What it means
General electrical fault in the secondary air injection pump relay A circuit. Without the relay, the air pump cannot be commanded and the system cannot complete its cold-start function.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The AIR control A circuit operates the relay that supplies power to the secondary air pump. The control module monitors this circuit so it can switch the pump on during cold starts. P0418 sets when the module detects an electrical fault in the control A circuit, which can leave the pump unavailable.
Common causes
Most likely first.
- Air pump relay failed
- Wiring or connector fault
Symptoms
- Check engine light on
- Failed emissions inspection
- Possible roughness during cold start
How to diagnose it
- Review freeze-frame data and related AIR codes to see whether the fault occurred when the pump was commanded on.
- Inspect the AIR pump relay A, its socket, and nearby wiring for corrosion, heat damage, loose terminals, or a poor connection.
- Check the relay and its control circuit using the manufacturer's test procedure, and verify the pump power supply and ground.
- Command the AIR pump on with a scan tool, if supported, and check whether the relay switches and power reaches the pump.
- If the relay, wiring, and pump circuit pass, check the control module output and connector for a driver fault.
Common fixes
- Clean and secure a loose or corroded relay A connection.
- Repair damaged wiring or terminals in the control A circuit.
- Replace a failed AIR pump relay A.
- Repair the control module driver circuit after confirming the relay, pump supply, and wiring.
Can I keep driving?
The vehicle is usually driveable, but the secondary air pump may not run during cold starts, increasing emissions and risking an inspection failure. If cold-start roughness appears, limit unnecessary short trips while the relay control A fault is diagnosed.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.