2003 Toyota Camry obd2 code P0420

The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

A code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for
An oxygen sensor is not reading (functioning) properly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings

Mark (mhpautos)

I have the same error code for the same model car/year. I had a mechanic check the car, and he concluded that because the grounds to the sensors were OK, and the front O2 sensor was normal, but the rear sensor scope pattern closely follows the front sensor, that the catalytic converter was defective. Problem is, the check engine light comes on, then goes off some time later. It's done this three or four times. I have a hard time believing that a catalytic converter could be intermittent, that's more likely and electrical or computer problem. Any thoughts?