Thermostat Not Working, Heat or Cool: Where to Start

By Ray Bennett · Updated 2 October 2026 · 7 min read · Checked against the sources at the end

Thermostat Not Working, Heat or Cool: Where to Start

Short answer

Four things go wrong and only two of them are the thermostat's own fault. A blank screen is power. On but nothing running is usually the mode, the setpoint or the five minute wait. Wrong times is a hold or a schedule. And there is one test that settles it: join R to one call wire and see whether the system obeys.

A thermostat not working is four different complaints wearing one name, and two of them are not the thermostat at all. A screen that is dead, a screen that works while nothing happens, a system that runs at the wrong times, and a room that never matches the number.

There is also one test that settles the argument in five minutes, and it is at the bottom of this page.

Before you start

What to carry: a small flat screwdriver, fresh batteries and a short piece of wire. Fifteen minutes.

Gas or electric? Where the thermostat gets its power differs. On a gas furnace there is a switch on the side of the cabinet and a breaker behind it. On an electric air handler it is the breaker alone. Either way that is the 24 volt supply the thermostat runs on, so a dead thermostat is often a system that has lost power rather than a thermostat that has failed.

Which half has stopped? Heat working and cooling not, or the other way round, is a very different fault from neither working. Note that before anything else.

Before any wire is touched, and one pair that must never meet. Switch the system off at the furnace or air handler and at its breaker before the thermostat comes off the wall. Twenty four volts will not hurt you, but two wires touching blows a small fuse on the control board and leaves the whole system dead, which looks far worse than what you started with.

Never join R to C. Those two are the transformer’s own pair, so bridging them is a dead short. Every other test on this page joins R to a call wire, which is exactly what the thermostat does inside itself.

And tape the cable to the wall or wrap it round a pencil before you pull anything. A cable that slips back inside the wall turns fifteen minutes into a hole in the plaster.

Which of the four is yours?

Four complaints, and only the first two are ever the thermostat's own doing.

Check 1

The screen is blank

Pull the thermostat gently off its wall plate. A battery compartment behind it means it can run on batteries, so put two fresh alkaline cells in, the right way round. Rechargeable cells sit at a lower voltage and cause a dim screen and dead buttons. Which size each model takes: thermostat batteries.

No compartment, and a wire on the C terminal, means it runs off the system. Then a blank screen means no 24 volts arriving, and there are three usual reasons: the front panel of the furnace not clicked properly shut, the switch on its cabinet, or the small blade fuse on the control board.

Also push the thermostat firmly back onto its plate until it clicks. They look seated when they are not.

For two common makes there is more detail on the Honeywell page and the Nest one.

Check 2

The screen works but nothing runs

The mode. Heat, Cool, Auto or Off, and Off is easy to leave on after a mild week.

The setpoint. Push it a good five degrees past the room, so there is a real call rather than a borderline one.

The fan switch. On ON the blower runs day and night pushing room temperature air about, which feels like a system that is not working.

The five minute wait. After any interruption the thermostat holds the outdoor unit off for up to five minutes to protect the compressor. People reset, rewire and reboot during that wait and then credit whatever they did last.

Heat works and cooling does not, or the other way round

This one is worth its own section because it points somewhere specific.

One side working proves the thermostat has power and proves the system has power. What is missing is the one wire, or the one setting, for the side that does nothing.

The system type. In the installer settings the thermostat has to know what it is driving. Set to furnace when it is a heat pump, or the changeover set to O when it should be B, gives you exactly this.

The Rh and Rc link. If the thermostat has both and there is only one red wire, the little jumper has to be in place or one side never gets power.

The wire itself. W for heat, Y for cooling, G for the fan. One loose strand at either end takes out one side and leaves the rest working: what each letter and colour does.

The test that settles it

Join R to one call wire at a time. What the system does next tells you who is at fault.

Check 3

Join R to one wire at a time

Switch the system off at the furnace or air handler and at the breaker. Take the thermostat off its plate so the bare wire ends are accessible.

With the power still off, join the R wire to one other wire with a short piece of wire or a paper clip. R to G for the fan. R to W for heat. R to Y, and G as well, for cooling.

Then switch the power back on and watch. Whatever you joined should happen, because you have just done by hand what the thermostat does with a switch.

If the system obeys, the thermostat is the fault. It is not sending the signal it should, and the fix is a new thermostat.

If nothing happens, the thermostat is innocent. The fault is in the furnace, the air handler or the outdoor unit, and this page is not where to look.

Power off again before you take the jumper out, and never join R to C. That is the short that blows the fuse on the board and leaves the whole system dead.

It runs, but not when you want it to

Hold. A display saying Hold or Permanent Hold means the schedule is switched off and it will sit at one temperature forever.

The schedule itself. A house that goes cold at ten at night is usually doing what somebody programmed years ago.

Eco, or Away. Smart thermostats drop the temperature when they decide the house is empty, and they get that wrong in predictable ways.

Recovery, or early start. The thermostat starting well before the scheduled time on purpose, so the house is at temperature when the schedule says.

It reads the wrong temperature

Look at what is near it before blaming it. Sunlight through a window, a lamp, a television, a supply vent blowing at it, a draught coming up through the hole in the wall behind it, or a kitchen doorway all shift the reading.

Stuffing a scrap of insulation into the wall opening behind the thermostat is free and stops that draught.

Most thermostats also have a temperature offset in the settings, and that is the right tool once the placement is ruled out rather than before.

Questions people ask

I set it to Cool and the outdoor unit never starts.

Give it the five minutes first. After that, do the R to Y test above, because it separates a thermostat that is not calling from an outdoor unit that is not answering: which fan has stopped.

The screen is on but the buttons do nothing.

Try fresh batteries first, even without a low battery warning. Some models also have a lock in the menu that somebody switched on.

It worked until the power cut.

The five minute delay first. Sensi’s advice after a surge is to wait a few minutes after your other electronics come back on, then reset the thermostat. A wired thermostat that still stays blank points at the breaker and the fuse on the board.

Can I fit a new thermostat myself?

Most people can. Photograph the wires before anything comes off, work letter to letter rather than colour to colour, and tape the cable to the wall so it cannot drop inside.

Do I need a smart one?

No. A basic programmable thermostat controls a system just as well. The smart ones add remote control and reports, and many of them need a C wire that plenty of houses do not have.

The room never gets to the number I set.

That is usually the system rather than the thermostat, especially in very cold or very hot weather. Start at the machine instead: the furnace checks.

Sources: Honeywell RTH2300 operating manual 69-2326EF (compressor protection, Hold, blank screen) and FocusPRO 6000 user guide 69-1921EFS (blank display checks, Adaptive Intelligent Recovery); Sensi (Copeland) "Why isn't my heating turning on?" and "Why isn't my cooling turning on?"; Carrier two stage furnace troubleshooting guide Form 58-3T (jumpering R to W, G and Y); Goodman *M9S80 installation instructions (thermostat location)