P0418 - Secondary Air Injection System Control 'A' Circuit
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System
Powertrain (Engine & Gearbox)
Related Fault Codes
Technical Description
P0418 is a standard OBD-II trouble code indicating an electrical malfunction in the Secondary Air Injection (AIR) system control circuit 'A'. This typically refers to the relay or solenoid that activates the air pump. During cold starts, the ECU activates the pump to inject oxygen-rich air into the exhaust stream, promoting rapid catalyst light-off. If the ECU detects an improper voltage signal (open circuit, short to ground, or short to power) on the driver circuit, P0418 is stored. This system is monitored via the oxygen sensors to verify flow; however, P0418 specifically points to a circuit failure rather than a flow issue.
Possible Causes
Faulty Secondary Air Injection (AIR) pump relay
Wiring harness issues (corrosion, open circuit, or short to ground)
Failed AIR pump motor (internal electrical failure)
Faulty Engine Control Module (ECM/ECU) or blown high-amp fuse VIDA.
Symptoms
Recommended Solution
- Visual Inspection: Check the AIR pump fuse (typically a high-amp fuse). Inspect the electrical connector at the pump for heat damage or corrosion.
- Measurements: Test the AIR pump relay. Resistance across the coil should be 40-80 Ohms. Measure the resistance of the pump motor (0.5-5 Ohms is normal; OL indicates an open circuit).
- Advanced Diagnosis: Use a scan tool to perform an 'Active Test' of the AIR system. Monitor the voltage drop across the relay contacts.
- Component Replacement: Replace the relay (common failure) or the air pump if electrical tests fail.
Mechanic's Tip
Before replacing expensive components, always double-check wiring and connections. 80% of all sensor faults are actually due to corroded contacts or broken wires.
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