Hi! I have an '06 Accord and the malfunction indicator lamp came on one night while my wife was driving on the freeway. She had gotten gas that morning, so after she got home I tightened the gas cap, but the light was still on the next day so she took it in to our Honda dealer and they told her we need a new catalytic converter ($1500), but the car was ok to drive as long as the light didn't start blinking. The next day, the light disappeared. That was a week ago. This morning the light came on again, and this evening it went back out. When we looked at the paperwork from the dealer, it says "P0420, found the catalytic converter was starting to fail". The car is not driving any differently and it's not making any noises. Could this just be a sensor issue? Wouldn't we feel some kind of difference if the catalytic converter were failing, and wouldn't the light stay on all the time? Any advice would be appreciated!

 

P0420 is very difficult to accurately diagnose. It could indeed be a poor performing catalytic converter, weak primary O2 or air/fuel sensor, exhaust leak, or faulty secondary (monitors converter performance) O2 sensor.

Do you want to self-diagnose? There is a video on youtube that may help.

Also carefully inspect for exhaust leaks.

Also shake the converter rigorously. If it rattles, the converter matrix has failed and a new converter is indicated.

If above suggest converter may be OK, then perhaps O2 sensors may be suspect. This takes advanced equipment (scanner or shop scope) to monitor O2 sensor and determine whether performance is degraded.

If you don't want to diagnose, you need to find a shop that will perform the diagnostics, but the cost of diagnostics can quickly add up.

good luck