Replacing thermostat wire

House Repair Talk

Help Support House Repair Talk:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Active Member
Feb 25, 2018
Reaction score
I have an A/C unit that apparently has a break in the thermostat wire that needs to be replaced. How much would running a new wire to the condenser cost approximately? Is $400 a fair price or is that excessive? Thanks for any feedback.
That would depend where you live both in terms of how long it would take a service person to get to you and also what is the prevailing costs where you live. Then it would be a matter of what was involved to get a wire from point A to point B. There would also be a cost involved with trouble shooting the problem in the first place. Where I live it is going to be 100 bucks to just get the service persons truck to my driveway.
DE103065, I apologize that this is long but I wanted to give you some ideas that may turn this into an easy DIY repair.

Lock off the power to the furnace & outdoor unit before proceeding. If you unhook any wires, label each wire by color so you know what goes where. Photos help, too.

A) Re-Purpose A Wire
Perhaps your cable has a spare wire that you can re-purpose. It'll be taped off or hanging loose at the thermostat. It is not unusual to have an extra wire in the cable. See the bottom of this post for how to find the broken wire in the cable.

B) The Easier Cable To Replace
Often the wire to the condensing unit comes from the furnace instead of the thermostat. Changing that one yourself is quite easy since nothing is in the wall. It most likely follows alongside of the refrigerant lines.

C) Splice & Save
Also note that such breaks are usually outside (animals &/or UV rays), so if you find it you can splice. UY connectors make splicing quick and watertight.

D) Not As Easy, But Do-Able
If the troublesome wire is between the thermostat and downstairs, replacing it is easy if you can see get to the subfloor below the thermostat. If the wire was ever replaced after the house was built, it's not stapled to the studs. You can tie the old the the new and pull while someone guides the wire.
If it's stapled, you can use a fish stick to shove the new wire up through the old subfloor hole.

Hope To Save You Some Money!

To Test Which Wire Is Broken:
1) Lock off power to the furnace and outdoor unit.
2) Label all wires so you know which goes where. Photos help.
3) Unhook both ends of all wires.
4) Choose one to be the "standard". For this example, we will use the Red
5) Connect the standard to one other wire with a baby blue wire nut or alligator clip. Binder clips work, too.
For this example we will use the White
6) Use an ohm meter or test light across the Red & White at the not tied end.
7-A) If you have low ohms or bright test light, both are good.
7-B) If you don't have a light, one is bad. TIE A DIFFERENT wire to the Red.
OK? The white is bad. Not OK? Red may be bad
8) If the Red & White are good, keep swapping which wire is tied to the Red until you find the bad guy.
Last edited: