Rick's Free Auto Repair Advice

What causes P0420 trouble code

What to check if you have a P0420 trouble code

A P0420 trouble code Catalyst System Efficiency Below Threshold (Bank 1), and P0430 Catalyst System Efficiency Below Threshold (Bank ) usually mean you’ll be in the market for a new catalytic converter. Yes, I know that many online sites tell you to fix a P0420 with a new oxygen sensor. In rare cases, that might fix the problem. But for the vast majority, it’s really wishful thinking and a waste of money. So let’s take a look as what causes a P0420 and P0430 code and how to test to make sure the catalytic converter is actually bad.

What causes a P0420 trouble code

When the ECM sets a P0420 trouble code, it’s really telling you that the catalytic converter isn’t doing its job of cleaning up the exhaust. The question is; is there and underlying problem that’s overloading the cat converter with either air or fuel? And, has the underlying problem gone on long enough that it’s caused permanent damage to the converter?

These underlying problems can damage a catalytic converter and cause a P0420 trouble code

Intake manifold air leaks

An intake manifold air leak after the MAF sensor allows unmetered air into the engine. The oxygen sensor detects a constantly lean exhaust and the ECM responds by adding fuel—but only to a certain point. After the ECM reaches maximum fuel trim, the engine experiences lean misfires and that causes excessive oxygen in the cat converter.

Fuel injector problems (leaks)

This is just the opposite of the intake manifold leak and results in flooding the catalytic converter with unburned fuel which causes overheating and even ceramic structure meltdown.

Incorrect spark plugs

Incorrect or improperly gapped spark plugs cause misfires which results in extra fuel entering the cat converter causing overheating and ceramic damage

Ignition timing

Incorrect ignition timing results in incomplete burn and misfires which also dumps unburned fuel into the catalytic converter

EGR problems

The EGR performs a vital role in reducing emissions of NOx. It doesn this by introducing exhaust gas into the cylinder during the intake stroke to act as ballast. The non-burnable exhaust gas prevents the combustion from reaching as high a temperature where oxygen and nitrogen mix to become NOx. However, when the EGR system isn’t working, combustion temperatures rise, causing excessive heat flow to the catalytic converter.

Oil or antifreeze entering exhaust

An internal oil or coolant leak works the same as adding excessive fuel to the combustion chamber. It causes the cat converter to overheat, which damages the ceramic structure.

O2 sensor not operating correctly

The ECM relies on the O2 sensor to help regulate the air/fuel mixture. If the O2 sensor isn’t working problem, the ECM can’t do a good enough job figuring out the best air/fuel ration.

Impact damage to converter

This one’s pretty self explanatory. You run over a parking catalytic converter impact damage lot curb, dent the converter, break the ceramic structure and you’ve pretty much ruined the converter

Silicone contamination

Using a non-catalytic converter approved silicon RTV compound can coat the surface of the ceramic structure with silicone. That literally kills the converter since it can’t react with either oxygen or fuel

In other words, catalytic converters don’t wear out, THEY’RE KILLED by all the above underlying problems. If you replace a catalytic converter without fixing the underlying problem, the new converter will fail as well.

How to test a catalytic converter

Perform a manifold vacuum test

Connect a vacuum gauge to the intake manifold. Note the reading at idle. Then raise RPMs to around 3,000. As you open the throttle, it’s normal to see a quick drop in vacuum, but it should then rise and level off. If the vacuum starts to drop off as you hold the RPMS steady, that’s the sign of a blocked cat converter. In other words, the passages through the ceramic honeycomb have either melted or become clogged or there’s an exhaust restriction in the muffler or resonator.

Perform a backpressure test

Remove the upstream oxygen sensor and insert a vacuum gauge adapter into the O2 sensor threads. Backpressure on a stock factory engine should be around 1.25-psi at idle. Raise idle to around 2,000 RPM and read the gauge. A reading of 3-psi or more indicates an exhaust restriction, either in the converter, muffler or resonator.

The temperature input versus output temp test is no longer considered a valid test

In the early days of catalytic converters, the rule was that the input versus output temperatures must be at least 100°F apart. But better fuel injection and fuel metering and more efficient combustion has resulted in much cleaner burning engines. So the difference in input versus output temperatures in a late model vehicle can often be as little as 50°F. At idle, the difference may actually be 0°F.

Tips to avoid silicone contamination:

NEVER use any type of RTV on exhaust flanges. If a gasket won’t seal it, don’t reach for any type of sealer. You’ll just kill the cat converter. Have an exhaust shop weld on a new flange so it will seal with a gasket.

NEVER use Teflon tape on an O2 sensor. High temperature teflon tape on O2 sensoranti-seize is the ONLY sealant that’s approved for O2 sensors.

NEVER add high temp silicone RTV to O2 sensor threads. High temperature anti-seize is the ONLY sealant that’s approved for O2 sensors.

silicone on O2 sensor

©. 2019 Rick Muscoplat

Posted on by Rick Muscoplat



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