P0420: Catalyst System Efficiency Below Threshold (Bank 1)

The catalytic converter on bank 1 is not cleaning exhaust efficiently.

Medium Severity

$150 – $2,500

Typical repair cost range

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Can I Keep Driving with Code P0420?

Yes, but get diagnosed within a few weeks. Long-term it can lead to full converter failure.

What P0420 Means

P0420 means the upstream and downstream O2 sensors on bank 1 are reading too similar to each other, which tells the computer the catalytic converter is no longer doing its job. The cat itself, the O2 sensors, or an exhaust leak can all trigger this.

Common Causes

Failing catalytic converter — see repair cost
Bad downstream O2 sensor — see repair cost
Exhaust leak ahead of the rear sensor
Long-term engine misfires damaging the cat — see repair cost

Diagnostic Tips

Rule out exhaust leaks and bad O2 sensors before replacing a $1,500+ catalytic converter.
On older cars, aftermarket cats are 50 to 70 percent cheaper than OEM.
Check state law, California requires CARB-compliant cats.

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Frequently Asked Questions

Can I just clear P0420 and pass emissions?

No. Most state programs check that monitors are set and the code stays gone after driving.

Will a fuel additive fix P0420?

Sometimes, briefly, but it does not repair a damaged catalyst. It can clean up sensors and fuel injectors which sometimes helps.