P0420 on a Mercedes-Benz: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

A Mercedes-Benz with P0420 is drivable short-term, but a failing converter will eventually hurt fuel economy and performance.

What P0420 means on a Mercedes-Benz

On a Mercedes-Benz, 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 sensor signals, and when they become too similar it stores P0420. On V6 and V8 models, bank 1 is the driver's side.

Is P0420 common on Mercedes-Benzs?

P0420 shows up on higher-mileage C-Class, E-Class, and S-Class models. Mercedes catalytic converters and oxygen sensors wear with age, and oil or coolant leaks onto the exhaust can shorten sensor and converter life.

Commonly reported on: C-Class, E-Class, S-Class, GLE.

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 on bank 1
common: Faulty rear oxygen sensor
common: Exhaust leak before the rear sensor
less common: Oil or coolant leak contaminating the sensor or converter

How to fix it

Fix	DIY?	Typical cost
Inspect the exhaust and O2 sensor wiring for leaks or damage	Yes	$50–200
Test and replace the rear oxygen sensor if faulty	Yes	$80–250
Replace the catalytic converter on bank 1	Mechanic	$800–2800

Frequently asked questions

Can I drive my Mercedes 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 have it diagnosed soon.

Do Mercedes models get P0420 from a sensor or the converter?

Both are common. Higher-mileage Mercedes often wear out the rear O2 sensor, and oil or coolant leaks can damage the converter, so a shop should test before replacing parts.

How much does it cost to fix P0420 on a Mercedes-Benz?

A rear O2 sensor is about $80–$250, while a converter replacement is typically $800–$2,800, with dealer labor on the higher end.