Truglia is the owner of Car Clinic, a state-of-the-art repair facility in Mahopac, N.Y. He is ASE A6 certified with a M.A. from Columbia University. (Vehicles were diagnosed by Kevin Quinlan and Alex Portillo. Contributions made by Gerry Truglia.)
Out of all of the different DTCs I see, P0420 is probably the most common money-making job I get. And, out of all the manufacturers I work on, I see P0420s on Toyotas the most.
There is more to this DTC than merely changing the catalytic converter and two oxygen sensors. We have to be very familiar with fuel trim, Mode 6, interpreting oxygen sensor feedback and reflashing to diagnose these DTCs. Before I get into Toyota specifics, we are going to lay some ground rules that will help you diagnose a P0420 on everything from a Toyota to a BMW.
P0420 equals changing the catalytic converter, right? Not always.
I find it useful to believe in a noble lie that catalytic converters will never go bad unless something makes the cat prematurely degrade. The reason this is a “noble lie” is because in reality many Nissans, Volkswagens, Subarus and Toyotas have catalytic converters simply made with a lesser lifespan than that found on many other manufacturers’ vehicles. However, when diagnosing a P0420, pretend that reality does not exist and temporarily believe in the lie: catalytic converters will never go bad unless something prematurely degrades the cat.
So, your approach to diagnosing a P0420 should be in reality fixing bad MAFs, exhaust leaks, ignition coils, vacuum leaks, and other conditions that create extreme fuel trim conditions that degrade catalytic converters. If you go down the list and find that the vehicle is maintained properly and no condition exists that screws up the fuel trim, only then can you condemn the cat.
Sure, sometimes catalytic converters just go bad. But just pretend that is not true when you are diagnosing that P0420 until you are confident the vehicle just needs a cat.
Understanding oxygen sensors and P0420s
Before the catalytic converter, the oxygen sensor’s voltage will zigzag up and down when graphed. To the contrary, the air-fuel sensor will be at a stable voltage. The post-cat oxygen sensor will be a straight line of voltage if the catalytic converter is good in most cases.
Post-cat oxygen sensors, when good, are at a steady voltage, usually between 500 to 700 mV. If it zigzags, mirroring the pre-cat oxygen sensor, the catalytic converter is highly suspect. Sometimes the post-cat oxygen sensor will have a gap of time between the pre-cat sensor switching voltage and itself. This is often normal during a sudden fuel event that the catalytic converter, even when good, cannot instantly clean up.