Possible causes

– Idle Air Control Valve harness is open or shorted
– Idle Air Control Valve circuit poor electrical connection
– Faulty Idle Air Control Valve

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)
– Engine rough idle

P1508 HONDA Description

The target idle speed is in the Engine Control Module (ECM) memory for various engine conditions (coolant temperature, A/C operation and so on). The ECM keeps the actual idle speed at the target idle speed by switching the Idle Air Control Valve IACV) ON/OFF to control the intake airflow. If the IACV output duty cycle (the length of ON time) is out of a specified range,
a malfunction is detected and a DTC is stored.