When an air conditioner won’t come on, check power, thermostat, filter, and the drain safety switch before calling a technician.
Your cooler sits silent while the room warms up. The good news: many no-start issues come from simple things you can confirm in minutes. This guide gives clear, safe checks you can do right away, why each step matters, and when to stop and book service.
AC Won’t Turn On: Quick Checks
Start with fast wins. Walk through the list below in order. Each step rules out a common cause without tools or guesswork.
| Symptom | Likely Cause | First Step |
|---|---|---|
| Thermostat lit but no cooling | Mode/temp not set, delay timer active | Set to COOL, fan AUTO, target at least 5°F lower; wait 5 minutes |
| Blank thermostat | Dead batteries or no low-voltage power | Replace batteries; check furnace/air-handler switch |
| Indoor blower runs, outdoor unit silent | Tripped breaker or service switch off | Reset breaker once; confirm outdoor disconnect is ON |
| Clicking, then shuts off | Safety switch tripped by a clogged drain | Empty pan, clear drain, and reset the float switch |
| Runs briefly, then quits | Dirty filter causing freeze or high pressure | Install a clean filter with correct airflow arrow |
| Nothing anywhere | Main breaker or blown fuse | Check panel; restore power if safe |
Confirm Power To Every Stop In The Chain
Cooling needs two power paths: high-voltage to the outdoor unit and low-voltage control power from the furnace or air handler. If either path is down, the system will sit idle.
Check The Main Panel And Outdoor Disconnect
Open your breaker panel and look for the double-pole breaker labeled for cooling. If it sits in the middle position, flip it fully OFF, then back ON once. Don’t repeat resets—recurring trips point to a real fault that needs a pro. Outside, open the metal disconnect box near the condenser and make sure the pull-out or switch is seated and ON.
Verify The Furnace Or Air-Handler Switch
There’s often a wall switch near the indoor unit that looks like a light switch. If it’s OFF, your thermostat may stay dark and the outdoor unit won’t receive the start signal. Flip it ON and wait a few minutes for the system to wake up.
Set The Thermostat So It Can Call For Cooling
Thermostat settings block many calls. Set mode to COOL, fan to AUTO, and the setpoint at least 5°F below room temp. Most models build in a short delay to protect the compressor after power changes, so give it a full five minutes before judging the result. If the screen is blank, replace the batteries or reseat the thermostat on its base to restore contact pins.
Smart models can lose Wi-Fi or schedules after outages. If a schedule parked the setpoint too high, use a HOLD or temporary override. If nothing helps, a gentle reset from the app or the unit’s menu can clear a stuck state without wiping your wiring profile.
Give The System Air: Filters And Vents
A clogged filter starves airflow, which can trigger safeties or ice the coil and stop cooling. Slide out the filter at the return grille or the furnace slot and replace it if you can’t see light through it. Size and MERV rating matter; too restrictive media can cause trouble in older blowers.
ENERGY STAR advises checking filters monthly and changing them on a regular cadence to protect both comfort and equipment. A clean filter helps the unit start and keeps energy use in check. See the official maintenance checklist for homeowner tasks and contractor items.
Clear The Condensate Drain And Reset The Safety
Most central systems include a float switch that stops cooling when the drain pan fills. This prevents water damage. If you see standing water near the indoor unit, the safety likely did its job.
How To Restore Drain Flow
Turn off power at the furnace switch. Remove the PVC cap on the drain line and vacuum the line from the outside termination, or pour a small amount of warm water followed by a bit of white vinegar to break slime. Empty the pan with a wet/dry vac. Once water clears, the float will drop and the system can call for cooling again.
Tip
A few drops of mild household bleach or vinegar in the drain line during the season can slow algae growth. Avoid harsh chemicals that can harm fittings or finishes.
Inspect The Outdoor Unit For Simple Blocks
Grass clippings and lint can pack the coil and stop the fan from moving heat. With power off at the disconnect, gently rinse debris from the fins from the inside out. Keep shrubs at least two feet away so the fan can breathe.
The U.S. Department of Energy notes that dirty coils and neglected maintenance reduce performance and can lead to failures. Their air conditioner maintenance page outlines coil and fin care, filter guidance, and why tune-ups matter.
Listen For The Contactors And Safeties
When the thermostat calls for cooling, you should hear a click at the outdoor unit as the contactor pulls in. If the click happens but the fan and compressor stay still, stop and call for service—live power is present. If you hear nothing, control power may be missing, a float may be tripped, or low-voltage wiring could be open.
Try A Safe Reset Sequence
Power cycles can clear short lockouts from brownouts or quick breaker flips. Set the thermostat to OFF. Turn the outdoor disconnect OFF and the furnace switch OFF. Wait two minutes. Turn the furnace switch ON, then the outdoor disconnect ON. After five minutes, set the thermostat to COOL and try again once.
When A Heat Pump Acts Like It’s Dead
Heat pumps share many parts with central cooling. Two differences catch people off guard: the defrost control board and outdoor temperature limits. In rare cases, a failed defrost sensor or board blocks operation. In extreme heat, some systems trip on high pressure until airflow and coil cleanliness are corrected. The checks above still apply before you call.
Common Repair Paths And What They Cost
Some fixes are DIY-friendly; others need a licensed technician with gauges and training. Use these ranges to plan. Local rates vary.
| Fix | Typical Range (USD) | DIY Or Pro |
|---|---|---|
| New thermostat batteries | $3–$12 | DIY |
| Clean/replace filter | $8–$40 | DIY |
| Unclog drain/float reset | $0–$250 | DIY or Pro |
| Clear outdoor coil | $0–$200 | DIY or Pro |
| Replace contactor/capacitor | $150–$450 | Pro |
| Low-voltage wiring repair | $120–$350 | Pro |
| Blower or fan motor | $400–$900 | Pro |
| Compressor or refrigerant leak | $900+ | Pro |
Prevent Breakdowns With Simple Habits
Small routines keep cooling ready when you need it. Set calendar reminders to check the filter. Keep the outdoor coil clean. Clear the condensate line at the start of the season. Book a spring tune-up so a pro can check refrigerant charge, electrical connections, and safety controls before heat arrives. Hold steady setpoints with a programmable thermostat and let schedules do the work. Keep shrubs trimmed to maintain free airflow outside.
When To Call For Service Right Away
Stop DIY and book a licensed technician if you see a smoking or swelling capacitor, scorched wires, a breaker that trips again after a single reset, ice covering the refrigerant line, or loud humming from the outdoor cabinet. Those signs point to conditions that can damage parts or pose a safety risk.
Fast Checklist You Can Save
Use this one-page routine the next time cooling stays off. It traces the same order a technician will follow, but in plain steps:
- Set thermostat to COOL, AUTO, and a lower setpoint; wait five minutes for the delay.
- Swap thermostat batteries if the screen is blank.
- Check the furnace/air-handler switch; make sure the blower door is closed.
- Reset the cooling breaker once. Confirm the outdoor disconnect is ON.
- Install a clean air filter and open supply and return vents.
- Clear the condensate drain and empty any pan; confirm the float drops.
- Inspect the outdoor coil; remove debris and give it breathing room.
- Try the safe reset sequence before a second test call for cooling.
- If power is present but motors won’t run, stop and call a pro.
Why These Steps Work
No-start calls often trace back to three buckets: no power, no call for cooling, or a tripped safety. The checklist covers all three in the simplest order. Power checks restore energy to both the indoor and outdoor sides. Thermostat settings and batteries let the control send its call. Filter and drain tasks remove the two most common roadblocks the system detects on its own. By the time you reach outdoor coil cleaning and reset steps, many homes are already cooling again.
Keep Energy Use In Check While You Troubleshoot
If your home warms up, close blinds on sunny windows and run ceiling fans to boost comfort. Once cooling returns, aim for steady setpoints instead of big swings. The energy programs above share ranges that balance comfort and cost, and a programmable thermostat can hold those settings for you.
Seasonal Startup Checklist
Before the first hot spell, run a drill: replace the filter, clear leaves from the outdoor pad, flush the drain, and test cooling. Label the breaker and the furnace switch so anyone at home can find them. Keep a photo of your filter size in your phone, and stash spare batteries near the thermostat. Prep turns a swelter-day scramble into a five-minute fix.