What it means
The engine control module has detected that the Bank 1 catalyst system is not meeting its monitored emissions-conversion efficiency threshold. The code does not by itself prove that the catalytic converter has failed; other engine, sensor, or exhaust faults can affect the monitor.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The engine control module compares oxygen-sensor behavior before and after the catalytic converter on bank 1 to assess emissions conversion. P0420 sets when the monitored Bank 1 catalyst efficiency falls below the module's threshold, though the code alone does not establish that the converter is the cause.
Common causes
Most likely first.
- Catalyst aged or contaminated
- Downstream oxygen sensor drift
- Exhaust leak before catalyst
- Long-term misfire damage
Symptoms
- Check engine light on
- Failed emissions inspection
- Possibly slightly increased fuel consumption
- Sulfur (rotten egg) smell from exhaust under load
How to diagnose it
- Read freeze-frame data and check for other engine or oxygen-sensor codes that could affect the Bank 1 catalyst monitor.
- Inspect the exhaust around the Bank 1 converter and oxygen sensors for leaks, damage, or loose connections.
- Review live data from the upstream and downstream Bank 1 oxygen sensors to check whether their behavior is consistent with the monitor fault.
- Check for active or recent misfires and mixture problems, which can impair the catalyst monitor and damage the converter.
- If the exhaust is sealed and engine operation is sound, test Bank 1 catalyst performance using the manufacturer's diagnostic procedure.
Common fixes
- Repair an exhaust leak ahead of the Bank 1 catalyst.
- Correct any misfire or mixture fault that is affecting the catalyst.
- Replace a faulty Bank 1 downstream oxygen sensor if testing confirms it is drifting.
- Replace the Bank 1 catalytic converter if testing confirms its efficiency is below specification.
Can I keep driving?
You can usually drive short distances if the engine runs normally and the light is steady, but the vehicle may fail an emissions inspection. Stop and investigate promptly if a flashing light, severe power loss, or a strong sulfur smell develops.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.