P0420 on a Hyundai: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

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

What P0420 means on a Hyundai

On a Hyundai, P0420 means the rear oxygen sensor on bank 1 reports the catalytic converter isn't cleaning the exhaust efficiently. The engine computer compares the front and rear O2 readings, and when they get too similar it sets P0420. Most Hyundai four-cylinders have a single bank, so the code covers the one converter.

Is P0420 common on Hyundais?

P0420 is fairly common on Elantras, Sonatas, and Santa Fes, particularly with high mileage. Some earlier 2.0L and 2.4L Theta engines had oil consumption issues that can clog the converter, and Hyundai's rear O2 sensors also wear out on their own.

Commonly reported on: Elantra, Sonata, Santa Fe, Tucson.

Symptoms

Check engine light on
Slightly worse fuel economy
Rotten-egg smell from the exhaust
Usually 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 before the rear sensor
less common: Oil consumption from the engine damaging 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	Mechanic	$600–2100

Frequently asked questions

Can I drive my Hyundai with P0420?

Yes for a short time, but it's best to diagnose it soon. A failing converter hurts fuel economy, and if oil consumption is the cause, driving on it makes things worse.

Why do Hyundai engines get P0420?

Higher-mileage Theta 2.0L and 2.4L engines can consume oil, and oil reaching the converter can clog it. The rear O2 sensor is another common failure on its own.

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

A rear O2 sensor is about $60–$200, while a converter replacement is typically $600–$2,100 depending on the model.