Bad Catalytic Converter Symptoms: Signs and Causes
Signs of a bad catalytic converter include a check engine light with code P0420 or P0430, a rotten-egg (sulfur) smell, sluggish acceleration from a clogged converter, rattling under the car, and a failed emissions test. A converter rarely fails on its own: misfires, oil burning or coolant leaks are usually what killed it.
Common symptoms
- Check engine light with P0420 or P0430 ("catalyst efficiency below threshold")
- Rotten-egg smell from the exhaust
- Poor acceleration or a top-speed limit: a clogged converter restricts exhaust flow
- Rattling under the car, loudest at startup: broken internal honeycomb
- Worse fuel economy
- Failed emissions inspection
Rule these out before replacing it
A P0420 code does not always mean the converter is bad. An exhaust leak ahead of the downstream oxygen sensor, or a lazy sensor, can trigger the same code. A good shop checks sensor readings and looks for leaks first.
- A failing downstream oxygen sensor
- An exhaust leak near the sensors
- Engine misfires or a rich fuel mixture that are still present
What kills catalytic converters
Fixing the underlying cause is essential, or a new converter will fail the same way.
- Misfires: unburned fuel overheats the converter
- Oil burning: worn rings or valve seals coat the converter
- Coolant leaks into the combustion chamber, often from a head gasket
- Physical damage from road debris
- Theft: a sudden, very loud exhaust usually means it was cut off
Frequently asked questions
How long does a catalytic converter last?
Often the life of the car, 10 years or more, if the engine runs well.
Can I clean a catalytic converter?
Fuel additives rarely fix a failed converter. They may help marginal cases but are not a reliable repair.
Is a catalytic converter covered by warranty?
Federal law requires an 8-year/80,000-mile warranty on the converter for many vehicles. Check your warranty booklet before paying.
Related repair costs
More guides
- How Long Does a Car Battery Last?
- Bad Alternator Symptoms: 7 Signs Your Alternator Is Failing
- Why Does My Car Shake When Braking?
- How Long Do Brakes Last?
Sources and methodology
Estimates combine typical book labor times, current aftermarket and OEM parts prices, and 2026 US shop labor rates. See exactly how we calculate them.
- U.S. Bureau of Labor Statistics: Occupational Employment and Wage Statistics, automotive service technicians (49-3023), by state
- U.S. Bureau of Labor Statistics: Automotive Service Technicians and Mechanics (pay and duties)
- Federal Trade Commission: Auto Repair Basics (estimates, warranties, your rights)
- NHTSA: Check for recalls before paying for a repair (our free recall check uses NHTSA data)
- ASE (National Institute for Automotive Service Excellence): technician certification