Lennox AC Not Cooling: Causes, Fixes & Repair Costs
Get this fixed by a local pro
Free, no-obligation quotes from licensed HVAC technicians in your area. Compare up to 3 estimates before anyone visits.
Lennox air conditioner not cooling: what it means
“Not cooling” is a symptom, not a Lennox fault code. Your system runs, or tries to run, but the house does not get cool. Unlike a lockout code, this symptom does not reset on its own.
Some Lennox systems also show diagnostic information that can narrow down the cause:
- Communicating systems (iComfort or S40 thermostat): alert codes may appear on the thermostat, such as high-pressure or low-pressure trips, or a compressor that will not start.
- Variable-capacity models (for example SL25XCV and EL18XCV): the outdoor control board flashes LED codes.
Note the exact code if one appears. Any pressure or compressor lockout shown there is held by the control and needs a technician to diagnose it.
This page covers split central air conditioners. If your outdoor unit is a heat pump, see the related heat pump page instead.
Lennox air conditioner not cooling: common causes, ranked by probability
- Thermostat setting or power issue. The thermostat is in the wrong mode, its batteries are dead, or the setpoint is above room temperature.
- Dirty air filter or blocked airflow. This reduces cooling and can freeze the indoor coil.
- Tripped breaker or outdoor disconnect. The indoor fan blows air, but the outdoor unit is dead.
- Failed run capacitor (dual capacitor). This is the most common electrical failure in the outdoor unit. The compressor or fan hums but does not run.
- Failed or pitted contactor. The outdoor unit does not engage even though the thermostat is calling for cooling.
- Frozen evaporator coil. Caused by poor airflow or low refrigerant. Ice blocks heat transfer.
- Low refrigerant from a leak. Indoor evaporator coil leaks are a well-documented issue across many brands, including some older Lennox coils.
- Dirty outdoor condenser coil or failed condenser fan motor. Causes high head pressure and high-pressure switch trips.
- Clogged condensate drain tripping a float or safety switch. This shuts cooling off on systems that have one.
- Compressor failure or a control board or inverter fault. This is the least likely cause and the most expensive. Inverter faults apply to variable-capacity models.
How to fix Lennox air conditioner not cooling: safe checks first
- Thermostat:
- Set it to COOL with a setpoint at least 3°F below room temperature.
- Replace the batteries if the display is weak or blank.
- Wait 5 minutes. Many Lennox systems have a compressor anti-short-cycle delay.
- Air filter: Replace it if it is dirty, and check that it faces the correct airflow direction.
- Breakers:
- Check the AC breaker and the furnace or air handler breaker at the panel.
- If one has tripped, reset it once.
- If it trips again, stop and call a technician.
- Outdoor disconnect: Make sure it is fully seated and in the ON position.
- Registers and returns: Open the supply vents and clear any furniture or rugs blocking the returns.
- Outdoor unit clearance: Remove leaves, grass clippings and debris from around the outside of the unit. Do not open the unit.
- Frozen coil: If you see ice on the refrigerant lines, turn the system OFF and set the fan to ON until the ice has fully melted.
- Condensate:
- Check whether the drain pan or float switch area is holding water.
- Clear the accessible condensate line opening, for example with a wet/dry vacuum at the outside drain end.
- Panels: Confirm the indoor unit’s access panels are fully seated.
Stop there. Do not open the outdoor unit. Capacitors hold a dangerous charge even when the power is off.
How a technician will diagnose it
- Confirm the cooling call. Verify that 24 V is present at the outdoor unit’s Y terminal and that the contactor pulls in.
- Test the electrical components. Measure the capacitor’s microfarads against its rating, check voltage across the contactor, and measure compressor winding resistance and amp draw.
- Check the condenser fan motor. Confirm it runs and draws normal amps.
- Measure temperature split. A 15–22°F drop between return air and supply air is typical.
- Check refrigerant charge with gauges. On TXV-equipped Lennox units, charge is checked by subcooling. On fixed-orifice units, it is checked by superheat.
- Find leaks if the charge is low. Expect an electronic leak search, bubble test or nitrogen pressure test. Insist on finding the leak before any recharge.
- Read stored fault history. On communicating systems this comes from the iComfort or S40 thermostat. On variable-capacity models it comes from the outdoor board.
- Inspect the coils. Look for dirt on the evaporator and condenser coils, and confirm static pressure and blower operation.
Symptom, cause and what to do
| Symptom | Likely cause | DIY action | Technician job |
|---|---|---|---|
| Nothing runs, thermostat blank | Dead batteries or no power | Replace batteries; reset the breaker once | Check transformer, fuse and wiring |
| Indoor fan runs, outdoor unit silent | Tripped breaker or disconnect, contactor, or no 24 V signal | Check the breaker and disconnect | Replace contactor; trace control wiring |
| Outdoor unit hums, fan not spinning | Failed run capacitor or fan motor | Turn power off at the breaker and call a technician | Replace capacitor or fan motor |
| Ice on lines or indoor coil | Dirty filter or low refrigerant | Replace filter; run fan only to thaw | Check charge, find leak, clean coil |
| Runs constantly, air only slightly cool | Low charge or dirty coils | Clear debris from around the outdoor unit | Leak repair and recharge; coil cleaning |
| Cooling stops, water near indoor unit | Clogged drain tripping float switch | Clear the condensate line | Clean pan; check float switch |
| Alert code on iComfort or S40 | Pressure or compressor lockout | Record the code | Diagnose the specific fault |
Lennox air conditioner not cooling repair cost
| Repair | Typical US cost |
|---|---|
| Filter or thermostat batteries | $5–$40 (DIY) |
| Service call and diagnosis | $90–$200 |
| Run capacitor | $150–$400 |
| Contactor | $150–$350 |
| Condenser fan motor | $350–$800 |
| Drain line clearing or float switch | $100–$250 |
| Coil cleaning | $100–$400 |
| Leak search | $200–$600 |
| Refrigerant recharge, R-410A | $250–$800 |
| Refrigerant recharge, R-454B on 2025+ systems | Often higher than R-410A |
| Evaporator coil replacement | $1,200–$3,000 |
| Outdoor control board (variable-capacity models) | $500–$1,500 |
| Compressor replacement | $1,800–$3,500+ |
On an older unit, compressor replacement often makes system replacement the better value. Warranty parts coverage can lower these costs considerably, but labor and refrigerant are usually not covered.
Related codes
- Lennox Heat Pump Not Cooling: Causes, Fixes & Costs
- Lennox Heat Pump Compressor Not Running: Causes & Fixes
- Lennox Heat Pump Outdoor Fan Not Spinning: Causes & Fixes
- Lennox Heat Pump Breaker Keeps Tripping: Causes & Fixes
- Lennox Heat Pump Runs Constantly: Causes, Fixes & Costs
- Lennox Furnace Blower Not Running: Causes & Fixes
- Lennox Furnace Leaking Water: Causes, Fixes & Costs
Parts & tools for this fix
As an Amazon Associate, fixme.vip earns from qualifying purchases. Prices and availability are on Amazon.
Frequently asked questions
Why is my Lennox AC running but blowing warm air?
The indoor blower is usually running while the outdoor unit is not cooling. Common reasons are a failed run capacitor or contactor, a tripped outdoor breaker or disconnect, low refrigerant, or a compressor that has stopped on internal overload. Check the breaker and thermostat yourself, and leave everything else to a technician.
Can I add refrigerant to my Lennox AC myself?
No. Handling refrigerant legally requires EPA Section 608 certification. Low refrigerant also means there is a leak. Adding refrigerant without finding and fixing the leak only delays the next breakdown and can damage the compressor.
My Lennox indoor coil is iced over. What should I do?
Set the thermostat to OFF and the fan to ON so the ice melts. This can take several hours. Check the filter and make sure the supply registers are open. If the coil ices up again after you restart the system, call a technician. Repeated icing usually means low refrigerant or an airflow problem inside the equipment.
Does the Lennox 10-year limited warranty cover a not-cooling repair?
Registered Lennox units usually carry parts coverage on the compressor and covered components, often 5 to 10 years depending on the series and whether the unit was registered. Labor and refrigerant are usually not covered unless you bought an extended labor plan. Check your registration status before you approve major work.