If you're trying to work out how to tell if your thermostat is bad, rule out the simpler causes first: a wrong setting, a power problem or the furnace itself can all look like a failed thermostat. The thermostat is the part you can see and touch, so it gets blamed first, but a new one won't fix any of those.
Signs your thermostat is bad
A blank or dead screen
Carrier says that when the display goes completely dark and the buttons do nothing, the device "has likely lost power," which "commonly indicates dead batteries or a tripped circuit breaker." Trane adds "loose or damaged thermostat wiring, a tripped circuit breaker, or a dead thermostat." A failed thermostat is on the list, but it comes last.
Readings that don't match the room
Carrier says that when the reading doesn't match how the room feels, "you might be dealing with a miscalibration issue," though sun or a draft hitting the thermostat can throw it off too. Lennox counts mismatched readings as a sign of a failing thermostat, along with unresponsive controls and "no signal being sent to the HVAC system."
You change the setting and nothing happens
The screen shows the new number and the furnace stays quiet. Carrier lists several possible causes: "a wiring issue, thermostat failure, HVAC control board issue or tripped safety switch." Only one of those is the thermostat.
Sometimes the system runs and won't stop. Trane's test: "Try turning the thermostat off. If your air conditioner or furnace continues to run, your thermostat may have bad wiring or need to be recalibrated."
A thermostat that keeps forgetting its settings is annoying, but Trane says that alone doesn't necessarily mean it has failed.
Rule out the settings
- Mode and setpoint. Lennox says to confirm it's on Heat, with the target higher than the current indoor temperature.
- Schedule. "If your thermostat is programmable, review the schedule," says Lennox. A program can be holding a different temperature from the one you think you set.
- Lockout and limits. Some models have a keypad lock, and some let the installer cap the heat setpoint. The Honeywell FocusPRO manual covers both. Your model's manual will say how to unlock it.
- Fan. If air keeps moving but feels cool between cycles, look for a fan set to ON instead of AUTO.
- Em Heat. Heat pump owners should rule out a heat pump left on Em Heat, which runs backup heat only.
Then give it time. Carrier notes that modern thermostats "often have time delays in place to protect equipment from short cycling," and says to "wait at least 10 minutes."
Then batteries and power
If it takes batteries, put in fresh ones. Lennox's advice is simple: "Replace the batteries with fresh ones to rule this out."
Hard-wired thermostats take a little more thought. Honeywell's support page for the RTH6360 runs in order: try it on batteries, and if it still won't come on, you may need a technician. If batteries bring it to life, check that the heating system has power: the breakers, plus the furnace's power switch and door switch. If the system has power and the thermostat still only works on batteries, a technician may be needed.
For the thermostat, Trane says the breaker to look at is "typically the same breaker as your indoor furnace or air handler, not the outdoor AC unit breaker." If it has tripped, Bryant says to "reset it once. If it trips again, leave it off and call a professional." Trane is more cautious: if the breaker has tripped, it says to consult an HVAC professional, because "A problem with the system could have caused the breaker to trip."
The FocusPRO manual's fixes for a blank display also include making sure the heating system's power switch is on and the "furnace door is closed securely." Carrier's FAQ says the same for gas furnaces. A door put back slightly off after a filter change is easy to miss.
Smart thermostats
Google's Nest help page says "A dark yellow light on your thermostat's display means that the battery charge is very low but it's being charged." And if the display "isn't fully seated on the base, your thermostat won't turn on and can't charge its battery." Press it firmly onto its base before assuming anything worse.
Is it the thermostat or the furnace?
A working screen and a silent furnace can point past the thermostat. Carrier's FAQ: "If the unit is showing heat/cool on with no air flowing, then the issue could be the HVAC unit." Lennox says to watch for "error codes or unusual sounds that may point to system issues rather than thermostat problems."
A dead smart thermostat can be a furnace problem in disguise. Google explains that "a clogged air filter can cause your system to overheat and automatically shut off. When this happens, your Nest thermostat can't draw power from your system to charge its battery." The thermostat is the casualty there, not the cause, so check the filter.
If the thermostat is clearly calling for heat and nothing answers, move on to a furnace that won't turn on.
Wrong readings: placement, dust and calibration
A thermostat only measures the air right around it, so sunlight, drafts and nearby heat skew it. Our short-cycling guide covers where a thermostat shouldn't be mounted.
Lennox says dust buildup can interfere with the sensors, and Carrier recommends "gently wiping the display with a dry microfiber cloth." Nothing wet, no spray.
To check accuracy, set a separate thermometer beside the thermostat, leave it a while, and compare. If they disagree consistently, suspect calibration. Carrier's FAQ: "Sometimes, this recalibration setting is available to the end user. If not, it may require a qualified contractor."
What not to do
- Don't jump or bridge the wires to make the furnace run.
- Don't put a meter on anything or test anything live.
- Don't pull wires off the base.
- Don't open up the furnace to chase the fault.
Lennox says "electrical troubleshooting should be handled by a licensed technician." Carrier says to "immediately stop troubleshooting and call a professional if you notice red flags like exposed or frayed wiring, persistent error codes that return after a reset, or any burning smells."
When replacing it makes sense
If the settings are right, the batteries are new, the furnace has power and the screen is still blank, the thermostat may simply have failed. Honeywell's advice for its Series 2000 thermostats is two sentences: "Check and replace the batteries. If the unit does not power up, replace it."
Age counts too. Carrier says, "Most home thermostats reliably last between 10 to 15 years."
The replacement has to suit your equipment. Trane warns that "not all thermostats are heat pump thermostats," and calls replacing one "a job best left to an HVAC technician due to the wiring and testing needed."
Checks before you call
- Heat mode, target above room temperature
- Schedule, keypad lock and setpoint limits
- Fresh batteries, if it takes them
- Breaker reset no more than once
- Furnace power switch on, door closed and secure
- Filter looked at, especially with a smart thermostat
- What the screen shows: blank, a warning light, an error message, or normal
- What the furnace does when you turn the heat up
Still not sure it's the thermostat?
Call 587-834-3668 and tell us what the thermostat screen shows and what the furnace does when you turn the heat up. We price the repair before any work starts. Call and we'll tell you honestly when we can get there. We dispatch from both Calgary and Airdrie. The furnace repair page explains a repair visit. In Airdrie, use furnace repair in Airdrie. If no room in the house is warming up, read about no heat issues.
