P0138: Oxygen Sensor High Voltage (Bank 1, Sensor 2)
Can you still drive? Yes, but address it soon. A persistently rich exhaust can damage the catalytic converter over time.
What Does P0138 Mean?
Code P0138 indicates that the downstream oxygen sensor on bank 1 is producing a consistently high voltage signal, typically above 0.9 volts. This suggests the sensor is reading a rich exhaust condition after the catalytic converter. The cause could be a faulty sensor, an actual rich condition, or a short circuit in the sensor wiring.
Need to read your codes?
An OBD-II scanner lets you read and clear diagnostic trouble codes from your vehicle's computer. Most scanners work with any car made after 1996.
Browse OBD-II Scanners on AmazonCommon Causes
- Faulty downstream oxygen sensor stuck rich
- Short to voltage in sensor wiring
- Fuel system running excessively rich
- Leaking fuel injector causing rich exhaust
- Coolant entering combustion chamber
Symptoms
- Check engine light on
- Poor fuel economy
- May see black exhaust smoke
- Possible catalyst efficiency code
Diagnostic Steps
- 1Monitor sensor 2 voltage with scan tool at idle and under load
- 2Check for rich-running conditions on bank 1
- 3Inspect sensor wiring for shorts to power
- 4Disconnect sensor and check for voltage with key on
- 5Check for related fuel system codes
- 6Test sensor by introducing lean condition
Recommended Parts & Tools
downstream oxygen sensor
wiring repair kit
fuel injector cleaner
Estimated Repair Cost
| Type | Estimated Cost |
|---|---|
| Total Repair Cost Range | $100 - $400 |
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Chat with AI About P0138Commonly Affected Vehicles
Related Recalls
1 Active Recall — 2026 Ford F-150
The following NHTSA safety recalls have been issued for this vehicle. Contact your dealer to check if your vehicle is affected.
ELECTRICAL SYSTEM: INTEGRATED TRAILER BRAKE CONTROL
Ford Motor Company (Ford) is recalling certain 2021-2026 F-150, 2022-2026 F-250 SD, F-350 SD, F-450 SD, F-550 SD, Maverick, 2024-2026 Ranger, 2023-2026 F-600 SD, 2022-2027 Lincoln Navigator, Expedition, and 2026 E-Transit vehicles. When towing a trailer, the integrated trailer module (ITRM) may lose communication with the vehicle, possibly causing a loss of brake and turn signal lights, or a loss of brake function. As such, these vehicles fail to comply with the requirements of Federal Motor Vehicle Safety Standard number 108, " Lamps, Reflective Devices, and Associated Equipment."
Remedy: The integrated trailer module software will be updated by a dealer, or through an over-the-air (OTA) update, free of charge. Owner notification letters are expected to be mailed March 17, 2026. Owners may contact Ford customer service at 1-866-436-7332. Ford's number for this recall is 26C10. Vehicle Identification Numbers (VINs) involved in this recall will be searchable on NHTSA.gov beginning March 17, 2026.
Data provided by the National Highway Traffic Safety Administration (NHTSA).
Frequently Asked Questions
What does high voltage on an O2 sensor mean?
A high voltage reading (above 0.9V) on an oxygen sensor means it is detecting a rich exhaust condition with excess fuel in the exhaust gases.
Should I replace the sensor or look for other problems first?
Always check for underlying rich-running conditions first. If the engine is running rich, a new sensor will still read high voltage. Fix the root cause before replacing the sensor.
Community Fix Reports
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