DTC P0418: Air Pump Relay Control Circuit: Description
WARNING: This page is about a different car, the 2003 Chevrolet Corvette. However, it is still accessible from the selected car via links, so may be relevant.
The secondary Air Injection (AIR) pump relay controls the high current flow to the AIR pump. This allows the Powertrain Control Module (PCM) driver to control only the low current that is used by the relay coil. Ignition 1 voltage is directly supplied to the AIR pump relay coil. The PCM controls the relay by supplying a ground to the control circuit of the AIR pump relay. When the PCM commands the relay ON, the voltage of the control circuit should be low, near 0 volts. When the PCM commands the relay OFF, the voltage potential of the circuit should be high, near the battery voltage. If the PCM monitor detects a voltage other than expected, DTC P0418 sets.