With the extreme sensitivity of current North American emissions controls, and the extremely low emissions limits that have been imposed, certain errors now crop up that didn't happen before. One of those is the P0420 error, which is supposed to mean that the catalytic converter is not working as well as it should. Unfortunately, it just means the downstream O2 sensor thinks too much oxygen is sneaking past the cat and tattles to the ECM.
A catalytic converter is an oxygen storage device. The only way it can function is if it can take up and release oxygen in the quantities required to convert engine emission gases to water and carbon dioxide. OBD-II specifications require that the catalytic converter be regularly tested by the ECM. Test failures tell the computer that the cat has lost some of its oxygen storage capability, which is what's meant by "below threshold".
Your car's computer is allowed to adjust the fuel/air mixture within a very narrow range in order to help keep the P0420 error from happening. If the needed adjustment exceeds that range, the error code will be set.
"Three-way catalytic converter (TWC) efficiency is measured by how well it can store oxygen.
"The ECM monitors converter efficiency by comparing the voltage values of the heated oxygen sensor 1 (HO2S-1) and heated oxygen sensor 2 (HO2S-2). Under normal operating conditions, the HO2S-1 should vary between 10 mV and 1065 mV and the HO2S-2 should remain relatively steady between 500 mV and 800 mV. This steady reading of the HO2S-2 indicates a correctly functioning catalytic converter.
"When all parameters have been met, the ECM will run a 5 second DTC P0420 diagnostic at idle. The ECM will command rich and monitor the time it takes the HO2S-2 to go rich. It will then command lean and monitor the time it takes the HO2S-2 to go lean. The longer it takes the HO2S-2 to change rich/lean means the converter is storing oxygen and is functioning properly. If the five second test fails, the ECM may take several tests during several ignition cycles to set the DTC. DTC P0420 sets when the ECM has determined that the catalytic converter is no longer efficient."
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
Some suggested steps for troubleshooting a P0420 code include:
* Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
* 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
Hope this helps.