P0418Powertrain
Powertrain · SAE
Secondary Air Injection Relay Control Circuit A Malfunction
Meaning
The ECM has detected a fault in the control circuit of relay A that supplies power to the secondary air injection pump. The relay switches load current to the air pump on ECM command; a fault in this circuit prevents pump operation.
Common causes
- Faulty secondary air pump relay (open winding, oxidised or welded contacts)
- Short circuit or open circuit in the relay control wiring
- Blown fuse in the air pump supply circuit
- Oxidised relay socket or damaged pins
- Faulty ECM driver output for the relay coil
- Missing supply voltage or ground at the relay socket
Symptoms
- MIL light on, DTC P0418 stored
- Secondary air pump does not start during cold start
- Secondary air injection system completely inoperative
- Elevated HC and CO emissions during warm-up phase
- Possible total absence of air pump noise at engine start
Severity
Medium severity: the air pump cannot activate, completely disabling the cold-start emission reduction system; no immediate driving risk but out-of-specification emissions result.
What to do
- Read all DTCs and check for additional codes related to the secondary air system or wiring
- Locate the air pump relay in the fuse box and swap it with an identical type for testing
- Verify correct supply voltage and ground at the relay socket terminals using a multimeter
- Check the ECM control signal to the relay coil (typically voltage pulling to ground when activated)
- Repair defective wiring or replace the relay, clear the code, and perform a cold-start cycle to confirm correct pump activation