P0420 on a GMC: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

A GMC with P0420 can usually be driven short-term, but a clogged converter will eventually hurt power and fuel economy.

What P0420 means on a GMC

On a GMC, P0420 means the rear oxygen sensor on bank 1 reports the catalytic converter isn't cleaning the exhaust as well as it should. The ECM compares the front and rear O2 readings, and when they track too closely it stores the code. On V8 trucks and SUVs, bank 1 is the driver's side.

Is P0420 common on GMCs?

P0420 is a frequent issue on Sierras, Yukons, and Acadias with higher mileage. GMC V8s share the Chevy platform, so intake gasket leaks and AFM-related misfires are a known route to a damaged converter, and the rear O2 sensor itself is a common failure.

Commonly reported on: Sierra, Terrain, Acadia, Yukon.

Symptoms

Check engine light on
Slightly worse fuel economy
Rotten-egg smell from the exhaust
Possible rough idle if there's an underlying misfire

Common causes

most common: Worn catalytic converter on bank 1
common: Faulty rear oxygen sensor
common: Exhaust leak before the rear sensor
less common: Misfire from an intake gasket leak or AFM issue that damaged the converter

How to fix it

Fix	DIY?	Typical cost
Check the exhaust for leaks and repair them first	Yes	$50–200
Test the rear oxygen sensor and replace it if faulty	Yes	$60–200
Replace the catalytic converter on bank 1	Mechanic	$600–2400

Frequently asked questions

Can I drive my GMC with P0420?

Yes for a short time, but don't ignore it. A failing converter hurts fuel economy, and if a misfire caused the code, driving on it can damage the converter further.

Why do GMC trucks get P0420 so often?

Higher-mileage Sierras and Yukons commonly wear out the bank 1 converter, and intake gasket leaks or AFM-related misfires can damage it faster, so the fix often starts with the root cause.

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

A rear O2 sensor is about $60–$200, while a converter replacement is typically $600–$2,400 depending on the model.