P0138 on a Volkswagen: Causes, Fixes & Repair Cost

🟢 Low severity · drivable

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

What P0138 means on a Volkswagen

On a Volkswagen, P0138 means the engine computer sees high voltage from the bank 1 downstream oxygen sensor behind the catalytic converter. This sensor only monitors converter efficiency, so high voltage is almost always a faulty sensor or a wiring short rather than a rich fuel mixture. Most VWs use a single bank, so bank 1 covers the whole engine.

Is P0138 common on Volkswagens?

P0138 is most common on higher-mileage VWs as the downstream sensor fails. Golf, Jetta, and Passat models see it regularly, and wiring in the hot exhaust area can short as its insulation breaks down.

Commonly reported on: Golf, Jetta, Passat, Tiguan.

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 with a Bosch or OE-quality part	Yes	$80–280
Repair shorted sensor wiring	Mechanic	$80–280
Replace the catalytic converter if it's failing	Mechanic	$900–2600