Emissions code P0420

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

How the light acts is an indication of the severity of the problem. Codes related to things that will not adversely affect emissions never turn on the Check Engine light. Minor problems that can affect emissions will turn the light on, then, if it's an intermittent problem that stops acting up, the light will go off while you're driving. If it's a little more severe, if the intermittent problem goes away, the light will be "latched" on until you turn the ignition switch off, then restart the engine. If it's still more severe, the light will stay on all the time, even if the problem is not currently acting up. The most severe is when the Check Engine light is flashing. That means stop the engine as soon as safely possible because too much unburned gas is going into the catalytic converter where it will be burned and cause it to overheat and be damaged.

This code is set when the oxygen sensor after the catalytic converter switches between "rich" and "lean" at close to the same rate as the sensor in front of the converter. That means no change is taking place in the composition of the exhaust gas as it passes through, so both sensor see the same thing. Normally the front sensor switches between the two states about twice per second and the rear sensor changes perhaps once every minute or two.

It's important to understand this defect is detected by the Engine Computer by comparing the signals from two properly-working sensors. You don't change the messenger when you don't like the message, and you don't replace the sensor when you don't like its readings, that is, of course, as long as the sensor is working properly. When it is not, a different fault code number would be set.

Definitely let the dealer look at this, and be sure you do not disconnect the battery or use a scanner to erase the fault code. Erasing a code never solves a problem. You want the mechanic to read the code for himself and document that on the back of the repair order. I worked for a very nice family-owned Chrysler dealership in the '90s, and I learned that when an intermittent problem is documented while the car is under warranty, if they can't find the cause or fix it now, Chrysler will usually cover it later when the car is out of warranty. Intermittent problems can be very frustrating to diagnose because any testing has to be done while the problem is occurring, otherwise, obviously, everything is working properly and there is no defect to find. Sometimes we have no choice but to throw random parts at a problem and hope one solves it, but that causes more angry customers when they have to come back than when we tell them they have to keep driving until the problem acts up more often.