Thermostat Not Working? What to Check and When to Call
QUICK ANSWER
Thermostat not working? Start with five checks: the batteries, the mode (COOL or HEAT), the set temperature, any schedule or HOLD setting, and the breaker for your AC or furnace. Then wait five minutes, because most systems pause before they restart. If the screen is still blank or the system still won’t respond, the cause could be a power problem, a clogged AC drain, a wiring fault or a failed thermostat. Castle Air Conditioning & Heating offers same-day thermostat repair across southwest Houston. We answer 24/7 at (713) 667-7606.
On this page
Thermostat not working? Five checks to make first
A lot of the thermostat calls we get end up being one of these. Each one takes a minute or two.
| Check | What to do | Why it matters |
|---|---|---|
| Batteries | Pop off the cover or pull the thermostat off its base. Put in fresh AA or AAA batteries. | Weak batteries can leave the screen on but stop the thermostat from sending a signal. |
| Mode | Set it to COOL in summer and HEAT in winter. Don’t use AUTO while you test. | In the wrong mode, the system either won’t start or will start the wrong thing. |
| Set temperature | Set it at least three degrees below the room for cooling, or above it for heat. | If the setting is close to the room temperature, the thermostat thinks the job is done. |
| Schedule or HOLD | Look for a schedule or a HOLD setting that’s overriding what you just picked. | A summer schedule can quietly change the temperature back every few hours. |
| Breaker | Find the breakers for the AC, furnace or air handler. Switch them all the way off, then back on. If one trips again, leave it off and call us. | If the indoor unit has no power, the thermostat goes dead too. |
After you change anything, wait five minutes. Most thermostats and outdoor units have a built-in delay that protects the compressor. A flashing snowflake or a “wait” message during that time is normal. It isn’t broken.
Thermostat screen blank? Here’s why
A blank screen is the most common way a thermostat not working shows up. It almost always means the thermostat has lost power. Here’s where that power goes missing, from most likely to least.
Power problems you can check yourself
Dead batteries. Many thermostats run on batteries, even ones that are also wired in. Replace them once a year, even if the screen looks fine.
A tripped breaker. Most thermostats get their power from the indoor unit. If its breaker trips, the screen goes blank.
Houston’s most common cause: a full drain pan
A full drain pan in the attic. This one is very common in Houston. Most homes here have the air handler in the attic. It has a safety switch that shuts the whole system off when the drain backs up, so water doesn’t pour through your ceiling. On many systems that switch also kills power to the thermostat. If your screen went blank on a hot, sticky day, check the drain pan first. Our page on a clogged AC drain line explains what to look for.
Problems at the furnace or air handler
A loose furnace door. The panel on the front of the furnace or air handler has a safety switch behind it. If it isn’t pushed all the way in, the system and the thermostat stay off. This often happens right after someone changes the filter.
A blown fuse on the control board. Inside the furnace or air handler is a small fuse that protects the thermostat wiring. A short in a wire can blow it. It’s a small part, but it’s inside a live unit, so leave it to a technician.
What your thermostat is doing, and what it usually means
A thermostat not working doesn’t always look the same. Find what you’re seeing in the left column.
| What you’re seeing | What it usually means | Thermostat or system? |
|---|---|---|
| Screen is blank | Dead batteries, a tripped breaker, a full drain pan or a blown fuse. | Usually the system |
| Says “cooling” but the air is warm | Usually the outdoor unit isn’t running or the coil is frozen. Sometimes it’s the wiring. | Usually the system |
| AC won’t shut off | Fan set to ON instead of AUTO, a set temperature that’s too low, or a stuck part in the outdoor unit. | Either |
| Turns on and off every few minutes | The thermostat sits in sun or near a vent, or the system is low on refrigerant. | Either |
| Room temperature reads wrong | Direct sun, a lamp, a draft or dust inside the cover throws off the reading. | Thermostat |
| Smart thermostat keeps going offline | Not enough power at the wall, usually a missing C-wire, or a Wi-Fi problem. | Thermostat |
| Heat pump blows cold air in HEAT | After a new thermostat, a wrong setting. Otherwise a valve in the heat pump. | Either |
If the third column says “system,” a new thermostat won’t fix it. That’s worth knowing before you buy one.
When the thermostat isn’t the problem
From inside the house, a thermostat not working and an AC not working can look exactly the same. But your thermostat is only the messenger. It tells the AC or furnace to start. If the screen is on, the setting is right and it says it’s cooling or heating, the thermostat is at least trying to start the system. The problem could still be in the thermostat, the wiring or the equipment.
Here’s a simple test for summer. Set it to COOL, five degrees below the room, and wait five minutes. Then go outside. Is the fan on top of the outdoor unit spinning? If not, the problem could be a failed capacitor or contactor, two small parts that wear out in Houston heat. It could also be a wiring problem, or the signal may not be reaching the unit at all. A technician can test which one it is. If the outdoor unit is running but the air inside is warm, look for ice on the copper line by the outdoor unit. Either way, it’s time to call a technician.
When the house is hot and getting hotter, our emergency and same-day AC repair page explains what to do while you wait. In winter, see heater not working instead.
New thermostat not working? Check the wiring and settings
Lots of people put in their own thermostat, and lots of those calls come to us a day later. A new thermostat not working is almost always one of these four things.
Wiring problems
No C-wire. Many smart thermostats need steady power from a wire called the C-wire, or from an approved power adapter. Plenty of older Houston homes don’t have a C-wire at the thermostat. Without it, a smart thermostat may reboot, lose Wi-Fi or drain its battery. A technician can add one or install an adapter at the air handler.
Wires on the wrong terminals. Before you take an old thermostat off, take a photo of the wires and the letters they connect to. Most mistakes happen when that photo doesn’t exist.
Setup problems
The wrong system type. During setup, the thermostat asks whether you have a gas furnace, electric heat or a heat pump. Pick the wrong one and it sends the wrong signals.
The heat pump setting. Heat pumps have a valve that switches between heating and cooling. Your thermostat has a setting, usually called O or B, that controls it. Most brands use O. A few, like Rheem and Ruud, use B. If it’s set wrong, your heat pump heats when you want cooling and cools when you want heat. Not sure which kind of system you have? Our guide to heat pumps vs. AC in Texas explains the difference.
The thermostat itself runs on low voltage. The equipment it controls doesn’t. If the fix means opening the air handler or furnace, that’s work for a licensed technician. Texas requires a state license for HVAC work, and Castle holds TACLA license 00051749C.
Why thermostat problems show up in spring and fall
In Houston, spring and fall are hard on thermostats. Look at what the temperature does in March, April, October and November.
In the shaded months, a Houston house can need heat at 7 a.m. and cooling by 3 p.m. People switch between HEAT and COOL, or set the thermostat to AUTO for the first time. A heater that sat unused since February gets called on. And thermostat batteries that are a year old finally give out. So those are the months to watch for a thermostat not working.
Most of the demand here is for cooling
It also helps to see how unevenly the work is split across the year.
How hard a Houston thermostat works, month by month
Blue is heating demand. Red is cooling demand. The longer the bar, the more your thermostat has to call for heat or air. Measured in degree days: how far each day’s average temperature sits above or below 65°F, added up for the month. NOAA 1991 to 2020 normals for Bush Intercontinental.
Add up the degree days and about 72 percent of Houston’s yearly heating and cooling demand is for cooling. That measures what the weather asks for, not how many hours your system runs. Still, most of what your thermostat asks for here is air conditioning.
Because most of that demand is cooling, summer is the hardest season on your system. A maintenance visit each spring and fall includes checking the thermostat, its wiring and the safety switches that can shut it down.
Thermostat repair or replacement: what we’ll recommend
Thermostat repair usually means fixing what’s around the thermostat, not the thermostat itself. When we come out for a thermostat not working, our technician will:
- Make sure the thermostat is getting power from the system.
- Look over each wire at the thermostat and at the air handler or furnace.
- Test the control board fuse, the drain safety switch and the door switch.
- Start the system right at the unit to see if the thermostat is the problem.
- Confirm the thermostat is set up for the system you actually have.
If the thermostat is fine and the problem is a wire or a switch, we fix that and leave your thermostat alone. If the thermostat itself has failed, we’ll tell you, and we can install a new one the same day. If the bigger problem is a system near the end of its life, our AC installation and replacement page explains what’s involved.
Thinking about a smart thermostat, or one that controls humidity? That’s a good time to talk about it. ENERGY STAR explains what to look for in a certified smart thermostat. Our post on humidity control thermostats covers which ones make sense in a Houston house.
Castle has been doing this work in southwest Houston since 1975. Maintenance plan members also get ten percent off repairs all year.
Thermostat repair across southwest Houston
We handle thermostat repair and AC and heating calls across Houston and Fort Bend County from our base in Fulshear. If your thermostat is not working and you’re in one of these areas, we can usually be out the same day.
For everything else we fix, see AC service and repair and heater service and repair.
Frequently asked questions
Why is my thermostat not working all of a sudden?
Most of the time it has lost power. Check the batteries, the breaker for your AC or furnace, and the panel on the front of the furnace or air handler. In Houston, a full drain pan in the attic is another common cause, because its safety switch can shut off the thermostat too.
Why is my thermostat screen blank?
A blank screen almost always means no power. The usual causes are dead batteries, a tripped breaker, a drain safety switch that shut the system off, a loose furnace door or a blown fuse on the control board. If new batteries and a reset breaker don’t bring it back, call a technician.
Why does my thermostat say cooling but the AC is blowing warm air?
The thermostat is calling for cooling, so the problem is usually in the AC, though it can be the wiring. Check whether the fan on the outdoor unit is spinning. If it isn’t, a capacitor or contactor has often failed. If it is spinning, the coil may be frozen or the system may be low on refrigerant. Both need a technician.
How do I reset my thermostat?
It depends on the model, so check your manual first. Many digital thermostats have a reset option in the settings menu. Others reset when they lose power for a minute. You can do that by taking out the batteries, or on a wired thermostat, by switching off the breaker for the furnace or air handler. A reset may erase your schedule. If a breaker trips again after you reset it, leave it off and call a technician.
Repair and replacement
Can a thermostat be repaired, or does it have to be replaced?
The thermostat itself is rarely repaired. Most thermostat repair work is on what’s around it: the wiring, the power supply, a fuse or a safety switch. If those are fine and the thermostat has failed, it gets replaced. We’ll check everything before recommending a new one.
Why won’t my thermostat reach the set temperature?
Often the thermostat isn’t the problem. A dirty filter, closed or blocked vents, low refrigerant or a system that’s too small for the house can all keep it from getting there. Check the filter and make sure the vents are open. If the system runs nonstop and still can’t reach the setting, it’s time for a technician.
Why is my new thermostat not working?
Usually one of four things: a missing C-wire, a wire on the wrong terminal, the wrong system type picked during setup, or the wrong O or B setting on a heat pump. A photo of the old wiring before it came off makes this much easier to fix.
How often should I change my thermostat batteries?
Once a year. A good time is when you switch from cooling to heating in the fall. Many thermostats warn you when batteries are low, but not all of them do.
Getting someone out
Is a thermostat not working an emergency?
In a Houston summer, it can be. If the house is getting hot and there’s a baby, an older adult or someone with a health condition at home, call us right away. Our post on how hot is too hot inside explains when heat becomes a health risk. On a mild day, it can usually wait for a same-day visit.
How fast can you come out for thermostat repair?
Same day in most cases, if you call in the morning. We answer 24 hours a day at (713) 667-7606. After-hours rates are different from daytime rates, and we confirm that with you before anyone heads out.
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Humidity Control Thermostats: What Works in a Houston House — if you’re replacing your thermostat anyway, here’s how to pick one that also helps with Houston humidity.
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