Fault code P1138 – a step-by-step guide to diagnosing and fixing it

Error code P1138 is displayed and the check engine light is on - what does this mean?

The DTC P1138 is recognized by the OBD system if there is a problem with the lambda sensor or the air-fuel mixture control circuit. This manufacturer-specific error code mainly affects the fuel injection system and the exhaust gas purification system. The exact definitions may vary depending on the vehicle manufacturer, but the code usually refers to a malfunction of the lambda sensor bank 1 sensor 1.

What causes can this error code have?

• Defective lambda sensor (bank 1, sensor 1)
• Damaged cable connections to the lambda sensor
• Dirty or clogged fuel injectors
• Defective mass air flow sensor (MAF sensor)
• Leaks in the intake system
• Faulty fuel pump or low fuel pressure
• Defective fuel pressure regulating valve
• Dirty air filter
• Loose or corroded plug connections
• Defective engine control unit (ECU)

What symptoms occur in parallel with fault code P1138?

• Engine performance: Reduced engine performance and poor acceleration
• Fuel consumption: Increased fuel consumption due to unoptimized mixture composition
• Idle problems: Unsteady idling or engine stalls when idling
• Starting behavior: Difficulty starting, especially when the engine is cold
• Exhaust odor: Unusual smell from the exhaust or black smoke
• Jerking: Engine jerks while driving or during acceleration

How to fix the problem of error code P1138?

• The repair requires a systematic diagnosis to identify the exact cause.
• Replace lambda sensor bank 1 sensor 1 if measured values are out of specification
• Check the wiring harness to the lambda sensor for damage and repair or replace defective cables
• Clean fuel injectors professionally or replace if heavily soiled
• Clean mass air flow sensor or replace with new MAF sensor if defective
• Check intake system for leaks and replace defective hoses or seals
• Measure fuel pressure and replace fuel pump or filter if pressure is too low
• Check air filter and replace with new filter if dirty
• Clean all electrical connections and check for tightness