P0420 on a Honda: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

You can usually drive a Honda for a while with P0420, but a clogged converter can reduce fuel economy and power over time.

What P0420 means on a Honda

On a Honda, P0420 tells you the rear oxygen sensor (sensor 2) on bank 1 sees the catalytic converter isn't cleaning the exhaust as well as it should. Honda's computer watches the difference between the front and rear O2 sensors, and when that difference gets too small it stores P0420. On four-cylinder Hondas there's only one bank, so the code covers the single converter.

Is P0420 common on Hondas?

P0420 shows up on a lot of higher-mileage Accords, Civics, and CR-Vs. Honda's rear O2 sensors are known to wear out and read slowly, and on older 2.4L engines, oil burning can eventually clog the converter.

Commonly reported on: Accord, Civic, CR-V, Odyssey.

Symptoms

Check engine light on
Slightly worse fuel economy
Rotten-egg smell from the exhaust
Little or no change in how it drives

Common causes

most common: Worn or clogged catalytic converter
common: Faulty rear oxygen sensor (sensor 2)
common: Exhaust leak near the rear sensor
less common: Oil burning from an older 2.4L engine damaging the converter

How to fix it

Fix	DIY?	Typical cost
Inspect the exhaust for leaks and repair them first	Yes	$50–200
Test and replace the rear oxygen sensor if it's slow	Yes	$60–200
Replace the catalytic converter (use an OEM or quality part)	Mechanic	$600–2000

Frequently asked questions

Can I keep driving my Honda with a P0420 code?

Yes for a short time. The car will still run, but a failing converter can hurt gas mileage and eventually clog, so get it looked at soon.

Do Hondas often get P0420 from a bad sensor instead of a bad converter?

Yes. The rear oxygen sensor on many Hondas fails or gets slow with age, and that alone can set P0420, so it's worth testing before buying a converter.

How much does it cost to fix P0420 on a Honda?

A rear O2 sensor runs about $60–$200, while a catalytic converter replacement is typically $600–$2,000 depending on the model.