P0418
Secondary Air Injection System Control A Circuit
If your code reader dashboard displays the generic DTC error code P0418, your vehicle is currently experiencing an active failure related to "Secondary Air Injection System Control A Circuit". Proper system troubleshooting is required to safely clear this warning.
Driver's Summary
Code P0418 means your vehicle detected a problem with the secondary air injection system control a circuit system. Typical symptoms include check engine light, failed emissions. You can continue normal driving, but schedule an inspection at your next service appointment to clear this code properly.
Symptoms
Check engine light, failed emissions
Common Causes
- Failed secondary air injection pump relay
- Defective air injection pump
- Open circuit in pump wiring
- Blown main pump fuse
How to Fix
- 1 Test and replace pump relay
- 2 Replace secondary air pump
- 3 Repair wiring harness
- 4 Replace maxi-fuse
Technical Explanation
Code P0418 is confirmed when the ECM's diagnostic algorithm detects a parameter deviation that persists across a defined number of consecutive drive cycles. EGR flow is verified by monitoring changes in MAP sensor readings before and after valve actuation; correct EGR flow produces a predictable pressure drop in the intake manifold that the ECM can measure with precision. The fault remains stored in memory even after the MIL is cleared; it becomes a confirmed DTC after failing two consecutive drive cycles, and the PCM logs a freeze frame record of the engine's exact operating state at the moment of detection.
Is It Safe to Drive?
Immediate safety risk is low with P0418 active. The primary concern is regulatory — this fault will cause a failed emissions test — and the secondary risk is that the small root cause (failed secondary air injection pump relay) becomes a larger problem if ignored for months.
Mechanic's Pro Tip
Before replacing any component on P0418, spend 5 minutes inspecting the wiring harness and connector first — corrosion, chafed insulation, and backed-out pins cause the majority of these faults and cost nothing to fix. Use a multimeter to measure voltage drop across the connector pins under load; anything above 0.1V indicates excessive resistance that will cause intermittent failures even after replacing the sensor.
Relay/Fuse: $20 - $50; Air pump: $150 - $400