P0138 Code: O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
🟢 Low severity · drivable
Safe to drive — this downstream sensor only monitors the catalytic converter.
What P0138 means
The bank 1 downstream oxygen sensor is reporting high voltage. Since this sensor sits after the catalytic converter and only checks converter efficiency, high voltage here is almost always a sensor or wiring fault — not a rich-running engine (unlike the upstream sensor, P0132).
Symptoms
- Check engine light on
- Usually no driving symptoms
Common causes
- most common: Faulty downstream oxygen sensor
- common: Wiring shorted to power
- less common: A failing catalytic converter allowing excess fuel through
How to fix it
| Fix | DIY? | Typical cost |
|---|---|---|
| Replace the bank 1 downstream oxygen sensor | Yes | $60–200 |
| Repair shorted sensor wiring | Mechanic | $50–150 |
Frequently asked questions
Does P0138 mean the engine is rich?
Not usually. The downstream sensor doesn't control fuel, so high voltage there is more likely a bad sensor or wiring than a rich mixture.
What causes P0138?
Most often a faulty downstream oxygen sensor.
How much does it cost to fix P0138?
Typically $60–$200 for a downstream oxygen sensor.
Related codes
P0136
P0136 Code: O2 Sensor Circuit Malfunction (Bank 1 Sensor 2)
P0137P0137 Code: O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
P0420P0420 Code: Catalyst System Efficiency Below Threshold (Bank 1)
P0138 by vehicle brand
Causes and repair costs for P0138 differ between makes. Pick your brand for the specifics.
Toyota
P0138 on a Toyota
HondaP0138 on a Honda
FordP0138 on a Ford
ChevyP0138 on a Chevy
NissanP0138 on a Nissan
BMWP0138 on a BMW
Mercedes-BenzP0138 on a Mercedes-Benz
VolkswagenP0138 on a Volkswagen
HyundaiP0138 on a Hyundai
SubaruP0138 on a Subaru
MazdaP0138 on a Mazda
JeepP0138 on a Jeep
GMCP0138 on a GMC
LexusP0138 on a Lexus
VolvoP0138 on a Volvo
Related guide: MAF Sensor Replacement Cost →
Updated 2026-08. This page is informational and not a substitute for professional diagnosis.