Quick Answer
If your thermostat screen is blank, replace the batteries first—this solves about 50% of cases. If there's no battery compartment (hardwired thermostat), check your HVAC circuit breaker. Other common causes include incorrect settings, a tripped safety switch, loose wiring, or a blown fuse on the furnace control board. Most thermostat issues are DIY-fixable or cost $200-$800 for replacement.
Your thermostat is the control center for your entire HVAC system. When it stops working—blank screen, unresponsive buttons, or simply not turning on your heating or cooling—nothing else works either. The good news: thermostat problems are some of the easiest HVAC issues to diagnose, and roughly half are solved by replacing batteries.
In this guide, we'll walk through every common thermostat problem, starting with the simplest fixes. Whether your screen is blank, the display works but the system won't respond, or the temperature readings seem wrong, we'll help you identify the cause and decide whether you can fix it yourself or need a professional.
Thermostat Screen Is Blank
A blank thermostat screen means no power is reaching the device. Work through these causes in order:
1. Dead Batteries
Many thermostats run on AA or AAA batteries, even if they're also hardwired to the HVAC system. When batteries die, the screen goes blank and the thermostat loses its connection to your heating and cooling equipment. Some thermostats display a low battery warning before dying completely—if yours went blank suddenly, batteries are the most likely cause.
Replace Batteries Annually
2. Tripped Circuit Breaker
Hardwired thermostats (no batteries) get power from the HVAC system's transformer. If the HVAC breaker trips, the thermostat loses power. Check your electrical panel for a tripped breaker labeled "HVAC," "Furnace," "Air Handler," or "AC." Flip it fully OFF, wait 30 seconds, then flip it back ON.
3. Blown Fuse on Furnace Control Board
Your furnace or air handler has a small 3-5 amp fuse on its control board that protects the thermostat circuit. If this fuse blows (often from a wiring short or a stuck contactor), the thermostat loses power even though the breaker is fine. This requires opening the furnace panel to check—if you're comfortable doing that, look for a small glass or blade fuse on the control board. Otherwise, call a professional.
Blown Fuses Have a Cause
Thermostat Has Power but System Won't Respond
4. Incorrect Settings
This sounds obvious, but it's surprisingly common—especially during seasonal transitions:
Thermostat Settings Check
- Mode is set to HEAT (winter) or COOL (summer), not OFF or FAN ONLY
- Temperature is set above room temp (heat) or below room temp (cool)
- Fan is set to AUTO (not ON)
- Schedule/program isn't overriding your manual setting
- Hold or vacation mode isn't active
5. Compressor Protection Delay
Modern thermostats have a built-in compressor protection delay—typically 5 minutes—after the system shuts off. This prevents the compressor from short cycling, which can cause damage. If you just changed settings or the system just shut off, wait 5-10 minutes before concluding it's not working. Some thermostats display "Wait" or a timer during this delay.
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Causes That Need Professional Repair
6. Loose or Corroded Wiring
The thermostat connects to your HVAC system via low-voltage wires (typically 24V). Over time, wire connections can loosen, corrode, or break—especially in older homes or after wall work. Symptoms include intermittent operation, the system running only heating or only cooling, or complete unresponsiveness.
A technician can check wire continuity, tighten connections, and identify any damaged wiring. This is a relatively quick and inexpensive repair in most cases.
7. Faulty Thermostat (End of Life)
Thermostats don't last forever. Most last 10-15 years before internal components start failing. Signs of a dying thermostat include temperature readings that are consistently off by several degrees, delayed response to setting changes, random program resets, or a screen that works intermittently.
Replacement cost: $200-$800 installed, depending on the model. Smart thermostats (Nest, Ecobee) cost more but offer energy savings of 10-15% through learning algorithms and scheduling.
8. Transformer Failure
The HVAC transformer converts your home's 120V power to the 24V that runs the thermostat and control circuits. If it fails, the thermostat gets no power regardless of batteries or breakers. This is diagnosed by measuring voltage at the thermostat wires—a technician can test this in minutes.
Transformer replacement is typically $150-$300 including parts and labor.
Thermostat Works but Temperature Seems Wrong
If your thermostat displays a temperature that doesn't match how your home feels, common causes include:
- Thermostat in direct sunlight—reads higher than actual room temp
- Thermostat near a supply vent—gets hit with conditioned air directly
- Thermostat on an exterior wall—influenced by outdoor temperatures
- Thermostat near a kitchen or heat-producing appliance
- Sensor drift—older thermostats lose calibration over time
Test accuracy by placing a separate thermometer next to the thermostat for 15 minutes. If they differ by more than 2-3°F, your thermostat sensor may need calibration or the thermostat may need relocation.
Thermostat Repair & Replacement Costs (2026)
Thermostat Fix Costs
| Fix | Typical Cost | DIY? |
|---|---|---|
| Replace Batteries | $3-$8 | Yes |
| Reset Circuit Breaker | Free | Yes |
| Correct Settings | Free | Yes |
| Replace Blown Fuse | $5-$15 | DIY if comfortable |
| Wire Repair/Reconnection | $100-$200 | No |
| Transformer Replacement | $150-$300 | No |
| Basic Thermostat Replacement | $200-$400 | No |
| Smart Thermostat Installation | $300-$800 | No |
Diagnostic fee at Kodiak: $89, waived when you proceed with repairs.
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Should You Repair or Replace Your Thermostat?
Repair vs. Replace Decision Guide
| Feature | Repair | Replace |
|---|---|---|
| Thermostat Age | Under 8 years | Over 10-15 years |
| Issue Type | Wiring or power issue | Internal component failure |
| Temperature Accuracy | Reads correctly | Consistently off by 3°F+ |
| Features | Happy with current features | Want smart/programmable |
| System Compatibility | Works with current system | Getting new HVAC system |
Smart Thermostats Save Money
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Written by
Kodiak HVAC Team
HVAC professional at Kodiak Heating & Cooling.