Car symptom
Rotten egg smell / P0420
A sulfur smell or a P0420 code points to the catalytic converter or the mixture feeding it. Fix the cause before replacing the cat.
Most likely causes
- Rich mixture / misfire
- Worn catalytic converter
- Faulty rear O2 sensor
- Exhaust leak before the cat
Checks to run
- Read fuel trims and codes
- Check for exhaust leaks
- Compare front/rear O2 signals
Tools you may need
OBD-II scanner
Confirm before replacing. These are hypotheses to check — never buy parts on a guess. If the fault involves brakes, airbags or fuel under pressure, use a professional.
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FAQ
It depends on the cause. Some causes are cheap and quick; others (like a head gasket or seized caliper) need attention soon. Work through the checks in order and confirm before replacing parts.
Start with the free checks listed here. If they involve high voltage, fuel under pressure, airbags or brakes, let a professional handle it.
The interactive diagnostic asks targeted questions to narrow it down, and every cause must be confirmed with a measurement — never guess.