DTC P0412: Air Relay Control Circuit: Description
WARNING: This page is about a different car, the 2002 Pontiac Firebird and 2002 Chevrolet Camaro. However, it is still accessible from the selected car via links, so may be relevant.
The Powertrain Control Module (PCM) controls the solenoid by grounding the control circuit via an internal switch called a driver. The primary function of the driver is to supply ground for the component that is being controlled. 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 is expected, the fault line status changes, and causes the DTC to set.