Control Wiring Troubleshooting – 24 Volt Open

Trying to troubleshoot control wiring is sometimes confusing and frustrating. But, if you have a step-by-step procedure it becomes easy and accurate. Below you find a procedure for troubleshooting a 24 volt open in an HVAC system.

Symptoms – With 24 volts present one or more control circuits (relay, contactor, blower motor relay, defrost control board, reversing valve)is not operational.

Troubleshooting Steps

Step 1 – Inspection

● Remove thermostat from the sub-base

○ Inspect for broken, loose, corroded, or disconnected wires.

● Remove air handler cover

○ Inspect for broken, loose, corroded, or disconnected wires.

● Remove condenser cover

○ Inspect for broken, loose, corroded, or disconnected wires.

○ Note – weed eater, lawn mower, and dog chewing damage is common where the control wiring exits the chase at the ground level at the condensing unit.

○ Note – rusted and corroded wire nut connections are common on previous repair or spliced control wiring.

Step 2 – With the thermostat still removed from the sub-base

● Disconnect and separate all control wiring from condenser.

● Disconnect and separate all control wiring at the air handler

● At the air handler, wire nut together all control wiring to the thermostat.

● At the air handler, wire nut together all control wiring to the condenser.

Step 3 – From Thermostat Sub-base to Air Handler

● Set your meter to the audible ohms (beep) setting.

● Alligator clip one lead to the common wire coming from the thermostat.

● One-by-one check:

○ Common to red.

○ Common to orange.

○ Common to yellow.

○ Common to green.

○ Common to any other wire that may be connected (2-stage system)

● Remove alligator clip from common and connect to the red wire.

● One-by-one check:

○ Red to orange.

○ Red to yellow.

○ Red to green.

○ Red to any other wire that may be connected (2-stage system)

● Remove alligator clip from red and connect to the orange wire.

● One-by-one check:

○ Orange to yellow.

○ Orange to green.

○ Orange to any other wire that may be connected (2-stage system)

● Remove alligator clip from orange and connect to the yellow wire.

● One-by-one check:

○ Yellow to green.

○ Yellow to any other wire that may be connected (2-stage system)

● Remove alligator clip from yellow and connect to the green wire.

● One-by-one check:

○ Green to any other wire that may be connected (2-stage system)

Step 4 – From the Condenser to the Air Handler

● Make the exact same check as listed in Step 3 with the wires going out to the condensing unit.

Step 5 – Diagnosis

● If your meter DID NOT beep in Step 4, the open or broken wire is between the air handler and the thermostat.

● If your meter DID NOT beep in Step 5, the open or broken wire is between the air

handler and the condenser.

● If your meter beeped on all wires, the field installed control wiring is good.

● The open or broken wire can be determined by noting on which wires your meter

did not beep.

○ Locate open or broken wire – repair or replace wiring.

○ Locate open or broken wire – repair or replace wiring.

○ Check for factory wiring opens in the air handler and condensing unit.!

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Ron Walker
Ron Walker

After retiring from the U.S. Marines and achieving his B.S. degree, Ron Walker entered the HVAC field. He has been an HVAC technician, service manager, and business owner. Working as a service manager, he spent many years training HVAC technicians to be more technically competent and really understand their trade. His passion for teaching and helping others resulted in the creation of HVAC Training Solutions, LLC.