How to Replace the Blue & Green Wires in a Low Pressure Switch for an Air Conditioner

Disconnect the electrical supply to the HVAC system. If the system cannot be physically separated from the mains, but only switched off at a main control panel, switch the circuit breakers off then tape them in the "off" position. This prevents another person from accidentally switching the power on while you are working.

Determine where both the blue and green wires are routed when they leave the low-pressure switch. Physically follow the wires, and at the same time measure them. Make a note of the wires’ lengths. Use the color-coded gauge reader behind the jaws of a pair of wire cutters to determine the gauge of the wires, and note this down, also.

Remove both ends of one of the two wires; which one -- either the blue or the green -- is not important. If the wire is routed loose, simply remove and replace it with the equivalent color. If the wire is routed through conduit, attach one end of the new wire to one end of the old wire, then extract it from the conduit; this automatically draws the new wire in as the old wire is drawn out.

Attach both ends of the replacement wire to the appropriate terminals, either using the prefitted contacts or installing new contacts per the manufacturer’s instructions.

Repeat the process with the second new wire. Reinstate the power supply. Test operation by using the thermostat to initiate the system.