following: A damaged or failed oxygen sensor The oxygen sensor wiring damaged or connected improperly The engine coolant...

Q: Car shaking when applying brakes and going slow. Check Engine Light is on and engine idles roughly.

It sounds like you may have a vacuum leak on your car. Check all vacuum lines and make sure they are in good condition and connected. If you don't see a leak you can spray some carburetor cleaner on the...

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...
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...
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...