P0420: Catalyst System Efficiency Below Threshold (Bank 1)
The downstream oxygen sensor is seeing nearly the same oxygen levels as the upstream sensor, meaning the catalytic converter on bank 1 isn't cleaning the exhaust as well as it should.
Green, yellow, red: three ways P0420 can look
The same code can be a quick fix or a reason to pull over. Match your situation to the closest example.
Keep an eye on it
Light on, car runs normally, nothing else set. Safe to drive while you plan the repair.
Schedule a repair
Rotten egg smell, rattling under the car, or you need inspection soon. Get it diagnosed. Rule out a sensor or leak before buying a converter.
Stop and get help
P0420 with misfire codes, a flashing light, or a glowing red converter. Stop driving. The underlying misfire is destroying the converter and can start a fire.
Symptoms you might notice
- Usually no driving symptoms
- Failed emissions inspection
- Sometimes a rotten egg smell
Common causes, cheapest first
- Aging catalytic converter
- Exhaust leak near the sensors
- Faulty downstream oxygen sensor
- Engine burning oil or misfiring and damaging the converter
What you can check before calling
- Look for exhaust leaks (ticking or soot)
- Check for any other codes, especially misfires
A code points to a system, not a part. Replacing parts based on the code alone is how people spend money without fixing the problem. Testing confirms the cause first.
Want the full story? Read our guide: P0420: is your catalytic converter really bad?.
Generic codes (second character 0) mean the same thing on every vehicle. Some manufacturers add their own details, so check service information for your exact vehicle.
Related codes
Look up another code
Cummins fault code (like 1922), J1939 SPN and FMI (like SPN 3251 FMI 0), or an OBD-II code (like P0420).
Questions
What does P0420 mean?
P0420 means "Catalyst System Efficiency Below Threshold (Bank 1)". The downstream oxygen sensor is seeing nearly the same oxygen levels as the upstream sensor, meaning the catalytic converter on bank 1 isn't cleaning the exhaust as well as it should.
Can I drive with code P0420?
It depends on what the vehicle is doing. Green: Light on, car runs normally, nothing else set. Safe to drive while you plan the repair. Yellow: Rotten egg smell, rattling under the car, or you need inspection soon. Get it diagnosed. Rule out a sensor or leak before buying a converter. Red: P0420 with misfire codes, a flashing light, or a glowing red converter. Stop driving. The underlying misfire is destroying the converter and can start a fire.
What causes P0420?
The most common causes are: aging catalytic converter; exhaust leak near the sensors; faulty downstream oxygen sensor.
Is P0420 a serious code?
It can be. P0420 with misfire codes, a flashing light, or a glowing red converter. Stop driving. The underlying misfire is destroying the converter and can start a fire.
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