Home AC, furnace & heat pump troubleshooter
Answer one question at a time. You start with the checks that are safe to do yourself, and stop at the point where a technician is needed, knowing what the repair should cost.
When a home AC runs but is not cooling, the cause is most often a clogged air filter, a failed capacitor on the outdoor unit, or low refrigerant from a leak. Check the filter and the thermostat first: they cost nothing to fix.
Choose what is happening to start.
AC not blowing cold air (running but not cooling)
Work through these 6 checks in order; they are safe for homeowners. If none fixes it, the likely causes and repair costs are below. Run it step by step →
- Check the thermostat settings. Mode on COOL, fan on AUTO (not ON), and the set temperature at least 3°F below the room. With the fan on ON, the blower keeps pushing air even when the AC is not cooling.
- Pull the air filter and look at it. If you cannot see light through it, replace it. A clogged filter starves the coil of air, which cuts cooling and can freeze the coil.
- Look at the outdoor unit while it runs. The top fan should spin and you should hear the compressor hum. Clear leaves, grass and anything within 2 feet. If the fan is still but the unit hums, turn the system off: that is usually a failed capacitor.
- Check both breakers. Central AC has one breaker for the indoor unit and one for the outdoor unit. Reset a tripped one once. If it trips again, leave it off and call a technician.
- Look for ice on the copper lines. Ice on the large copper line or the indoor coil means the system froze. Switch cooling OFF and the fan to ON for 2–4 hours to thaw it before running it again.
- Measure the temperature split. After 15 minutes of running, the air from a supply vent should be about 15–20°F cooler than the air going into the return. Much less than that points to refrigerant or coil problems.
| Likely cause | Repair cost |
|---|---|
| Dirty air filter The most common cause, and a $10 fix. | DIY |
| Failed capacitor Outdoor unit hums but the fan does not spin. | $100–$250 |
| Dirty condenser coil The outdoor unit cannot shed heat on hot days. | $100–$400 |
| Low refrigerant (leak) Small temperature split, ice on lines, hissing. | $200–$1,500 |
| Bad contactor Outdoor unit will not start at all. | $200–$450 |
| Failing compressor Fan runs, compressor does not; breaker trips. | $600–$2,500 |
AC not turning on
Work through these 4 checks in order; they are safe for homeowners. If none fixes it, the likely causes and repair costs are below. Run it step by step →
- Check the thermostat screen. A blank screen usually means dead batteries or a tripped breaker on the air handler. Replace the batteries first.
- Check the breakers and the outdoor disconnect. Reset a tripped breaker once. Also check the pull-out disconnect box next to the outdoor unit and the service switch near the indoor unit (it looks like a light switch).
- Look at the condensate drain pan. Many systems have a float switch that shuts the AC off when the drain backs up. Water in the pan means the drain line is clogged: clearing it often brings the AC back.
- Make sure the furnace or air handler door is closed. A safety switch behind the access panel cuts power when the panel is loose.
| Likely cause | Repair cost |
|---|---|
| Thermostat batteries or settings Blank or unresponsive thermostat. | DIY |
| Tripped breaker or blown fuse One reset is fine; repeated trips mean an electrical fault. | $75–$300 |
| Clogged condensate drain (float switch) Water in the pan, AC dead. | $75–$250 |
| Failed thermostat Fresh batteries, still no response. | $140–$350 |
| Control board Power is present but nothing starts. | $150–$700 |
AC leaking water inside the house
Work through these 4 checks in order; they are safe for homeowners. If none fixes it, the likely causes and repair costs are below. Run it step by step →
- Turn the AC off and protect the area. Turning it off stops more condensate from forming. Put towels or a pan down if water reaches a ceiling or floor.
- Clear the condensate drain line. Find the PVC pipe where it exits outside and pull the clog out with a wet/dry vacuum for 1–2 minutes. Then pour a cup of white vinegar into the access tee near the indoor unit.
- Check the filter. A clogged filter can freeze the coil, and the melting ice overflows the pan.
- Check the condensate pump, if you have one. A small box with a tube near the unit. Pour water in; it should start pumping. If it does not, it has failed.
| Likely cause | Repair cost |
|---|---|
| Clogged condensate drain line Algae and sludge build up every season. Most common cause. | $75–$250 |
| Frozen coil melting Dirty filter or low refrigerant. | DIY |
| Failed condensate pump Pump does not run when filled. | $100–$450 |
| Rusted or cracked drain pan Common on units older than 10–15 years. | $200–$600 |
AC frozen: ice on the lines or coil
Work through these 4 checks in order; they are safe for homeowners. If none fixes it, the likely causes and repair costs are below. Run it step by step →
- Turn cooling OFF, fan ON. Let it thaw for 2–4 hours. Never chip the ice off: you can puncture the coil.
- Replace the air filter. Restricted airflow is the number-one cause of a frozen coil.
- Open vents and clear the returns. Closing too many supply vents or blocking the return grille with furniture lowers airflow.
- Run it again and watch for 30 minutes. If ice comes back with a clean filter and open vents, the cause is refrigerant or the blower. That needs a technician.
| Likely cause | Repair cost |
|---|---|
| Dirty filter / blocked airflow Fix and the ice does not return. | DIY |
| Low refrigerant (leak) Ice returns even with good airflow. | $200–$1,500 |
| Weak blower motor Low airflow at every vent. | $300–$900 |
| Dirty evaporator coil Years without cleaning. | $100–$400 |
AC outside unit not running
Work through these 3 checks in order; they are safe for homeowners. If none fixes it, the likely causes and repair costs are below. Run it step by step →
- Confirm the thermostat is calling for cooling. COOL mode, set below room temperature. Wait 5 minutes: many thermostats delay the outdoor unit to protect the compressor.
- Check the outdoor breaker and disconnect. Reset once if tripped. Repeated trips mean an electrical or compressor fault.
- Listen at the unit. A hum with no fan movement usually means a dead capacitor. Turn power off. Capacitors hold a dangerous charge even when off: do not touch it.
| Likely cause | Repair cost |
|---|---|
| Failed capacitor Hum, no spin. The most common summer failure. | $100–$250 |
| Bad contactor Silent unit with power at the disconnect. | $200–$450 |
| Condenser fan motor Compressor runs, fan does not. | $200–$700 |
| Tripped breaker / fuse Disconnect fuses can blow. | $75–$300 |
| Compressor failure Trips the breaker on every start. | $600–$2,500 |
Thermostat blinking "Cool On"
Work through these 4 checks in order; they are safe for homeowners. If none fixes it, the likely causes and repair costs are below. Run it step by step →
- Wait five minutes. A flashing "Cool On" usually means the thermostat is running its compressor-protection delay. After a power blip or a settings change it waits up to about 5 minutes before starting the AC. If it starts cooling after that, nothing is wrong.
- Replace the thermostat batteries. Weak batteries can keep the thermostat from sending a steady call for cooling, even when the screen still lights up.
- Check the AC breaker and outdoor disconnect. If the outdoor unit has no power, the thermostat keeps calling for cooling that never arrives. Reset a tripped breaker once.
- Check the drain pan and the furnace door. A float switch tripped by a clogged drain, or a loose access panel, opens a safety circuit and blocks cooling while the thermostat keeps waiting.
| Likely cause | Repair cost |
|---|---|
| Normal compressor delay Stops flashing within about 5 minutes. | DIY |
| Weak thermostat batteries Flashing returns, screen dims. | DIY |
| Tripped breaker or fuse Outdoor unit silent. | $75–$300 |
| Clogged drain (float switch) Water in the drain pan. | $75–$250 |
| Thermostat or wiring fault Keeps flashing for hours. | $140–$350 |
| Control board Power is fine but the call never reaches the unit. | $150–$700 |
Furnace not turning on (no heat)
Work through these 5 checks in order; they are safe for homeowners. If none fixes it, the likely causes and repair costs are below. Run it step by step →
- Check the thermostat. HEAT mode, set a few degrees above the room temperature, fresh batteries.
- Check the furnace power switch and breaker. There is usually a light-style switch on or near the furnace. Make sure it is ON and the breaker has not tripped.
- Replace the filter. A clogged filter overheats the furnace and trips the high-limit safety, which shuts the burners down.
- Check the gas supply. The valve handle on the gas pipe should be parallel to the pipe. If your gas stove works, gas service is on.
- Read the status light. Most modern furnaces blink an error code through a small window in the lower panel. Match the blink pattern to the chart on the inside of the furnace door, and give that code to the technician.
| Likely cause | Repair cost |
|---|---|
| Thermostat or settings Batteries, wrong mode, or a dead thermostat. | $140–$350 |
| Failed ignitor Inducer runs, burners never light. | $100–$250 |
| Dirty flame sensor Burners light, then shut off within seconds. | $80–$250 |
| High-limit switch tripping Overheating, usually from a clogged filter. | $100–$375 |
| Draft inducer motor No startup hum, or a loud grinding motor. | $400–$1,100 |
| Control board No status light, or erratic behavior. | $300–$650 |
Furnace blowing cold air
Work through these 4 checks in order; they are safe for homeowners. If none fixes it, the likely causes and repair costs are below. Run it step by step →
- Check the fan setting. If the thermostat fan is on ON, the blower runs between heating cycles and moves unheated air. Set it to AUTO.
- Give it two minutes. It is normal for the first 30–90 seconds of air to be cool while the heat exchanger warms up.
- Watch the burners through the window. If they light and go out after a few seconds, the flame sensor is the usual suspect.
- Replace the filter. An overheating furnace shuts off its burners but keeps the blower running, blowing cool air.
| Likely cause | Repair cost |
|---|---|
| Fan set to ON Fixed at the thermostat. | DIY |
| Dirty flame sensor Short burner flame-outs. | $80–$250 |
| Overheating / limit switch Clogged filter or blocked vents. | $100–$375 |
| Ignitor Burners never light. | $100–$250 |
| Gas valve Ignitor glows but no gas flows. | $200–$800 |
Furnace short cycling (turns on and off)
Work through these 3 checks in order; they are safe for homeowners. If none fixes it, the likely causes and repair costs are below. Run it step by step →
- Replace the filter. Overheating trips the limit switch, the furnace cools, restarts, and repeats.
- Check the thermostat location. A thermostat near a supply vent, a lamp or a sunny window reaches its set point too fast.
- Open supply vents and clear returns. Restricted airflow causes overheating.
| Likely cause | Repair cost |
|---|---|
| Clogged filter / overheating The most common cause. | DIY |
| Dirty flame sensor Ignites, runs seconds, stops. | $80–$250 |
| Thermostat Bad location or failing sensor. | $140–$350 |
| Oversized furnace Heats the house in minutes, then stops. Address at replacement. | — |
Heat pump not heating
Work through these 4 checks in order; they are safe for homeowners. If none fixes it, the likely causes and repair costs are below. Run it step by step →
- Check the thermostat mode. HEAT, not COOL. "EM HEAT" or "AUX" means backup electric strips are running, which works but costs much more.
- Replace the filter. Heat pumps deliver gentler, lower-temperature heat, so airflow matters even more than with a furnace.
- Clear the outdoor unit. Remove snow, leaves and ice around and on top of it. Keep 2 feet clear.
- Check both breakers. One feeds the outdoor unit, one the indoor air handler and its backup heat strips.
| Likely cause | Repair cost |
|---|---|
| Thermostat mode or settings Wrong mode or failing thermostat. | $140–$350 |
| Failed capacitor Outdoor unit hums but does not run. | $100–$250 |
| Low refrigerant (leak) Runs constantly, weak heat. | $200–$1,500 |
| Defrost / control board Outdoor coil stays iced over. | $150–$700 |
| Compressor Outdoor unit fan runs, no heat. | $600–$2,500 |
Heat pump frozen: ice on the outdoor unit
Work through these 4 checks in order; they are safe for homeowners. If none fixes it, the likely causes and repair costs are below. Run it step by step →
- Know what is normal. A light white frost in cold, damp weather is normal. The unit melts it every 30–90 minutes in a defrost cycle (you may see steam).
- Check for water dripping onto it. A leaking gutter dripping on the unit builds thick ice that defrost cannot clear.
- Pour warm (not hot) water to clear it. With the system off, rinse the ice away. Never chip or scrape the fins.
- Watch the next cycle. If a solid block of ice comes back within hours, the defrost system has failed.
| Likely cause | Repair cost |
|---|---|
| Defrost control / board Never enters defrost. | $150–$700 |
| Low refrigerant Coil runs too cold and ices up fast. | $200–$1,500 |
| Outdoor fan motor Fan stopped, coil ices. | $200–$700 |
| Water dripping on unit Fix the gutter. | DIY |
Heat pump blowing cold air
Work through these 3 checks in order; they are safe for homeowners. If none fixes it, the likely causes and repair costs are below. Run it step by step →
- Check whether it is defrosting. During defrost the system briefly runs in cooling mode for a few minutes. Cool air for 2–10 minutes in winter is normal.
- Hold your hand at a vent for a minute. Heat pump air feels lukewarm (about 90–100°F) compared with a furnace. That is normal too.
- Confirm HEAT mode. If the thermostat was swapped recently, the O/B wire may be configured backwards, so the system cools when it should heat.
| Likely cause | Repair cost |
|---|---|
| Normal defrost or lukewarm air No repair needed. | DIY |
| Thermostat configuration Cools in heat mode after a thermostat change. | $140–$350 |
| Low refrigerant Air stays cool for long periods. | $200–$1,500 |
| Control board Stuck in defrost or wrong mode. | $150–$700 |
When to stop and call a technician
- A breaker trips a second time after one reset.
- You see ice return after a full thaw with a clean filter.
- You smell burning, gas, or hear banging at furnace startup.
- The fix involves refrigerant, gas valves, or the capacitor (it holds a dangerous charge).
A diagnostic visit costs $75–$200. Tell the technician which checks you already did; it saves time on the clock. See repair prices by part →
Questions homeowners ask
Why would my AC be running but not cooling?
The usual causes, in order of likelihood: a clogged air filter, the thermostat fan set to ON, a dirty outdoor coil, a failed capacitor (outdoor fan not spinning), or low refrigerant from a leak.
Should I turn off my AC if it is not cooling?
Yes. Running a system that is not cooling can freeze the coil or overheat the compressor, turning a small repair into a big one. Switch cooling off and leave the fan on if you see ice.
How do I reset my central AC?
Turn the thermostat off, switch off the AC breaker for 30 seconds to a minute, switch it back on, then set the thermostat to COOL and wait 5 minutes. Some outdoor units also have a red reset button near the bottom of the cabinet. Reset only once; if it fails again, there is a fault to fix.
Why did my AC suddenly stop blowing cold air?
A sudden stop on a hot day is very often a failed capacitor or contactor at the outdoor unit: inexpensive parts ($100–$450) that fail under heat. A tripped breaker or a full condensate float switch can do the same.
Why is my thermostat blinking "Cool On"?
Most thermostats flash "Cool On" while they wait out a compressor-protection delay of up to about 5 minutes after a power interruption or a setting change. That is normal. If it keeps flashing much longer, check the batteries, the AC breaker and the condensate drain float switch.
Why is my AC not turning on?
Start with the thermostat (mode, set point, batteries), then the two breakers and the outdoor disconnect, then the condensate drain: a clogged drain trips a float switch that shuts the AC off. If all of those check out, a failed contactor, capacitor or control board is the usual cause.
Why is my AC leaking water inside?
Almost always a clogged condensate drain line. Other causes are a frozen coil melting (dirty filter or low refrigerant), a failed condensate pump or a rusted drain pan. Turn the system off and clear the drain line first.
Why is my furnace blowing cold air?
Check that the thermostat fan is set to AUTO, not ON. If it is, the furnace may be overheating from a clogged filter, or a dirty flame sensor is shutting the burners off seconds after they light.
Is ice on my heat pump normal?
A light frost in winter is normal and melts in the next defrost cycle. A thick block of ice that does not clear within a few hours is not; it points to a defrost control, fan, or refrigerant problem.
Estimates for planning only. Prices vary by region, home, and contractor. Gas, refrigerant, and high-voltage work must be done by a licensed technician.