Rick's Free Auto Repair Advice

Fix Code P0420

Fix Code P0420

A P0420 code means your catalytic converter has failed an efficiency test. In other words, it’s not removing harmful combustion byproducts from the exhaust. Many people think that the way to fix code P0420 is to replace the oxygen sensors. That’s wrong. In fact, the vehicle computer can’t even come to the conclusion that your catalytic converter has failed the efficiency test if your oxygen sensors are bad. So don’t waste your time and money on oxygen sensors.

What to do if you get a P0420 code?

First, check for exhaust system leaks upstream from the catalytic converter. Air getting into the exhaust stream can throw off the catalytic converter efficiency test, resulting in a false P0420 code.

If you find an exhaust leak, fix it. Then clear the trouble and see if it returns

Check for high long term fuel trims

If the computer detects a lean air/fuel mixture it will add fuel. It will continue to add fuel until it sees the right oxygen level in the exhaust. However, a long term fuel trim above 10% is an indication of a problem that can cause a P0420

How to test for exhaust leaks to fix P0420 code

See this article of how to find an exhaust leak. Use a household shop vac. Connect the hose to the blow port and connect to your car’s tailpipe using duct tape. All you want to do is pressurize the exhaust. Turn on the vacuum (car engine off). They use a spray bottle filled with soapy water to spray the exhaust system from the engine all the way down to the catalytic converter. If you see bubbles blowing, you’ve found the leak. Have the leak repaired. Then clear the P0420 code and drive the vehicle to see if it returns.

If the P0420 code returns, the catalytic converter is dead

Your job is to find out what killed it

Check for:

Impact damage from striking a curb or rock
Excessive oil burning
Internal coolant leak

What goes wrong with a cat converter?

image of catalytic converter

Cut-away of a typical catalytic converter

Catalytic converters have a ceramic honeycomb inside and the surfaces contain a coating of precious metals like platinum, Palladium, Rhodium, and other non-precious metals. Those compounds react with unburned fuel in an oxidation/heating process that completes the burn and neutralizes oxides of nitrogen (a component of smog). It’s similar in concept to a municipal garbage burner where they add treat the smoke with extra heat to reduce emissions.

image of inside of catalytic converter

Cat Converter showing honeycomb in good condition

However, if too much unburned fuel enters the cat converter, the oxidation process creates too much heat and the converter starts a runaway reaction. If that continues, it causes a meltdown

P0420, P0430, flashing check engine light

Melted ceramic honeycomb. This is a destroyed converter caused by ignoring engine problems.

where the ceramic honeycomb actually self-destructs. Once that happens, the cat converter is toast. The point here is that cat converters don’t die on their own, they’re always murdered. Excess fuel, bad valve stem seals that leak oil into the combustion chamber, coolant leaks that send coolant into the exhaust stream—all of those engine problems can kill a cat converter. If you replace a converter without fixing the underlying problem, you’ll just have to replace it again.


© 2012 Rick Muscoplat



Posted on by Rick Muscoplat

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