P0138: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
Key takeaways
- P0138 means the downstream oxygen sensor on bank 1, the one behind the catalytic converter, is sending a voltage higher than it should.
- The engine rarely runs differently, so it's usually safe to drive, but the car will fail an OBD emissions inspection.
- A signal wire rubbing against a heater or power wire, or water in the connector, can cause it, so check wiring before buying a $150–$400 sensor.
- If the engine is actually running rich, the sensor is telling the truth, and the fix is upstream: a leaking injector or high fuel pressure, $250–$600 per injector.
- Subaru, Ford, GM and Nissan have bulletins naming P0138, from revised sensors to connector corrosion to repair procedures.
What P0138 means
The oxygen sensor behind your catalytic converter is reporting a voltage that’s too high for too long. The scanner name is O2 Sensor Circuit High Voltage (Bank 1, Sensor 2). Sensor 2 is the downstream sensor, after the converter. Its main job is to tell the engine computer whether the converter is working. A normal downstream sensor holds fairly steady somewhere under about 0.9 volts. High voltage means “rich,” lots of fuel and no oxygen. The computer sets P0138 when the reading stays above its limit, often after a deceleration when the sensor should drop low. Bank 1 is the side of the engine with cylinder 1; on a 4-cylinder it’s the only bank.
Is it safe to drive?
Usually yes. The downstream sensor has little say over fueling on most cars, so the engine drives the same. The car will fail an OBD emissions inspection, and some cars show slightly worse fuel economy. What changes the answer: a rough-running engine, a strong fuel smell, black smoke, or a flashing check engine light. Those mean the engine really is running rich or misfiring, and unburned fuel can overheat the catalytic converter. Get that checked soon.
Causes, ranked
| Cause | How you confirm it | Typical cost (US independent shop, 2026) |
|---|---|---|
| Wiring or connector fault (signal shorted to heater or power wire, water or corrosion in the plug) | Wiring melted near the exhaust, green corrosion in the plug; voltage reads high with the sensor unplugged | $100–$250 |
| Blown fuse or shorted heater circuit feeding voltage into the signal | Fuse check; heater codes such as P0141 set too | $0–$20 fuse; wiring as above |
| Downstream O2 sensor failed or contaminated | Live data stuck high while the engine is not rich; sensor tip coated with coolant or oil residue | $150–$400 |
| Engine actually running rich (leaking injector, high fuel pressure) | Upstream sensor and fuel trims also show rich; fuel smell | $250–$600 per injector |
| Engine computer software (some makes) | A bulletin for your exact model and year names P0138 | $100–$200, often $0 under warranty |
What to check first
- Read all the codes. If heater codes are there too, start with the fuse and the wiring. If rich or misfire codes are there, fix those first and the downstream reading may settle.
- Check the O2 heater fuse. The owner’s manual fuse chart names it. A blown fuse means there’s a short somewhere, so look for it before replacing the fuse.
- Inspect the harness under the car. Look for wires touching the exhaust, a melted clip, or water in the connector. Unplug it, dry it, look for green corrosion.
- Watch live data. A shop compares the upstream and downstream sensors and fuel trims. If the whole engine looks rich, the sensor is fine.
- Replace the sensor last, once wiring and fueling check out. Clear the code and drive a normal mixed trip to confirm.
- Check bulletins. Subaru has released revised oxygen sensors for 2020–2023 Outbacks; Ford names corrosion or water intrusion at the connector on 2021–2022 Bronco Sport 2.0L; GM and Nissan have repair bulletins for the 2018–2019 Equinox and Terrain and the 2018 Armada and QX80.
What it costs
The cheap outcome is a fuse or a cleaned and resealed connector, under $20 in parts. The common outcome is a wiring repair at $100–$250 or a downstream sensor at $150–$400 installed. The expensive outcome is fixing a real rich condition upstream, $250–$600 per injector. Paying for a look at live data first is what keeps you from buying a sensor that was reporting the truth. Our O2 sensor check engine light guide covers the whole sensor family.
Related codes
P0138 often appears with P0141 (heater circuit for the same sensor) and P0137 (low voltage, same sensor). If P0420 comes up later, the converter test may be reading bad data from this sensor, so fix P0138 first. If the car feels fine otherwise, see check engine light on but car runs fine.
What owners report to NHTSA
11 complaints filed with NHTSA mention P0138, 10 of them since 2015, across 9 different models. The component owners most often filed it under: engine.
Most-reported models
| Vehicle | Complaints | Model years reported |
|---|---|---|
| CHEVROLET MALIBU | 3 | 2017 |
Manufacturer service bulletins that name P0138
50 technical service bulletins and manufacturer communications filed with NHTSA mention P0138, from 13 makes. A bulletin means the manufacturer has documented a known cause or a fix procedure for that model. Examples:
- SUBARU OUTBACK (2020, 2021, 2022, 2023), bulletin 11000828: This bulletin announces the availability of new oxygen sensors developed to address the following Diagnostic Trouble Codes (DTCs); P0137 (O2 Sensor Circuit Low Voltage Bank 1 Sensor 2, P0138 (O2 Sensor Circuit High Volta…
- FORD MAVERICK (2022), bulletin 10216960: Some 2022 Maverick vehicles built on or before 4-Apr-2022 equipped with 2.0L engine may exhibit a charging system warning indicator with diagnostic trouble codes (DTCs) U012D, U0284 and/or U042E stored in the powertrain …
- FORD BRONCO SPORT (2021, 2022), bulletin 10218901: Some 2021-2022 Bronco Sport vehicles equipped with 2.0L engine may exhibit a malfunction indicator lamp (MIL) with diagnostic trouble codes (DTCs) P0036 and/or P0138 stored in the powertrain control module (PCM). This ma…
- JEEP GLADIATOR (2022), bulletin 10223805: Flash: Powertrain Control Module (PCM) Updates This bulletin involves reprogramming the PCM with the latest available software. Customers may experience a Malfunction Indicator Lamp (MIL) illumination. Upon further inves…
- FORD F-650 SD (2020), bulletin 10181940: Some 2020 F-Super Duty vehicles equipped with the 6.2L or 7.3L engine built on or before 05-Jun-2020 may exhibit an illuminated malfunction indicator lamp (MIL) with diagnostic trouble codes (DTCs) P0132, P0135, P0138, P…
- FORD F-250 SD (2020), bulletin 10199477: Some 2020 F-Super Duty vehicles equipped with a 6.2L or 7.3L engine, 10R140 transmission, and built on or before 05-Jun-2020 may exhibit an illuminated malfunction indicator lamp (MIL) with diagnostic trouble codes (DTCs…
Sources: NHTSA Office of Defects Investigation complaint database and manufacturer communications file, snapshots September 2026. A complaint or bulletin that mentions a code is a report about that vehicle, not a statement that the code has one cause.
Frequently asked questions
Where is bank 1 sensor 2?
It's the oxygen sensor after the catalytic converter on the side of the engine with cylinder 1. On a 4-cylinder there's only one bank, so it's simply the rear sensor, often reached from under the car.
Can I drive with P0138?
Usually yes. The downstream sensor mainly checks the catalytic converter, so the engine runs normally. If the engine feels rough, smells of fuel, or the light flashes, the problem is bigger than the sensor and needs a look soon.
How much does it cost to fix P0138?
A fuse or connector fix can be under $20 in parts; wiring repair runs $100–$250 and a new downstream sensor $150–$400 at an independent shop. If the engine is running rich, the upstream fix can reach $600 per injector.
Will a new O2 sensor fix P0138?
Often, but not always. If the wiring is shorted or the connector has water in it, a new sensor will set the same code. Check the harness and look at live data first.
Can P0138 damage the catalytic converter?
The sensor fault itself won't. But if the high reading is real and the engine runs rich for months, extra fuel can overheat and damage the converter.
References
- NHTSA ODI complaints flat file (FLAT_CMPL)
- NHTSA manufacturer communications / technical service bulletins flat files
- 40 CFR 86.1806-17: Onboard diagnostics (EPA OBD requirements, incl. oxygen sensor and catalyst monitoring)
- 13 CCR 1968.2: Malfunction and diagnostic system requirements (California OBD II)