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P0420 Catalyst System Efficiency Below Threshold (Bank 1)
What it means
The computer compares the oxygen sensors before and after the catalytic converter. On bank 1 the downstream sensor is switching too much like the upstream one, meaning the converter isn't storing/cleaning exhaust as well as it should.
Severity: Moderate
Can I drive it? Usually safe to drive short-term, but you'll fail emissions and a failing converter can get worse. If the light flashes or you smell rotten eggs/feel power loss, get it checked soon.
Common causes (most likely first)
- Worn-out catalytic converter
- Faulty downstream (rear) oxygen sensor
- Exhaust leak near the O2 sensors
- Engine misfires or rich running that damaged the converter
- Outdated engine software (some makes have a reflash)
Likely fixes
- Check for exhaust leaks and other codes first (misfire/fuel codes)
- Test/replace the downstream O2 sensor
- Replace the catalytic converter (use CARB-compliant in CA/strict states)
DIY parts cost
$40–$600
Approximate estimate
Shop repair cost
$200–$2,500
Approximate estimate, parts + labor
DIY difficulty
Moderate
Varies by vehicle
Cost ranges are rough, approximate U.S. estimates and vary widely by vehicle, region, and parts brand. Always confirm the diagnosis before buying parts.
Parts you may need
- Downstream oxygen sensorCheck price
- Catalytic converterCheck price
- O2 sensor socketCheck price
- Exhaust gasketCheck price
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