The Secondary Air Injection System (SAI) is composed of an air pump that is driven by a belt or electric pump. This system also includes pressure sensors and check and switching valves. All of them are controlled by the PCM (Powertrain Control Module).
For the system pressure control, the PCM relies on input signals from one or more of the pressure sensors, which report that value. If the system pressure drops below the level established by the manufacturer, the pump will be activated. Then, it will open the appropriate valves to allow compressed ambient air to be pumped into the exhaust manifold.
The Diagnostic code P0418 OBDII is set when the PCM detects a malfunction of the secondary air injection pump relay “A” circuit, which is interpreted as a fault in the UPS System.