Test the A/C switch (see page 21-52). Is the A/C switch OK? YES - Replace the heater fan switch. NO - Replace the push switch. Make sure the A/C switch is OFF. Measure the voltage between the No. 9 terminal of under-dash fuse/relay box connector F (12P) and body ground with the under-dash fuse/relay box connectors connected. UNDER-DASH FUSE/RELAY BOX CONNECTOR F (12P) Wire side of female terminals Is there battery voltage? YES - Repair open in the wire between the under-dash fuse/relay box and the A/C pressure switch. NO -Refer to the multiplex control system (see page 22-268). Check for proper A/C system pressure. Is the pressure within specifications? YES - Replace the A/C pressure switch. NO - Repair the A/C pressure problem.