I think it relates to this:
During a cold start, the secondary air injection system supplies the air to the engine exhaust port so that the CO and HC burns and an increase in the temperature of the three way catalyst is facilitated. The three way catalyst is quickly activated and the emission gas during the cold start is reduced. When all of the following conditions are satisfied, the ECM activates the air pump, opens the air cut solenoid valve and supplies the secondary air to the exhaust port. The secondary air injection system operates for approximately a few tenths of a second depending on the engine coolant temperature and other conditions. When the system is deactivated, the ECM closes the air cut solenoid valve and stops the air pump to prevent the emission gas from blowing back.
< Activating conditions >
When starting the engine
Battery voltage: More than 10 V
Engine coolant temperature at engine start is between 15 - 35 °C (59 - 95° F) and has lowered 45°C (113° F) or more since the latest engine stop.
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