AC Running Constantly / Won't Shut Off
Air conditioner runs continuously without cycling off, even when target temperature is reached.
Quick Overview
DIY thermostat fix: 5 minutes | Filter/coil cleaning: 1 hour | Thermostat replacement: 1-2 hours | System replacement: 1-2 days
Summer
Common Symptoms
You might be experiencing this problem if you notice any of these signs:
- AC never shuts off even when target temp reached
- Unit runs 24 hours straight
- High electricity bills from constant operation
- Thermostat shows correct temperature but AC still running
- Fan never stops blowing
- No cycling on and off like normal
- Outdoor unit runs non-stop
- House eventually cools but unit keeps running
- Thermostat fan set to ON instead of AUTO
Step-by-Step Diagnosis
Follow these steps to identify the exact cause of your problem:
- Check thermostat fan setting - should be AUTO not ON
- Verify actual room temperature matches thermostat reading
- Set thermostat 5°F higher than current temp to test shutoff
- Check air filter condition
- Inspect outdoor unit for dirt or debris
- Verify thermostat location not affected by heat sources
- Test if compressor and fan both run constantly
- Check for proper insulation and air sealing in home
- Measure temperature drop at supply vents
- Note outdoor temperature during issue
- Check if issue started after specific event
- Verify correct thermostat wiring
Most Common Causes (Ranked by Likelihood)
Thermostat fan set to ON instead of AUTO
HIGH LIKELIHOODDirty air filter restricting airflow
HIGH LIKELIHOODUndersized AC unit for home
MEDIUM LIKELIHOODThermostat calibration problem
MEDIUM LIKELIHOODLow refrigerant reducing cooling capacity
MEDIUM LIKELIHOODDirty condenser coil reducing efficiency
HIGH LIKELIHOODThermostat stuck or contactor welded closed
LOW LIKELIHOODExtreme heat exceeding system capacity
MEDIUM LIKELIHOODPoor insulation or major air leaks
MEDIUM LIKELIHOODDIY Solutions
What You'll Need:
- Replacement air filter
- Garden hose
- Coil cleaner
- Thermometer
- Caulk gun (for sealing)
Time Required: 45-90 minutes
Step-by-Step Instructions:
- Check thermostat and switch fan from ON to AUTO
- Replace air filter if dirty or clogged
- Clean outdoor condenser coil thoroughly
- Remove all debris from around outdoor unit
- Test thermostat by setting temp 5°F above room temp
- Verify AC shuts off within 2-3 minutes
- Check thermostat location - move if near heat sources
- Seal obvious air leaks around windows and doors
- Close blinds during peak sun hours
- Verify all registers are open and unblocked
- If extreme heat, set thermostat 5°F higher temporarily
- Consider running ceiling fans to reduce AC load
- Check temperature differential at vents (15-20°F is normal)
- If no improvement, schedule professional load calculation
⚠️ Safety Warnings
- Turn off power before cleaning outdoor unit
- Don't force thermostat switches - may indicate wiring problem
- Never wire thermostat yourself if unsure - fire hazard
- Extreme heat above 100°F may prevent reaching very low temps
When to Call a Professional
Don't attempt DIY repairs if you experience any of these situations:
- Thermostat set to AUTO but unit still won't shut off
- AC clean with new filter but runs non-stop
- Contactor stays engaged even when thermostat off
- Thermostat shows much lower temp than actual room temp
- Wiring appears damaged or corroded
- System is clearly undersized for home
- Temperature drop at vents less than 15°F
- Issue started suddenly without weather change
- Smart thermostat shows constant call for cooling
- Multiple thermostats in home showing different readings
Prevention Tips
Stop this problem before it starts with these preventive measures:
- Always use AUTO fan setting on thermostat
- Size AC properly with Manual J load calculation
- Replace filters monthly during peak cooling season
- Schedule annual AC maintenance
- Improve home insulation and air sealing
- Install programmable or smart thermostat
- Keep thermostat away from heat sources and sunlight
- Add attic insulation to R-38 or higher
- Seal ductwork in unconditioned spaces
- Use window treatments to block solar heat gain
Frequently Asked Questions
Is it bad if my AC runs constantly?
Yes, continuous operation without cycling indicates a problem. It causes excessive wear, higher energy bills ($100+ extra per month), and means your home isn't being properly cooled. Normal ACs should cycle on and off, running about 15-20 minutes per cycle in moderate weather.
Why does my AC run all day when it's hot outside?
If outdoor temps exceed 100°F, some runtime increase is normal, but the AC should still cycle off periodically. If it truly never stops, the unit is undersized, has low refrigerant, dirty coils, or your home has insulation/sealing problems. Most systems should maintain 20-25°F below outdoor temp.
Should I turn off my AC if it won't stop running?
Check the thermostat fan setting first - if ON, switch to AUTO. If that doesn't fix it and the unit runs constantly for 24+ hours, turn it off to prevent damage and high bills. Schedule immediate service to diagnose thermostat, contactor, or sizing issues.
Related Problems
AC Short Cycling
AC turns on and off repeatedly in short bursts instead of running proper cooling cycles.
High Energy Bills
Unusually high electric or gas bills compared to previous months or years, excessive HVAC energy usage.
AC Not Cooling / Not Blowing Cold Air
Air conditioner runs but produces warm or room-temperature air instead of cooling properly.