P0138 on a Toyota: Causes, Fixes & Repair Cost

🟢 Low severity · drivable

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

What P0138 means on a Toyota

On a Toyota, P0138 means the engine computer sees high voltage from the bank 1 downstream oxygen sensor behind the catalytic converter. Since this sensor only checks converter efficiency, high voltage is almost always a faulty sensor, a wiring short, or a converter problem — not a rich-running engine.

Is P0138 common on Toyotas?

P0138 is most common on higher-mileage Toyotas as Denso downstream sensors fail with age. Corolla, Camry, and RAV4 models see it regularly, and wiring damaged by rodents is a known cause on Toyotas because the wiring insulation can be attractive to them.

Commonly reported on: Camry, Corolla, RAV4, Tacoma.

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 another wire
common: Wiring damaged by rodents
less common: Failing catalytic converter affecting the downstream reading

How to fix it

Fix	DIY?	Typical cost
Replace the bank 1 downstream oxygen sensor with an OE-quality Denso unit	Yes	$70–250
Repair shorted or rodent-damaged sensor wiring	Mechanic	$100–350
Replace the catalytic converter if it's failing	Mechanic	$800–2500