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Catalytic Converter Problems: Symptoms, Diagnosis & Repair

The catalytic converter is a critical emissions component that converts harmful exhaust gases into less harmful substances. When it fails, you'll typically see a P0420 or P0430 code, smell rotten eggs, lose power, or fail an emissions test.

What Is It?

The catalytic converter contains precious metals (platinum, palladium, rhodium) that act as catalysts to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) into carbon dioxide (CO2), water vapor (H2O), and nitrogen (N2).

Common Causes

Engine Misfires (Root Cause)

Very Common

Unburned fuel from misfires enters the catalytic converter and ignites inside, overheating and melting the substrate. This is why fixing misfires promptly is critical.

Contaminated Catalyst

Common

Coolant (from head gasket leak), oil (from worn rings), or lead/silicone contamination coats the catalyst metals, reducing efficiency.

Physical Damage

Moderate

Road impacts, speed bumps, or rust can damage the converter housing or break the ceramic substrate inside.

Age and High Mileage

Common

Catalytic converters naturally lose efficiency over time. Most last 100,000-150,000 miles under normal conditions.

How to Diagnose

  1. 1

    Scan for codes: P0420 = Bank 1 efficiency below threshold, P0430 = Bank 2.

  2. 2

    Compare upstream and downstream O2 sensor waveforms — they should be different. Similar patterns = converter not working.

  3. 3

    Check exhaust backpressure with a gauge — high backpressure indicates a clogged converter.

  4. 4

    Smell the exhaust — a strong rotten egg (sulfur) smell indicates the converter is struggling.

  5. 5

    Before replacing the converter, check for root causes: misfires, oil burning, coolant leaks.

  6. 6

    Use an infrared thermometer to check converter inlet vs outlet temperature — outlet should be hotter.

Estimated Repair Cost

$500 to $2,500+ depending on vehicle

When to See a Mechanic

Catalytic converter replacement requires professional diagnosis to confirm failure and identify root causes. Replacing the converter without fixing the underlying issue (misfires, oil burning) will destroy the new one.

Frequently Asked Questions

What does a bad catalytic converter sound like?

A rattling sound from underneath the vehicle indicates the ceramic substrate has broken apart. You may also notice reduced exhaust flow or a hissing sound if the converter is cracked.

Can I drive with a bad catalytic converter?

A failing converter won't damage the engine, but a clogged one reduces power significantly. You'll also fail emissions testing. If the substrate has collapsed, it can block exhaust flow completely.

Related Parts

Shop catalytic converters →Shop oxygen sensors →Shop fuel injectors →

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Still Not Sure What's Wrong?

Our AI mechanic can give you a personalized diagnosis based on your specific vehicle and symptoms.