Disconnect the lower solenoid vacuum hose from the vacuum manifold and connect the vacuum pump.
Disconnect the upper solenoid vacuum hose from the air control valve A.
Disconnect the 2-pin connector near the solenoid valve.
Create a vacuum.
If the valve does not hold vacuum, it should be replaced.
If the solenoid valve holds vacuum, then connect the positive battery terminal to the Green/Black terminal of the 2-pin connector and the negative battery terminal to the Black terminal.
Next, create a vacuum.
If the solenoid valve holds a vacuum, it should be replaced.
If not, you should disconnect the battery from the 2-pin connector.
Next, reconnect the vacuum hose.
Warm up the engine to operating temperature until the cooling fan comes on.
Measure the voltage between the Green/Black (+) and Black (-) terminals on the main wiring harness while the engine is running at approximately 2500 RPM.
If the voltage alternates between 0 and 12 volts, the vacuum line should be checked for proper connection, cracks, plugs, or a disconnected hose.
If not, then measure the voltage between the Green/Black(+) terminal and ground while the engine is running at approximately 2500 rpm.
If the voltage alternates between 0 and 12 volts, then the break in the Black wire between the solenoid valve and point G4 should be repaired.
If not, then you should check for a break in the Green/Black wire between the electromagnetic valve and terminal 29 of the control unit. If the wire is normal, then you should refer to the diagnostics section of the control module inputs.
Original article on the website HondaBook