P0138 on a BMW: Causes, Fixes & Repair Cost

🟢 Low severity · drivable

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

What P0138 means on a BMW

On a BMW, P0138 means the DME sees high voltage from the bank 1 downstream oxygen sensor, the one behind the catalytic converter. Because this sensor only checks converter efficiency, high voltage is almost always a faulty sensor or a wiring short — not a rich-running engine. On inline-six BMWs, bank 1 is the only bank; on V8s it is the driver's side.

Is P0138 common on BMWs?

P0138 is most common on higher-mileage BMWs as the post-cat sensor fails or its wiring wears through against the body. 3 Series and X5 models see it regularly, and exhaust leaks near the sensor can also confuse the reading.

Commonly reported on: 3 Series, 5 Series, X3, X5.

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 or chafed
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	$90–320
Repair shorted sensor wiring	Mechanic	$100–350
Replace the catalytic converter if it's failing	Mechanic	$900–2800