DTC P0418: Air Pump Relay Control Circuit: Description
WARNING: This page is about a different car, the 2001 GMC Savana, 2001 Chevrolet RV Cutaway, 2001 Chevrolet Cutaway, and 2001 Chevrolet Chevy Express. However, it is still accessible from the selected car via links, so may be relevant.
Powertrain Control Module (PCM) controls relay by grounding control circuit via an internal switch called a driver. Primary function of driver is to supply ground for controlled component. Each driver has a fault line which PCM monitors. When PCM commands a component on, voltage of control circuit should be low, about zero volts. When PCM commands control circuit to a component off, voltage potential of circuit should be high, about battery voltage. If fault detection circuit senses a voltage other than what PCM expects, fault line status changes causing DTC to set. Relay controls high current flow to Secondary Air Injection (AIR) pump and AIR solenoid. This allows PCM driver to only have to control relatively current used by relay.