Jim Grant's Tech Tips
Toyota Corolla, Emission Code P0420
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.
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.!