P0138 on a Ford: Causes, Fixes & Repair Cost

🟢 Low severity · drivable

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

What P0138 means on a Ford

On a Ford, 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.

Is P0138 common on Fords?

P0138 shows up on higher-mileage Fords as the downstream sensor wears out. F-150 and Explorer models with V6 and V8 engines see it most, and wiring in the exhaust area can chafe or short against the body or heat shields.

Commonly reported on: F-150, Escape, Explorer, Focus.

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 against the body
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	$70–250
Repair chafed or shorted sensor wiring	Mechanic	$80–300
Replace the catalytic converter if it's failing	Mechanic	$800–2500