P0420 on a Subaru: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

A Subaru with P0420 is drivable short-term, but a clogged converter will eventually hurt power and fuel economy.

What P0420 means on a Subaru

On a Subaru, P0420 means the rear oxygen sensor on bank 1 sees the catalytic converter isn't cleaning the exhaust as well as it should. Subaru's computer compares the front and rear O2 sensors, and when the readings stay too similar it stores P0420. On the flat four-cylinder engines, bank 1 covers the single converter up front.

Is P0420 common on Subarus?

P0420 is a familiar code on Outbacks, Foresters, and Imprezas with higher mileage. Subaru catalytic converters and oxygen sensors wear with age, and some boxer engines burn enough oil to shorten converter life.

Commonly reported on: Outback, Forester, Impreza, Crosstrek.

Symptoms

Check engine light on
Slightly worse fuel economy
Rotten-egg smell from the exhaust
Often no 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: Oil burning from the engine contaminating the converter

How to fix it

Fix	DIY?	Typical cost
Inspect the exhaust for leaks and repair them first	Yes	$50–200
Test the rear oxygen sensor and replace it if slow	Yes	$60–200
Replace the catalytic converter	Mechanic	$600–2200

Frequently asked questions

Can I drive my Subaru with a P0420 code?

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

Why is P0420 so common on Subarus?

Subaru boxer engines and their exhaust parts wear with mileage, and the rear O2 sensor is a known weak point. Some engines also burn oil, which can clog the converter faster.

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

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