Fault Code P0138: High Voltage from Oxygen Sensor (O2) – Sensor 2, Bank 1
What is the P0138 Code?
Fault code P0138 is a generic OBD-II code related to the powertrain. It indicates an abnormally high voltage from the heated oxygen sensor (HO2S) located after the catalytic converter on engine bank 1.
This downstream sensor (B1S2) monitors the catalyst’s efficiency by comparing oxygen levels with those from the upstream sensor. Code P0138 is triggered when the engine control module (ECM) detects that the sensor voltage consistently exceeds 999 mV for more than 2 minutes.
Symptoms of P0138 Fault
- Malfunction Indicator Lamp (MIL/Check Engine) illuminated
- Often no visible symptoms
- Possible slight increase in fuel consumption
- Unstable engine idle in some cases
Main Causes of Code P0138
Electrical Issues
- Short circuit in the O2 sensor signal circuit
- Damaged wires or melted insulation
- Oxidized or corroded connector
Mechanical Failures
- Faulty or aging oxygen sensor
- Excessively high fuel pressure
- Sensor ground circuit issue
Diagnosis and Solutions
Preliminary Steps
- Check the condition of connectors and wiring
- Visually inspect the sensor for damage
- Use a diagnostic tool to read real-time data
Corrective Solutions
- Replace the faulty O2 sensor (most common solution)
- Repair or replace damaged wiring
- Clean corroded electrical connectors
- Check and adjust fuel pressure if necessary
FAQ – Frequently Asked Questions
Can I drive with a P0138 code?
Limited driving is possible, but prompt diagnosis is recommended to prevent potential damage to the catalyst.
How much does the repair cost?
Replacing an O2 sensor typically costs between €150 and €400 depending on the vehicle model.
Conclusion
Code P0138 usually indicates an electrical problem or a faulty oxygen sensor. Professional diagnosis is recommended to accurately identify the cause before any replacement. Inspection of wiring and connectors should always be the first step.
Need help with this fault code? Consult a qualified mechanic for accurate diagnosis and reliable repair.