DTC P0418: Air Pump Relay Control Circuit: Description

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

A voltage is directly supplied to the relay coil of the secondary Air Injection (AIR) pump. The Powertrain Control Module (PCM) controls the relay by grounding the control circuit via an internal switch that is 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 PCM receives a different voltage, from the fault detection circuit, than expected, 2 events occur. The fault line status changes, and the DTC sets. The relay controls the high current flow to the AIR pump. This allows the PCM driver to control only the relatively low current that is used by the relay.