Lennox AC Not Cooling? Common Causes & How to Fix It

First, the fast checks

If your Lennox AC is running but not cooling, start with the five checks that resolve most no-cool calls:

1. Thermostat - set to COOL, target below room temp, fresh batteries. On an iComfort communicating thermostat, a blank screen can point to a control issue.
2. Air filter - a clogged filter chokes airflow and freezes the coil. Replace it if it's dirty (how to).
3. Breakers - reset the indoor and outdoor breakers once; repeated trips mean call a pro.
4. Outdoor unit - confirm the fan is spinning, clear debris, check the disconnect.
5. Frozen coil - ice on the lines means shut the AC off (fan ON) to thaw, then run with a clean filter.

Still not cooling? Here's what's most likely on a Lennox.

The Lennox-specific causes we see most

• Proprietary control-board or iComfort faults. Lennox uses proprietary parts, so a board fault on a high-efficiency unit needs a tech who can source the specific component quickly - the single most important factor in a fast Lennox repair.
• Inverter or variable-speed issues on the high-efficiency Signature (SLP/XC) systems - weak or no cooling from the variable-speed compressor or blower.
• Failed run capacitor or contactor - the most common no-cool cause on any AC; cheap and fast to fix.
• Low refrigerant from a leak - shows up as ice on the lines and weak cooling.

See the full list on our Lennox repair page, and our general guide to why an AC stops cooling covers the rest.

When to call a pro

Call a licensed tech if you suspect an inverter, board, or refrigerant issue, the coil keeps freezing, or the breaker trips repeatedly. Because Lennox parts are proprietary, an upfront flat-rate quote matters here - and a tech who can get Lennox parts fast. More on that in our Lennox brand review.

Lennox AC down today? Get matched with a licensed local pro for same-day Lennox repair - upfront pricing, no overtime fees.