DTC P0418: Air Pump Relay Control Circuit: Description

WARNING: This page is about a different car, the 2002 GMC Savana and 2002 Chevrolet Chevy Express. However, it is still accessible from the selected car via links, so may be relevant.

The Powertrain Control Module (PCM) controls the relay by grounding the control circuit via an internal switch called a driver. The primary function of the driver is to supply the ground for the controlled component. Each driver has a fault line which the PCM monitors. When the PCM commands a component ON, the voltage of the control circuit should be low, near 0 volts. When the PCM commands the control circuit to a component OFF, the voltage potential of the circuit should be high, near the battery voltage. If the fault detection circuit senses a voltage other than what the PCM expects, the fault line status changes causing the Diagnostic Trouble Code (DTC) to set. The relay controls the high current flow to the secondary Air Injection (AIR) pump and the AIR solenoid. This allows the PCM driver to only have to control the relatively low current used by the relay.