I'm hoping to replace my thermostat with an Arduino-powered one of my own. The furnace is a Trane XV90 Upflow 2-stage condensing gas-fired one with upgraded inducer motor using Trane Furnace Kit KIT15017. No cooling.

I'm following this guide to understanding thermostat wiring, which recommends:

Measure the voltage between the terminal C and the rest of the terminals. Set your thermostat on the different modes (AC, HEAT…) and check what terminals are energized on each function.

This will give you a pretty good idea of the terminals that you will have to connect to activate the different modes.

But I don't have a C terminal. Here's what I have: enter image description here

So I have two issues:

  • No exposed C wire for measuring with multimeter
  • Accessing the terminals requires removing the thermostat. Won't this turn off my heat, and I won't be able to measure any activity anyway?


So here's the wiring at the furnace: enter image description here

Sanity check passes that the colors/labels align on this end, but it bothers me it doesn't match the wiring diagram from the manual:

enter image description here

For the silver lining, I spy that this end labels "B" as "B/C." Am I on to something?

Second edit: As for how to measure with a multimeter during operation, I postulate I could probe these furnace-side terminals instead of thermostat-side terminals. The system senses the furnace burner door being opened (required to access terminals) with a simple switch, foolable with an inch of duct tape. Now I can probe while the thermostat is plugged in, giving commands, and outputting system status.

Can I get a sanity check that my logic is correct before I proceed? Once I proceed, I can quickly solve the prior issue (whether 'B' is my common, I guess?) by sensing voltage difference between it and the 'R' wire.

  • Don't be surprised when it's not actually 24v "Typical specifications for the mains are ±10%, so the output of the 24 VAC transformer will vary from 21.6VAC to 26.4VAC from this factor alone." - If the t-stat doesn't take batteries, you 100% have a C(B) wire. - Red to blue : anywhere from ~20 to 30+ volts.
    – Mazura
    Commented Nov 13, 2021 at 23:06
  • This unit and t-stat are properly wired. All you should need is the legend for the terminals of the Arduino, which I've no exp with. Keeping in mind your W2 is black (usually brown); everything else is what it should be
    – Mazura
    Commented Nov 13, 2021 at 23:11
  • @Mazura Aside from not matching the docs, which is fine with me, I still have the lingering question about proper measurement with a multimeter + duct tape over that door sensor. I know you're not a fan of multimeters, but I'm far less experienced so thorough testing prior to changes will ease my mind.
    – armani
    Commented Nov 13, 2021 at 23:41
  • I have a magnet that's made for that switch; up to you if you're comfortable working in proximity to 120v. If you don't get the same reading between R and C on the furnace as you do directly across the transformer terminals, then the wire is bad (unlikely). I'd hate to be one of the two random people giving you a sanity check, and I don't gamble, but my money's on B/C is your common.
    – Mazura
    Commented Nov 13, 2021 at 23:55
  • @Mazura I poked around and confirmed your suspicions, leaving me with one final mystery: B/C to R is always energized, W1 and W2 energize when the heat is on... but G never energizes even when the system is clearly blowing warm air through the house.
    – armani
    Commented Nov 14, 2021 at 0:21

1 Answer 1


B is your C

Your thermostat base uses B to denote what other folks denote as a C wire; we can confirm this by looking at how it's wired to the B/C terminal on your control board via the blue wire in your thermostat cable.

And your approach to testing is sound

Your approach to testing the thermostat connections at the furnace-side with the interlock defeated is generally how it is done, yes. Do be careful to remember to remove the duct tape flag before returning the furnace to service, though!

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