Rick's Free Auto Repair Advice

Toyota Scion P0420

Fix code P0420 Toyota Scion

If you own a 4-cylinder Toyota or Scion vehicle and get a check engine light with trouble code P0420, welcome to the club. This has been a persistent problem with these vehicles. To get right to the point, the problem is bad software. Toyota has updated it. But that’s not all. If you don’t maintain this vehicle EXACTLY as Toyota says, you can accelerate this problem. It all starts with misfires and not changing your oil.

Misfires send unburned fuel down into the catalytic converter where it’s burned off. However, excess fuel can overheat the honeycomb inside the converter and cause it to melt. Also, going too long on an oil change can vent excess hydrocarbon into the intake (through the PCV system) and that also throws too much “fuel” into the cat converter.

Now the answer is not always a new converter. Here’s how a shop should diagnose the problem. The oxygen sensor in front of the cat converter should show a constantly changing rich/lean signal. The computer uses that to alter air/fuel requirements. But the post-cat sensor should show little to no change between rich/lean. That means it’s doing its job of cleaning up the exhaust. All the shop has to do is put a scan tool on the vehicle and check the switch rate of the post cat oxygen sensor to get a feel for how the cat is working. At the same time, they should also check the “fuel trim.” Fuel trim is the amount the computer has to constantly add to the engine to make it run right. The third test they can run is a temp check using an infrared non-contact thermometer.

If the post cat sensor shows rapidly changing values from rich to lean, that means the cat is not functioning. If you continue to drive it that way, you’ll eventually destroy the entire cat and cause a total meltdown. That will increase exhaust back pressure and you’ll feel a loss of power, rough idle, stalling, coughing, etc.

So here’s the drill when you get a P0420. Change the plugs and wires. Change the oil. Replace the PCV valve. Replace the air filter. Then clear the code and try driving it again. If you still get a trouble code, have the shop check the pre and post oxygen sensors and fuel trims to see if they’re in spec. If not, you’re going to bite the bullet and buy a new cat converter. BUT, make sure the shop also updates the software with the latest version from Toyota. If they don’t, you’ll be doing this all over again.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat


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