The short answer
When a Lennox AC or heat pump in Northern Virginia runs but won’t cool, check the thermostat, the filter, the breakers and the outdoor unit first. Lennox’s own before-you-call list covers exactly those. If the outdoor fan isn’t spinning, there’s ice on the copper line, or the air coming out is barely cool, it’s usually a capacitor, a contactor, a refrigerant leak or a dirty coil, and that’s a job for a tech.
Lennox’s own checklist
Lennox prints this list in its heat pump homeowner manual under “check the following before calling for service.” It works for a Lennox AC too:
- Every disconnect switch is ON. That’s the switch box on the wall by the outdoor unit and the switch at the furnace or air handler.
- The temperature setting is right. A few degrees cooler than the room, or it has no reason to run.
- The system switch is right. COOL or AUTO, not HEAT or OFF.
- Breakers and fuses. Reset a tripped breaker once. If it trips again, leave it off and call.
- Access panels are on. A panel off its switch shuts the unit down on purpose.
- The filter is clean. A choked filter starves the indoor coil of air, and that’s how coils freeze.
- Have the model number handy. Photo the rating plate on the outdoor unit. It saves a step on the phone.
Don’t open the outdoor unit’s electrical panel. The capacitor inside holds a charge even with the power off.
The $67 Heating Check
We check your furnace, heat pump or boiler before winter. Through December 31. See the offer
What to look and listen for
The outdoor unit hums, but the fan on top isn’t turning. That’s most often a weak capacitor, the part that gives the fan and compressor their kick to start. Don’t poke the fan to get it going. AC fan not spinning explains why.
The indoor fan runs, the outdoor unit is silent. Check the outdoor disconnect and breaker. If both are on, it’s often a burnt contactor, the relay that sends power to the outdoor unit.
Ice on the big copper line or the indoor coil. Turn cooling off and run the fan alone to thaw it, then change the filter. If it ices again, it’s usually low refrigerant or weak airflow.
The system shut off and there’s water in the pan. A float switch stops cooling when the condensate drain backs up, so the water doesn’t end up in your ceiling.
It runs all afternoon and the air is only cool-ish. A dirty outdoor coil, low refrigerant, or a blower wheel caked with dust. Rinse the outdoor coil gently with a garden hose and keep shrubs back; Lennox’s manual says to keep shrubbery trimmed and debris off.
An alert on the thermostat. Photo it. Lennox AC and heat pump error codes decodes the 400-series outdoor alerts. On newer Lennox systems with R-454B refrigerant, a leak sensor indoors stops cooling on purpose and shows E150.
What it usually turns out to be
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| The cause | How a tech confirms it | Our price |
|---|---|---|
| Weak or failed capacitor | Measures it against its rating | $240 to $370 |
| Burnt or stuck contactor | Voltage across it while the thermostat calls for cooling | $290 to $425 |
| Condenser fan motor | Good capacitor, motor still won’t run | $1,100 to $1,500 |
| Low refrigerant from a leak | Pressures and temperatures at the outdoor unit | $190 to $740 |
| Clogged condensate drain, float switch tripped | Water in the pan, switch open | $175 to $420 |
| Blower wheel caked with dust | Weak airflow at the vents, dust on the blades | $300 to $440 |
| Thermostat failed or set up wrong | Calls for cooling never reach the equipment | $300 to $640 |
| Control board | No output from a powered board | $550 to $1,180 |
A note on refrigerant: if it needs a top-up again next summer, it’s leaking, and topping it up every year is the expensive way to fix it. Ask for the leak to be found.
The diagnostic is a flat $99, and you see the price in writing before we start. AC repair costs in Northern Virginia has the full list, and AC blowing warm air covers every brand. If the system is a heat pump, it heats your house all winter too: heat pump repair.
Our heating check is $67 per system, through December 31: furnace, heat pump or boiler. On a furnace, for $229 (regularly $467) we also pull the blower wheel and clean it blade by blade. On a heat pump, that check is where a weak capacitor or contactor gets caught before it strands you on backup heat.
If the repair is a big one on an older furnace, get a second look before you spend it. Our free second opinion costs you nothing, and the estimator shows what a new system costs with $1,500 off already in the price. Thinking about brands? Lennox vs Trane lays out the difference.
Every Lennox code in one place: Lennox error codes. Other brands too: furnace and AC error codes.
The system we put in first
Built to take a beating. Put in by our crew.
Heat lamps, snow and rain on one outdoor unit: that’s Trane’s own film. We’re a Trane Comfort Specialist, on Trane’s own dealer locator, and when you pick a Trane, our crew puts it in.
Films: Trane’s own.
Call or book a Lennox repair
Call (703) 997-9757. We answer day or night. Or book online and pick a time. The diagnostic is a flat $99, and you see the repair price in writing before we touch anything. Furnace running fine and you want to keep it that way? Book the $67 heating check from goswiftpro.com/cookiesoffer (book by November 30) and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276. More on Lennox repair in Northern Virginia.
Questions people ask
Why is my Lennox AC running but not cooling?
The usual causes are a dirty filter, a dirty outdoor coil, a weak capacitor that stops the outdoor fan or compressor, or low refrigerant from a leak. Check the filter, the breakers and the thermostat first.
How do I reset my Lennox AC?
Set the thermostat to OFF, turn the outdoor unit’s breaker off and back on, then set the thermostat back to COOL. Give it several minutes; most outdoor units wait before the compressor starts again.
Why is my Lennox AC frozen?
Ice usually means too little air over the indoor coil, from a dirty filter or a dirty blower, or low refrigerant. Turn cooling off and run the fan to thaw it, then change the filter. If it freezes again, call.
How much does it cost to fix a Lennox AC that won’t cool?
A capacitor runs $240 to $370, a contactor $290 to $425 and a refrigerant top-up $190 to $740. The diagnostic is a flat $99, and you see the price in writing first.
