FYI
When you start the engine if the coolant temperature, measured by the coolant temp sensor, is less than 80 deg F, the PCM grounds the Air Pump Relay and the pump operates. In addition, the PCM, power train control module, controls a switchover solenoid (via vacuum) that opens the switchover valves. When the switchover valves are open and the Air Pump is running, atmospheric air is injected into the exhaust manifolds through a port in each cylinder head.
From the Student Reference Book