I am Wiring a New Honeywell RTH6580WF Thermostat but there is no Yellow Wire at Thermostat. The old Trane thermostat only had 4 connections and the blue (C wire) was connected to the Y terminal.

thermostat end

The furnace is wired with 5 wires connected (including a yellow) as you would expect.

furnace/ac end

I’ve pulled a bit of the multi wire out of the wall at the thermostat and stripped some covering to see if the yellow wire was cut off, but no luck.

Can I jumper the yellow wire to another color in the thermostat?

  • We're probably going to need a picture of the wiring inside the furnace, There is usually a diagram inside the removable panel.
    – JACK
    Feb 27, 2022 at 18:28
  • Can you trace the furnace and thermostat ends of the wiring to figure out where they join up? Feb 28, 2022 at 2:21

2 Answers 2


That thermostat requires at least five wires to operate (assuming you use heat AND cool). 2 for power, 1 for heat, 1 for AC, 1 for fan. So jumpering the wire won’t help.

Kits exist that can provide “virtual wires” by multiplexing signals through existing wires (such as the Fast-Stat Model 3000), but in my experience, you’re much better off pulling physical wire from the furnace to the thermostat.

Often, the existing thermostat wire can be used to pull the new wire. If the existing wire feels loose in the wall when tugging gently from the wall/ceiling penetration near the furnace (like it will pull out freely), twist-splice the existing and new wires together at the thermostat, wrap thinly and tightly with electrical tape and pull from furnace end.


Wire colors for t-stats (and most other circuits) below 50v are not color regulated by the NEC. There are "industry standards" that match color to letter of terminal, but in this situation the phrase "wire doesn't care about the color of the insulation" is code compliant.

That being said, if anything wire is not terminated to color expected I take the time to make sure I understand circuitry to make sure the is no splice or otherwise damaged wire that may cause peculiar function.

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