P0138 on a Mercedes-Benz: Causes, Fixes & Repair Cost

🟢 Low severity · drivable

Safe to drive — this downstream sensor only monitors the catalytic converter, so a Mercedes with P0138 can be driven while you arrange the repair.

What P0138 means on a Mercedes-Benz

On a Mercedes-Benz, P0138 means the engine computer sees high voltage from the bank 1 downstream oxygen sensor, located after the catalytic converter. This sensor only checks converter efficiency, so high voltage is almost always a faulty sensor or a wiring short rather than a rich-running engine.

Is P0138 common on Mercedes-Benzs?

P0138 is most common on higher-mileage Mercedes models as the post-cat sensor ages. C-Class and E-Class models see it regularly, and wiring in the exhaust area can short after years of heat exposure.

Commonly reported on: C-Class, E-Class, GLK, ML.

Symptoms

Check engine light on
Usually no change in how it drives
Possible reduced fuel economy
Sometimes no noticeable driving symptoms

Common causes

most common: Faulty bank 1 downstream oxygen sensor
common: Sensor wiring shorted to power
common: Exhaust leak before the sensor
less common: Failing catalytic converter

How to fix it

Fix	DIY?	Typical cost
Replace the bank 1 downstream oxygen sensor	Yes	$100–350
Repair shorted sensor wiring	Mechanic	$100–350
Replace the catalytic converter if it's failing	Mechanic	$900–2800