P0420 on a Mazda: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

A Mazda with P0420 can usually be driven short-term, but a failing converter will eventually hurt fuel economy.

What P0420 means on a Mazda

On a Mazda, P0420 means the rear oxygen sensor on bank 1 reports the catalytic converter isn't cleaning the exhaust well enough. The PCM compares the front and rear O2 sensor signals, and when they become too similar it sets P0420. Most Mazda four-cylinders have a single bank, so the code points to the one converter.

Is P0420 common on Mazdas?

P0420 is common on higher-mileage Mazda3s, Mazda6s, and CX-5s. Mazda's oxygen sensors wear with age, and the converter can lose efficiency over time, especially on cars that see lots of short trips.

Commonly reported on: Mazda3, Mazda6, CX-5, CX-9.

Symptoms

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

Common causes

most common: Worn catalytic converter
common: Faulty rear oxygen sensor
common: Exhaust leak before the rear sensor
less common: Misfire or rich fuel condition that damaged the converter

How to fix it

Fix	DIY?	Typical cost
Check for exhaust leaks and repair them before replacing parts	Yes	$50–200
Test and replace the rear oxygen sensor if faulty	Yes	$60–200
Replace the catalytic converter	Mechanic	$600–2200

Frequently asked questions

Can I drive my Mazda with P0420?

Yes, usually for a short time. The engine will keep running, but a failing converter can reduce fuel economy and eventually clog, so get it checked soon.

Do Mazdas get P0420 from a bad sensor or a bad converter?

Both are possible. Higher-mileage Mazdas often wear out the rear O2 sensor, and the converter can lose efficiency with age, so a shop should test the sensor readings before replacing the converter.

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

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