Rick's Free Auto Repair Advice

Symptoms of a bad catalytic converter

Bad catalytic converter symptoms

Check engine light on with P0420, P0430 trouble code is a symptom of a bad catalytic converter

P0420 and P0420 trouble codes mean that your catalytic converter has failed the efficiency test. The test consists of rapidly changing the air/fuel mixture from lean to rich and back again and tracking the results with the pre-converter oxygen sensor and post converter sensor. That tells the computer how well the catalytic converter did it converting the extra fuel into CO and CO2 and how well it did in storing extra oxygen.

Many people believe that replacing the oxygen sensor will fix a P0420 or P0430. It rarely does and the reason is simple; the computer needs a functioning pre-cat sensor in order to determine that the converter has failed. In other words, since it compares the pre-cat sensor to the post-cat sensor, it makes no sense that the pre-cat sensor would cause the P0420 or P0430. If the pre-cat sensor didn’t detect the rapid change in air/fuel mixtures, the computer would set an oxygen sensor code, not a converter code.

An exhaust leak after the pre-cat sensor but before the converter, on the other hand, would cause a false P0420 or P0430. That would cause the post-cat sensor to see a constantly lean condition, making the computer think that the cat converter is dead.

A starting problem is a symptom of a bad catalytic converter

When a catalytic converter overheats, the ceramic honeycomb can melt and obstruct exhaust flow. In addition, excess fuel oil, or engine coolant can clog the catalytic converter and restrict exhaust flow. That prevents the engine from breathing properly and that makes it much harder for the engine to start. You would experience this as the engine cranking but not firing up.

catalytic converter P0420

Poor gas mileage is also a symptom of a bad catalytic converter

Just like above, a clogged or partially melted catalytic converter prevents the engine from breathing properly, and that dramatically reduces acceleration and engine performance. If you notice that you have to press the accelerator pedal farther to get the same speed, that’s a sign you need to check out the converter.

A rattling sound coming from the catalytic converter

An overheated catalytic converter can melt and break apart. Once the ceramic honeycomb breaks, it rattles when going over bumps. Tap on the catalytic converter with a rubber mallet. If it rattles, it’s damaged.

catalytic converter impact damage

Impact damage on catalytic converter from road debris or parking lot curbs

The exhaust smells like rotten eggs

A dead catalytic converter no longer works to convert exhaust into non-polluting gasses. A sulfur or rotten eggs smell is a symptom of a bad catalytic converter.

If you suspect your catalytic converter is the cause, see this post on how to test a catalytic converter

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