P0420 Code: Catalyst System Efficiency Below Threshold (Bank 1)

🟠 Moderate severity · drivable

You can usually keep driving short-term, but a failing catalytic converter can eventually hurt performance and clog, so get it diagnosed soon.

What P0420 means

The downstream oxygen sensor on bank 1 is telling the engine computer that the catalytic converter isn't cleaning the exhaust as efficiently as it should. The computer compares the upstream and downstream oxygen sensors; if they read too similarly, the converter is underperforming and it stores P0420.

Symptoms

Check engine light is on
Reduced fuel economy
Possible rotten-egg smell from the exhaust
Often no noticeable driving symptoms early on

Common causes

most common: Worn or failing catalytic converter
common: Faulty downstream oxygen sensor (bank 1, sensor 2)
common: Exhaust leak before or near the downstream sensor
less common: Engine misfire or running rich that damaged the converter
less common: Coolant or oil contamination of the converter

How to fix it

Fix	DIY?	Typical cost
Inspect for exhaust leaks and repair them before replacing anything	Yes	$50–200
Test the downstream oxygen sensor with a scan tool and replace it if faulty	Yes	$60–200
Replace the catalytic converter (bank 1)	Mechanic	$600–2500

Frequently asked questions

Can I drive with a P0420 code?

Yes, usually for a short time. The engine will still run, but continuing to drive a failing converter can hurt fuel economy and eventually cause it to clog, so have it diagnosed soon.

Will a P0420 clear itself?

Sometimes the light goes off on its own if the converter was only marginally underperforming, but it usually comes back. The underlying problem won't fix itself.

How much does it cost to fix a P0420?

Most repairs run $600–$2,500 depending on the cause. A bad oxygen sensor is the cheap fix ($60–$200); a catalytic converter replacement is the expensive one.