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.
You will likely not notice any drivability problems, although there may be
A code P0420
may mean that one or more of the following
- 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
Some suggested steps for troubleshooting a P0420
- Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair
- Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor
in front of the catalytic converter normally has a fluctuating waveform. The
waveform of the sensor behind the converter should be more steady).
- Inspect the downstream heated oxygen sensor (HO2), replace if necessary
- Replace the catalytic converter
One thing to note is that many vehicle manufacturers offer a longer warranty
on emissions-related parts. So if you have a newer car but it's out of it's
bumper-to-bumper warranty, there still may be warranty on this type of problem.
Many manufacturers give a five year, unlimited mileage warranty on these items.
It's worth checking into.