Fault code P0412 – a step-by-step guide to diagnosing and fixing it

Error code P0412 is displayed and the check engine light is on - what does this mean?

The OBD/EOBD system has detected a fault in the secondary air system circuit "A". This DTC P0412 indicates a problem with the secondary air system switching valve or its electrical control. The error code can occur with different manufacturers, although the exact designation may vary slightly.

What causes can this error code have?

• Defective secondary air system switching valve
• Damaged wiring to the switching valve
• Corroded or loose plug connections
• Defective secondary air pump
• Blocked secondary air lines
• Faulty engine control unit (ECM/PCM)
• Blown fuse in the secondary air system circuit
• Defective secondary air bypass valve

What symptoms occur in parallel with fault code P0412?

• Check engine light: Permanent display of the check engine light
• Increased emission values: Deteriorated emission values during the main inspection
• Unsteady idling: Engine runs unevenly at idle speed
• Cold start problems: Difficulty starting when the engine is cold
• Reduced engine power: Noticeable loss of power while driving
• Increased fuel consumption: Noticeable increase in fuel consumption

How to fix the problem of error code P0412?

• The repair requires a systematic diagnosis of the secondary air system.
• Replacement of the defective secondary air system switching valve after prior functional testing
• Repair or replace the damaged wiring between the ECM and the switching valve
• Cleaning and securing all plug connections in the secondary air system circuit
• Replacing the secondary air pump if a pump defect is detected
• Clean or replace blocked secondary air lines and hoses
• Checking and, if necessary, replacing the associated fuses
• Professional diagnosis and repair of the engine control unit in the event of ECM faults