Catalytic Converter Failure (P0420)
Catalytic converter failure, commonly indicated by a P0420 check engine light, reduces engine performance and increases emissions due to an inefficient or damaged catalytic converter.
Last updated: 2026-05-15
What goes wrong
The catalytic converter is a crucial component in your exhaust system, responsible for converting harmful pollutants into less toxic gases. When it fails, often due to degradation of the internal catalyst material, it can restrict exhaust flow and prevent the engine from running efficiently. This often triggers a P0420 diagnostic trouble code, indicating that the catalyst system efficiency is below the threshold.
Symptoms to watch for
- Check Engine Light (P0420)
- Reduced engine power
- Poor fuel economy
- Sulfur or rotten egg smell from exhaust
- Failed emissions test
- Rattling sound from underneath the vehicle
Root cause
Over time, catalytic converters can degrade due to oil or coolant contamination, exhaust rich conditions (often from misfires), or simply age and mileage. The internal ceramic honeycomb structure, coated with precious metals like platinum, palladium, and rhodium, can melt or break apart, leading to reduced efficiency or blockages.
Buy time with these
- • Regular oil changes and maintenance to prevent oil burning, which can contaminate the catalytic converter.
- • Address engine misfires or rich running conditions promptly to avoid overloading the converter with unburnt fuel.
Permanent fix
- Replace the faulty catalytic converter with a new OEM or high-quality aftermarket unit. Ensuring proper engine tune-up and addressing any underlying issues that may have contributed to the converter's failure (e.g., misfires, oil consumption) is crucial for longevity.
- Inspect and replace oxygen sensors if they are also faulty, as they play a critical role in monitoring converter efficiency and can sometimes trigger false P0420 codes if they are not reading correctly.
FAQ
What causes catalytic converter failure (p0420)?
Over time, catalytic converters can degrade due to oil or coolant contamination, exhaust rich conditions (often from misfires), or simply age and mileage. The internal ceramic honeycomb structure, coated with precious metals like platinum, palladium, and rhodium, can melt or break apart, leading to reduced efficiency or blockages.
How much does it cost to fix?
National average $1,500–$2,500. Cost varies significantly based on whether an OEM or aftermarket catalytic converter is used, and labor rates in your area. Some states have stricter emissions requirements, which might necessitate more expensive, compliant catalytic converters.
Is this a DIY repair?
Difficulty 3/5. Replace the faulty catalytic converter with a new OEM or high-quality aftermarket unit. Ensuring proper engine tune-up and addressing any underlying issues that may have contributed to the converter's failure (e.g., misfires, oil consumption) is crucial for longevity.
What are the symptoms?
Check Engine Light (P0420); Reduced engine power; Poor fuel economy; Sulfur or rotten egg smell from exhaust; Failed emissions test; Rattling sound from underneath the vehicle