A DIYers guide to the dreaded P0420 catalytic converter code

What to do when you get a P0420 catalytic converter code

If you’ve got a check engine light with either a P0420 P0420 catalytic converter code or a P0430 trouble code, don’t automatically replace the oxygen sensor. Why? Because a herP0420 catalytic converter code is almost NEVER caused by a bad oxygen sensor. Here’s a step by step guide to dealing with a P0420 or P0430 code

Why it’s never the oxygen sensor

Here’s how the computer tests the efficiency of your catalytic converter. Once it’s up to operating temperature, the computer commands a quick rich/lean, rich/lean change to the fuel. It expects to see a rich/lean rich/lean reading from the upstream oxygen sensor. If it sees that, it starts the second portion of the test by watching the downstream sensor. If the cat is working properly, the downstream sensor should only show a steady reading or one that moves up/down just a small amount. If the downstream sensor readings mirror the upstream, it’s set a 420 or 430 code.

If the upstream sensor doesn’t report the rich/lean, rich/lean, it will set and OXYGEN sensor code to indicate that the upstream sensor has failed.

In other words, the computer can’t even test the cat if the upstream sensor is bad.

Step 1: Check for physical damage

Impacts, dents, scrapes or tears can cause an internal failure. Use a rubber mallet to rap on the converter. If you hear rattling, that’s the sign of a shattered ceramic structure. There’s no way to fix a shattered ceramic structure. You have to replace the converter.

Step 2: 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 3: Check for missing heat shields

Heat shields help hold heat in to keep the converter running at the right temperature. Most people think the shields don’t do anything or can’t affect converter operation. They’re wrong.

Step 4: Check for exhaust leaks

An exhaust leak can definitely cause the a P0420 catalytic converter code or P0430. If you see black soot around any exhaust fitting ahead of the converter, fix it before you do anything else. If you don’t see a leaking joint, use a shop vac and pressurized the exhaust while using a spray bottle of water and soap. See this post on how to check for an exhaust leak.

Step 5: Check for high fuel trims

Fuel trim is just a fancy term that means the computer has made a modification to the factory software to make the engine run better. Fuel trims are defined as short term or long term. If the computer sees the need to add fuel over a long period of time, you’ll see it reflected in the long term fuel trim (LTFT) readings. Anything over +10% indicates a problem. In most cases a high long term fuel trim means the engine is getting too much unmetered air, so the computer has to add more fuel. In other words, the engine most likely has a vacuum leak or an air duct leak between the air filter box and the throttle body. LTFT is a great way to spot a problem before spending a lot of money on parts.

Step 6: Check for technical service bulletins

In many cases, the carmaker has come out with a software update to fix false converter codes. You can buy a short subscription to alldatadiy.com or eautorepair.net to check the service bulletins. Or you can check with your public library to see if they offer free auto access to those shop manuals. PLEASE do this before you replace parts. I can’t tell you how many times people have jumped to conclusions and replaced parts only to find out that a software update is available that would have fixed the problem for a lot less money.

Step 7: 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 8: Check for high 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.

Why it’s not the oxygen sensor

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