Fault code P0042 – a step-by-step guide to diagnosing and fixing it

Error code P0042 is displayed and the check engine light is on - what does this mean?

The OBD/EOBD system detects a fault in the heating circuit of the lambda sensor downstream of the catalytic converter (bank 1, sensor 2). This DTC P0042 indicates that the voltage in the heating circuit of the downstream lambda sensor is too low, which can impair exhaust gas purification and engine control.

What causes can this error code have?

• Defective lambda sensor heater
• Damaged cable to the lambda sensor
• Corroded or loose plug connections
• Defective fuse in the heating circuit
• Faulty engine control unit (ECU)
• Short circuit in the electrical wiring
• Defective lambda sensor after the catalytic converter

What symptoms occur in parallel with fault code P0042?

• Increased fuel consumption: The vehicle consumes more fuel than usual because the mixture control is not functioning optimally.
• Poor exhaust values: Increased pollutant values are measured during the emissions test.
• Unsteady idling: The engine runs unevenly or jerks when idling.
• Loss of power: Noticeable reduction in engine power, especially at higher engine speeds.
• Check engine light: The yellow warning light in the dashboard lights up continuously.

How can the problem with error code P0042 be rectified?

• Repairing trouble code P0042 requires a systematic diagnosis of the lambda sensor heater.
• Replace the lambda sensor completely after the catalytic converter - Replace the defective sensor including heater with a new original part or equivalent spare parts
• Check and repair wiring and plug connections - Check all cables for damage, corrosion or loose contacts and replace defective cables
• Replace the fuse in the lambda sensor heating circuit - Replace the relevant fuse and check whether the fault code disappears
• Diagnose the engine control unit and replace if necessary - Have the ECU professionally tested and replace with a programmed control unit if defective
• Carry out a complete electrical check of the heating circuit - Measure the voltage and resistance in the entire system and rectify any electrical problems found