Air Injection: Description and Operation

Secondary Air Injection System Description

The Secondary Air Injection System aids in the reduction of hydrocarbon exhaust emissions during a cold start. This occurs when the start-up engine coolant temperature (ECT) is between -12 and +38°C (10-100°F), the intake air temperature (IAT) is greater than -12°C (+10°F) and it has been more than 10 s since the last start. The secondary air injection pump operates 5-60 s after start-up.

The engine control module (ECM) activates the secondary air injection system by simultaneously completing the ground paths for the secondary air injection pump and the secondary air injection shutoff and check (solenoid) valve relay. This action closes the relays' internal contacts. The pump and shutoff and check valve are in turn energized. The pump turns ON and the valve opens.

The secondary air injection pump sends pressurized fresh air into the pipes/hoses, through the open shutoff and check valve and into the exhaust manifold. The extra air accelerates the catalyst operation, helping it to reach operating temperature faster. The secondary air injection pump remains ON for a short period of time after the shutoff and check valve is commanded OFF. When the pump is commanded OFF it will not run or be activated until the next vehicle cold start. When the secondary air injection system is inactive, the shutoff and check valve is closed to prevent air/exhaust flow in either direction.

The ECM monitors the secondary air injection system pressure by tracking the voltage signal which is provided by pressure sensor in the exhaust.

The ECM utilizes a 3 phase diagnostic routine to test the secondary air injection system:

During phase 1, DTCs P0411 and P2430 run and both the secondary air injection pump and the shutoff and check valve is activated. Normal secondary air function occurs. Expected system pressure is 6-14 kPa (0.9-2.0 psi) above BARO.

During phase 2, DTCs P2430 and P2440 run and only the secondary air injection pump is activated. The shutoff and check valve is closed. Pressure sensor performance and shutoff and check valve deactivation are tested. Expected system pressure is 22-30 kPa (3.2-4.4 psi) above BARO.

During phase 3, DTC P2444 runs and neither the secondary air injection pump nor the shutoff and check valve is activated. Secondary air injection pump deactivation is tested. Expected system pressure equals BARO.

The secondary air injection system includes the following components:

* The secondary air injection pump-The electric secondary air injection pump supplies pressurized, filtered air to the secondary air injection shutoff and check valve. The secondary air injection pump is a turbine type pump that is permanently lubricated and requires no periodic maintenance.

Note: The secondary air injection shutoff and check valve and the pressure sensor are serviced as an assembly.

* The secondary air injection shutoff and check valve assembly-The shutoff and check valve assembly have direct current (DC) motor operated valves. When the valve motor is energized by the secondary air injection shutoff and check (solenoid) valve relay, the valve opens, pressurized air from the secondary air injection pump flows through the valve and is directed into the exhaust manifold.

* The secondary air injection pressure sensor-The pressure sensor is integral to the secondary air injection shutoff and check valve assembly. The sensor is a 3-wire sensor that measures the secondary air injection system pressure at the inlet of the shutoff and check valve assembly.

* The secondary air injection pump relay-The relay supplies high current and battery voltage to the secondary air injection pump. The ECM commands the relay ON by supplying a ground to the relay control circuit.

* The secondary air injection shutoff and check (solenoid) valve relay-The relay supplies high current and battery voltage to the secondary air injection shutoff and check valve. The ECM commands the relay ON by supplying a ground to the relay control circuit.

* The pipes and hoses-The secondary air injection pump inlet hose carries filtered air from the engine intake air cleaner to the secondary air injection pump inlet. The secondary air injection pump pipe carries the air from the pump outlet to the shutoff and check valve which in turn feeds the exhaust manifold.

System Fault Detection

The ECM monitors the secondary air injection system for faults during cold start operation. When the system's pressure or relay circuits operations vary too far from the expected values, a DTC will set. Diagnostics detect the following conditions:

* A partially or fully blocked or leaking secondary air injection system

* A malfunctioning secondary air injection pump

* A malfunctioning secondary air injection shutoff and check valve assembly

* A malfunctioning secondary air injection pressure sensor

* A restricted exhaust system

* A malfunctioning secondary air injection pump and secondary air injection valve relay

The following DTCs set when a secondary air injection system fault is detected:

* DTC P0411-A secondary air injection system insufficient airflow fault condition has been detected.

* DTC P0412-A secondary air injection valve relay coil circuit fault condition has been detected.

* DTC P0418-A secondary air injection pump relay coil circuit fault condition has been detected.

* DTC P2430-A secondary air injection pressure sensor signal stuck in range fault condition has been detected.

* DTC P2431-A skewed air injection pressure sensor signal has been detected.

* DTC P2432-A secondary air injection pressure sensor signal voltage below the minimum range of the sensor fault condition has been detected.

* DTC P2433-A secondary air injection pressure sensor signal voltage is above the maximum range of the sensor fault condition has been detected.

* DTC P2440-The shutoff and check valve is stuck open or a system leak between the pump and the valve has been detected.

* DTC P2444-A secondary air injection pump stuck ON fault condition has been detected.