Step by Step Guide to diagnose a P0420 or P0430 code

Learn how to diagnose a P0420 or P0430 code to confirm or deny the converter is bad

When your vehicle throws a P0420 or P0430 code, it’s telling you there’s a problem with the catalytic converter’s efficiency. These codes translate to “Catalyst System Efficiency Below Threshold” for Bank 1 or Bank 2. While a failed catalytic converter is often the issue, don’t rush to replace it just yet. In this article, I give you a step-by-step diagnostic guide to professionally diagnose whether the catalytic converter code means you have a bad converter or if the problem is caused by another issue.

WARNING AGAINST REPLACING PARTS OR USING A CHEMICAL “FIX”:

Oxygen Sensor Replacement

Before we dive in, let me give you an important warning: don’t fall for the bad advice floating around online. In almost every case—99% of the time, to be exact—the oxygen sensors are not the root cause of a P0420 or P0430 code. Unfortunately, many DIYers believe replacing O2 sensors will solve the problem, but it almost never does. Here’s why: your car’s ECM won’t even set a P0420 or P0430 code unless the oxygen sensors are functioning properly (I’ll explain this in detail in the section titled Understand How the ECM Sets the P0420 or P0430 Code).

Replacing the O2 sensors to fix one of these codes is usually wishful thinking. While there are rare exceptions—and I’ll address those below—don’t waste money on parts before performing proper tests to pinpoint the real issue.

Catalytic Converter Cleaner “Fixes” in a Bottle

Catalytic converter cleaners are often marketed as a quick and inexpensive solution for resolving P0420 or P0430 codes, but their effectiveness is limited and highly situational. They rarely work, and even in cases where they work to turn off the check engine light, it’s only temporary. The light and codes come back in a few weeks.

A step-by-step approach to diagnose a P0420 or P0430

1. Check for Technical Service Bulletins (TSBs)

Before touching a tool, check for manufacturer TSBs related to your specific vehicle and this code. Sometimes, software updates can fix false P0420/P0430 codes. You can find these service bulletins on services like AlldataDIY.com or eAutoRepair.net. Some local libraries provide access to these services for free. Why invest time and energy if the carmaker has already come up with a fix?

2. Perform These Under-Car Visual, Leak, and Wiring Inspections

Examine the catalytic converter, exhaust, and wiring for:

1) Overheating: Look for bluish discoloration.
2) Physical Damage: Check for dents, impacts, or cracks.
3) Exhaust Leaks: Inspect for any exhaust system leaks.
4) Wiring Issues: Ensure oxygen sensor connections and harnesses are intact.

If you notice physical damage or overheating, the catalytic converter is already compromised. Don’t automatically replace it without finding out WHY it overheated in the first place. If you skip that step, you’ll just destroy the replacement converter.

Catalytic Converters don’t overheat on their own. Overheating is always caused by a serious air/fuel mixture problem, an oil burning problem, an internal oil or coolant leak, or poisoning due to the use or silicone sealants. In other words, fix the underlying problem before replacing an overheated converter.

You can tell immediately that this catalytic converter has overheated and is most likely damaged internally. If you see signs of overheating and the engine lacks power, the internal substrate is melted and clogged, and the catalytic converter must be replaced.

If you see impact damage like this, tap the catalytic converter with a rubber mallet and listen for rattling. If you hear rattling, the internal structure has shattered from the impact, and the catalytic converter must be replaced.

3. Rattle Test

Even if there are no signs of impact or overheating, the internal structure may still be damaged. Using a rubber mallet, gently tap around the entire catalytic converter. If you hear rattling, it indicates a broken or degraded internal substrate, which means it must be replaced.

4. Test for Exhaust Leaks

An exhaust leak can cause a false P0420 or P0430, so you it’s important to eliminate en exhaust leak as a possible cause of the codes.

Use a shop vacuum in blow mode to pressurize the exhaust system:

Seal the shop vac hose to the tailpipe with duct tape and turn on the shop vac.
Spray all joints and connections with soapy water.
Look for bubbles indicating leaks, and fix them before proceeding.

5. Temperature Differential Test

The catalytic converter works by burning off excess fuel. So this test involves checking inlet and outlet temperatures using an infrared thermometer.

With the engine running and the catalytic converter running ar around 400°F, check the temperature of the converter near the inlet, middle, and outlet. For older vehicles, this difference might exceed 150°F; for modern cars, even 20°F can indicate a properly operating converter. However, if there’s no difference in temperature or the outlet temperature is cooler than near the inlet, the catalytic converter is likely dead.

6. Wiring

Check the wiring harness to the upstream and downstream oxygen sensors. Look for signs of rodent chewing, chaffing, or exposed wires. Next, disconnect the electrical connectors and check for signs of corrosion, bent pins, or melted connectors. Fix any issues you find before proceeding.

Now It’s Time to Move On to Live Data and Vacuum Testing

Check Fuel Trim Data

Using a scan tool, examine short- and long-term fuel trims:

Long-term fuel trim values above 10% indicate an engine or air/fuel-related problem, often due to vacuum or intake leaks. Solve these problems first—high fuel trims can lead to catalytic converter failure over time. See this article for the most likely causes of high long-term fuel trims.

Check for a clogged exhaust

1) Connect a vacuum gauge to the intake manifold.

This entire vacuum gage kit costs only $14 at Amazon

They’re inexpensive (around $14 on Amazon) and can tell you a lot about the condition of your exhaust and engine.

2) Start the engine and warm it up. Keep the engine at about 2,000 rpm. The needle should remain steady. If the RPM starts to fall and the needle falls with it, that’s a sign of a serious restriction in the exhaust. The restriction can be a clogged catalytic converter, a clogged resonator, or muffler.

3) Locate the clogged component: Disconnect the downstream oxygen sensor and repeat the vacuum gauge test. If you get a slightly better reading, that would indicate a problem with the resonator or muffler. If there’s no change, chances are the catalytic converter is clogged and must be replaced.

These images show what a clogged catalytic converter looks like. This is what’s preventing your engine from breathing.

8. Conduct a Volumetric Efficiency (VE) Test

Finally, conduct a VE test to evaluate your engine’s breathing efficiency. This test can pinpoint airflow issues caused by intake restrictions, clogged exhaust systems, or faulty sensors. You’ll need a scan tool with live data capabilities for this step. See this article for a step-by-step for conducting a volumetric efficiency test.

Understand how the ECM sets the P0420 or P0430 code

The ECM tests the converter by commanding a rapidly changing rich/lean air/fuel mixture. It then looks at the data from the upstream oxygen sensor to make sure it’s seeing the rapid changes. If it does, the upstream oxygen sensor passes the first part of the test. If not, it will set an upstream oxygen sensor trouble code.

Next, it looks at the downstream oxygen sensor. If the catalytic converter is doing its job, it will use the oxygen stored by the precious metals to burn off the excess fuel from the rich mixture, and it will store the excess oxygen present in the lean mixture. So, the downstream sensor should report non-fluctuating data. If the ECM sees fluctuating data that corresponds to the fluctuating data from the upstream oxygen sensor, it concludes that the catalytic converter has failed the test. Keep that in mind as we look at other possibilities for the cat converter codes.

Note: Ricksfreeautorepair advice receives a commission for any products ordered through the Amazon link posted in this article.

©, 2013 Rick Muscoplat