Jim Grant's Tech Tips

'01 Toyota Corolla, Emission Code P0420

Q: I own a ‘01 Toyota Corolla with an emission code P0420 for catalyst system fault. Where should I start? I’ve already erased the code and it has returned.

Mike

A: Your vehicle has 2 oxygen sensors. One is mounted in front of the converter and used primarily for monitoring and fine-tuning of the fuel to the engine. The second oxygen sensor is mounted behind the converter and is used mainly to monitor the efficiency of the converter. While you’re driving your vehicle down the road at just the right speed, time and temperature the computer takes a look at the 2 oxygen sensors and compares their values. If the 2 sensors match up too closely the computer will throw a flag and turn on the check engine light. You have to remember that the oxygen sensor in front of the converter has exhaust gases that are ripe from the engine. The second sensor is after the converter and if the converter is working correctly this sensor will be reporting less information, it won’t be as active, because the converter is using the remaining oxygen to clean the exhaust. What your computer is reporting is that the 2 sensors are looking too much alike. You just may have a bad converter. But don’t jump there just yet. There are a couple of other conditions that can falsely set this code and you do not want to replace that expensive converter just to have the same code return. First there can be no exhaust leaks. If there are fix them. The heater circuit or the heating element in the sensor behind the converter has to be checked. Yes there is a heater in the oxygen sensor. The heater is used to get the oxygen sensor hot and functioning quickly to reduce exhaust emissions. If the wiring to or the heating element itself is bad you can get a false converter code. So that part of the system must be diagnosed and repaired if needed before the converter is deemed inefficient. It gets back to that just because the computer says so, doesn’t mean it’s true stuff.!