What does it mean?
OBD II fault code P0418 is a generic code that is defined as “Secondary air injection pump relay A -circuit malfunction”, and is set when the PCM (Powertrain Control Module) detects a failure or malfunction in the SAI ( S econdary A ir I njection) circuit that is associated with the relay labelled “A”. Note that some applications use more than one relay in the SAI system, and that this code refers specifically to relay “A” and its associated circuit.
Refer to the manual for the application to determine which relay is labelled “A” on SAI systems that use multiple relays. Most modern applications use a dedicated air pump to slightly compress ambient air, before blowing it into the exhaust manifold(s) to aid in oxidizing harmful exhaust emissions.
Typically, SAI systems consist of an air pump that can be either belt driven or electrically operated, pressure sensors, various hoses, one-way check valves, switching valves, relays, fuses, bypass valves, and anti-backfire valves that are all controlled by the PCM. In practice, the PCM monitors the system pressure via inputs from one or more pressure sensors.
When the PCM detects that the SAI system pressure is approaching or falls below a level set by the manufacturer, it will activate the SAI pump (in the case of electrically operated pumps) and open/switch the appropriate valves to allow compressed ambient air to be pumped into the exhaust manifolds. Note however that requirements based on engine temperature, engine speed, as well as vehicle speed have to be met before the PCM will activate the SAI system.
To reduce loads on the electrical system, the PCM feeds power to the SAI pump via a relay (relay “A” in this case) that delivers battery voltage through a fuse or fusible link to the SAI pump motor. The image below shows a simplified schematic drawing of a typical secondary air injection system. Note the position of relay “A” relative to other components in the system.