DTC P0418: Air Pump Relay Control Circuit: Description

The Powertrain Control Module (PCM) controls the relay by grounding the control circuit via an internal driver switch. The primary function of the driver is to supply 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 is expects, the fault line status changes causing the DTC to set. The relay controls the high current flow to the AIR injection pump. This allows the PCM driver to only have to control the relatively low current that the relay uses.