P0138

What it means

P0138 is the generic code for a high voltage fault in the oxygen sensor circuit on Bank 1, Sensor 2, which is the sensor after the catalytic converter on the side of the engine containing cylinder 1. The control unit saw the sensor signal stay above its expected range, typically above about 1.0 volt, for longer than it should. This can be caused by a wiring short, a contaminated or failed sensor, or a real fault that makes the exhaust run very rich.

Symptoms

• Engine warning light on
• Little or no change in drivability in many cases
• Poor fuel economy if a rich running fault is present
• Smell of fuel or black smoke from the exhaust if the mixture is very rich
• Possible failed emissions test
• Other fuel trim or misfire codes stored alongside

Possible causes

• Faulty or contaminated downstream oxygen sensor (Bank 1 Sensor 2)likely
• Sensor signal wire shorted to voltage or to the heater supply, or chafed wiring near the exhaustlikely
• Corroded, damaged or loose connector at the sensor or control unitpossible
• Engine running rich due to a fuel system fault such as leaking injectors, high fuel pressure or a faulty upstream sensorpossible
• Silicone, coolant or oil contamination of the sensor elementpossible
• Fault in the engine control unit sensor circuit or reference voltagerare

How to diagnose

1. Read all stored codes and freeze frame data, noting engine load, temperature and fuel trim values at the time the fault set
2. Check for other codes, especially fuel trim, misfire or upstream sensor faults, and deal with those first
3. Inspect the sensor wiring and connector along its full length for heat damage, chafing, water ingress and corrosion
4. Confirm the sensor is on Bank 1 and is the downstream one, as the layout depends on the engine
5. Look at live data for the sensor voltage with the engine warm, and compare with the expected switching behaviour for that model
6. Check short term and long term fuel trims for signs of a rich condition
7. Unplug the sensor and check the signal and reference circuits for shorts to voltage or to each other, following the wiring diagram for the vehicle
8. Check for exhaust leaks near the sensor and signs of coolant or oil contamination on the sensor tip

How to fix

• Repair or replace damaged wiring and renew faulty connector terminals
• Replace the downstream oxygen sensor if it is faulty or contaminated, using the correct type for the engine
• Correct any rich running fault found, such as leaking injectors or fuel pressure problems
• Fix the source of any coolant, oil or silicone contamination before fitting a new sensor
• Repair or replace the control unit only after wiring and sensor faults have been fully ruled out

Can I keep driving?

This code for other makes

• P0138DodgeHO2S12 High Voltage

Nearby codes

• P0137GenericO2 Sensor Circuit Low Voltage Bank 1 Sensor 2
• P0139GenericO2 Sensor Circuit Slow Response Bank 1 Sensor 2