Vehicles are equipped with an air pump to reduce tailpipe emissions at start-up. This pump is activated through a relay, which is driven to the ground by the Powertrain Control Module (PCM). Also, the PCM grounds the vacuum control solenoid circuit of the air switching valve. Then, the vacuum is applied to both diaphragms of the switching valve, then they open the shut-off valve.
These circuits are enabled simultaneously when the Air Injection System (AIR) operation is required. Once the AIR system is working, the air pump makes it possible that fresh air comes into the exhaust stream, so catalyst operation is accelerated.
On the other hand, if the system is switched off, the shut-off valves will prevent airflow in either direction.
The OBDII DTC P0412 refers to a problem in the secondary air injection system switching valve control solenoid circuit. If the PCM observes an unexpected voltage, this code will be set.