Fault code P0138 – a step-by-step guide to diagnosing and fixing it

Error code P0138 is displayed and the check engine light is on - what does this mean?

The OBD/EOBD system has detected that the voltage of the rear oxygen sensor (bank 1, sensor 2) is too high. This DTC P0138 indicates a fault in the downstream O2 sensor or its circuit, which is located behind the catalytic converter. The fault code can occur with various manufacturers and affects the exhaust gas purification system and the engine control unit.

What causes can this fault code have?

• Defective rear oxygen sensor (bank 1, sensor 2)
• Damaged or corroded cable connections to the O2 sensor
• Short circuit in the sensor wiring
• Defective engine control unit (ECU)
• Dirty or contaminated oxygen sensor
• Leaks in the exhaust system upstream of the sensor
• Incorrect fuel/air mixture
• Defective catalytic converter

What symptoms occur in parallel with fault code P0138?

• Increased fuel consumption: The vehicle consumes more fuel than usual due to the faulty mixture control.
• Poor exhaust emission values: Increased emission values are detected during the exhaust emission test.
• Unsteady idling: The engine runs unevenly or rough at idle.
• Loss of power: Noticeable reduction in engine power, especially at higher engine speeds.
• Jerking when accelerating: The vehicle jerks or stutters during acceleration.

What symptoms occur in parallel with fault code P0138?

• First of all, a thorough diagnosis should be carried out to identify the exact cause.
• Replace the oxygen sensor: Replace the defective rear O2 sensor (bank 1, sensor 2) with a new original part, observing the correct tightening torques
• Repair wiring harness: Locate damaged or corroded cables to the oxygen sensor, repair severed areas or replace the complete wiring harness
• Clean plug connections: Remove all connectors to the O2 sensor, clean with contact spray and reconnect firmly to eliminate loose contacts
• Seal the exhaust system: Repair leaks in the exhaust system upstream of the sensor using new seals or welding work
• Check engine control unit: Test the ECU for defects and have it repaired or replaced if necessary
• Clean the fuel system: Perform a professional fuel system cleaning to remove contaminants that could affect the sensor