P0138 code meaning
O2 Sensor Circuit High Voltage Bank 1 Sensor 2
Powertrain system Generic code P01xx codes Check engine light
What P0138 means
The oxygen sensor after the catalytic converter on bank 1 (sensor 2) is a switching sensor (about 0.1–0.9 V) used for converter monitoring and fine mixture trim. P0138 means the signal stays above the upper limit. A zirconia sensor can hardly produce more than about 1 V, so readings above 1.05–1.15 V point to an outside voltage such as the heater supply on the signal wire; GM also sees an open circuit here because of its bias voltage. Ford: above 1.1 V for 10 s; GM: above 1050 mV. Subaru again uses a functional test: the minimum voltage never falls below 250 mV within 200 s after fuel cut-off, so the sensor looks permanently rich. Monitored continuously, two trips. When it fails, the control module stops the downstream trim and cannot evaluate the converter; there is no limp mode.
P0138 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyEasy, DIY possible
- Typical labor0.3 to 1.5 hours
- Clears after repairAfter a drive cycle
Symptoms of P0138
- Check engine light on (usually after two drive cycles)
- Catalyst monitor readiness not set
- Usually no noticeable change in driving
Common causes of P0138
- Most likelySignal wire shorted to heater supply or battery voltage
- PossibleSensor faulty internally (heater-to-signal short)
- PossibleSignal circuit open or high-resistance (GM bias logic)
- Less likelyGenuinely rich exhaust or oil/coolant contamination
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Oxygen sensor bank1 sensor2Oxygen sensor wiringCatalytic converter
What each part of P0138 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 1 | Area: fuel and air metering. |
| 38 | The exact fault in that area. |
What to do next
- Note P0138 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0138
What does P0138 mean?
P0138 means "O2 Sensor Circuit High Voltage Bank 1 Sensor 2". The oxygen sensor after the catalytic converter on bank 1 (sensor 2) is a switching sensor (about 0.1–0.9 V) used for converter monitoring and fine mixture trim. It is a generic code, so it means the same on every make.
Is it safe to drive with P0138?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P0138 fail an emissions test?
Yes, it can. P0138 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0138 to fix?
Difficulty: easy. A typical repair takes 0.3 to 1.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P0138 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.