Rick's Free Auto Repair Advice

P0420 — Test a catalytic converter

Test a catalytic converter

So you have a check engine light with either a P0420 or P0430 and you want to know if the problem is an oxygen sensor or the catalytic converter. First, it’s almost never an oxygen sensor. How do I know that? Because the computer switches the oxygen sensors rapidly in order to test the catalytic converter.   If the oxygen sensors aren’t working properly, the computer would set oxygen sensor codes and it wouldn’t be able to complete the catalytic converter test. So here’s how to test a catalytic converter.

Step 1 to test a catalytic converter — Check for technical service bulletins

Check for technical service bulletins. Many times, the carmaker will discover a software issue that’s causing a false or premature P0420 or P0430 trouble code and they’ll issue a software update to correct the problem. Why waste time testing if the carmaker has already issued a service bulletin?

Step 2 to test a catalytic converter — check for oil or coolant in the exhaust

Have you been using oil or losing coolant? Oil or coolant that is burned in the combustion chamber can cause a P0420 or P030 code. It makes no sense to replace a catalytic converter only to have the replacement fail again because you haven’t dealt with the root cause of an internal leak.

Step 3 to test a catalytic converter — check for exhaust leaks

An exhaust leak can cause the computer to set a false P0420 or P0430

Step 4 to test a catalytic converter — check for physical damage

Impacts, dents, scrapes or tears can cause an internal failure

Step 5 to test a catalytic converter — check for missing heat shields

Heat shields held hold heat in to keep the converter running at the right temperature.

Step 6 to test a catalytic converter — check for discoloration

A red or blue discoloration is an indication of a severe overheating condition. An internal overheat can melt the ceramic structure and cause the converter to fail. In addition to discoloration, check for ceramic brick failure by tapping the converter with a rubber mallet. Listen for a hollow sound or rattling that would indicate an internal breakup.

Step 7 to test a catalytic converter — check for backpressure

Start the engine and let it run until it reaches operating temperature. Connect a vacuum gauge to a port on the intake manifold. The engine should show a normal vacuum (18 and 22 in-Hg). Then perform a quick Snap-throttle maneuver where you quickly open the throttle all the way and then release. The gauge should drop to 0 and then come back to normal vacuum within 3-seconds. That would indicate no restriction in the exhaust. If it takes longer for the normal vacuum to return, that’s an indication of a clogged exhaust.

If you’re unsure whether the blockage is in the muffler/resonator or the catalytic converter, disconnect the muffler from the catalytic converter and repeat the test.

What to do if the catalytic converter fails

Find the cause of the failure. Catalytic converters are designed to last the life of the vehicle. If they fail early, it’s always due to an impact or engine malfunction that damaged the internal portion of the converter. Fix the root cause before replacing the bad converter.

©, 2020 Rick Muscoplat



Posted on by Rick Muscoplat

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