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.
How the engine computer detects it
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.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine light on (usually after two drive cycles)
- Catalyst monitor readiness not set
- Usually no noticeable change in driving
Common causes
- Signal wire shorted to heater supply or battery voltage Most likely
- Sensor faulty internally (heater-to-signal short) Possible
- Signal circuit open or high-resistance (GM bias logic) Possible
- Genuinely rich exhaust or oil/coolant contamination Less likely
How to diagnose P0138
- Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
- Inspect the parts involved: oxygen sensor bank1 sensor2, oxygen sensor wiring, catalytic converter.
- Work through the causes above from most to least likely, cheapest checks first.
- Clear the code and drive a full drive cycle to confirm the repair held.
Vehicles that report P0138 most
| Vehicle | Years | Reports | Share | Median miles |
|---|---|---|---|---|
| Chevrolet Malibu | 2017 | 3 | — |
Counts of owner complaints to the NHTSA that mention P0138 (11 in total), not failure rates: popular models get more complaints simply because more of them are on the road. Models with fewer than 3 complaints are left out.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 1
- Fuel and air metering
- 38
- Specific fault
Questions drivers ask
Can I drive with code P0138?
Usually yes for a short time, if the car drives normally, the temperature gauge reads normal and the check engine light is steady. If the light flashes, the engine loses power or overheats, or other warning lights come on, pull over and have it checked.
Will my car pass an emissions inspection with P0138?
Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.
Will P0138 clear by itself?
The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.
Can I fix P0138 myself?
Often yes, with an OBD-II scanner and basic tools. Difficulty: easy. Start with the cheapest checks before replacing expensive parts.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.