Lennox Heat Pump Not Cooling: Causes, Fixes & Costs
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Lennox heat pump not cooling: what it means
“Not cooling” is a symptom, not a single fault code. On most Lennox heat pumps paired with a conventional thermostat, nothing is displayed. The system simply runs without lowering the indoor temperature, or the outdoor unit doesn’t run at all.
On communicating systems, the thermostat may show an alert that points toward the cause. This applies to iComfort, S30 and S40 controls paired with variable-capacity units like the XP25 or SL25XPV. Examples include high-pressure, low-pressure or compressor-related alerts. Alert numbers and reset behavior vary by model and control generation, so look up the exact alert in that unit’s installation manual.
Pressure-switch lockouts on many Lennox outdoor controls hold after repeated trips within one thermostat call. These usually clear when the thermostat call ends or power is cycled. The number of trips before lockout, and how the lockout clears, vary by control. Check your unit’s manual for the exact behavior. A homeowner should reset only once.
A heat pump in cooling mode depends on these parts working together:
- the outdoor compressor and fan
- the reversing valve being in cooling position. Lennox energizes the valve in cooling through the O terminal.
- a correct refrigerant charge
- clean coils
- the indoor blower moving air across the coil
If heating also fails, see the related heat pump not-heating page as well.
Lennox heat pump not cooling: common causes, ranked by probability
- Thermostat setting or wiring issue. Possible problems:
- The thermostat is in Heat or Auto with the wrong setpoint.
- The O/B setting is wrong after a thermostat replacement. Lennox uses O, so the valve is energized in cooling. If the thermostat is set to B, the unit heats when cooling is called.
- Restricted airflow. A clogged filter, closed registers or a dirty indoor coil can cause poor cooling and eventually a frozen evaporator coil.
- Failed run capacitor (dual capacitor). This is a very common Lennox outdoor-unit failure. The compressor or outdoor fan hums but won’t start, or the fan doesn’t spin.
- Contactor failure. Pitted or burned contacts, or a failed coil, keep the outdoor unit from starting.
- Dirty or blocked outdoor coil. Cottonwood, grass clippings or debris raise head pressure. The unit may trip on high pressure, and some models then show an alert.
- Low refrigerant charge from a leak. Signs include weak cooling, ice on the suction line, or low-pressure lockouts. Leaks often occur at the coils or brazed joints.
- Reversing valve problem. The valve may be stuck in heating position, or its solenoid coil may have failed. The outdoor unit runs, but the house gets warmer.
- Outdoor fan motor failure. The compressor runs, but the fan doesn’t, and the unit trips on high pressure or thermal overload.
- Defrost/outdoor control board or communicating control fault. This mostly applies to variable-capacity and communicating models. An inverter or board fault keeps the compressor off.
- Compressor failure. The compressor may have a locked rotor, shorted windings or failed valves. This is less common, but it is the most expensive outcome.
How to fix Lennox heat pump not cooling: safe checks first
- Thermostat: Confirm it is set to Cool and the setpoint is several degrees below room temperature. Replace the batteries if it has them.
- Recent thermostat change: If the problem started after a new thermostat was installed, tell the technician. The O/B setting is a likely culprit, but don’t rewire it yourself.
- Air filter: Replace it if it’s dirty. If the indoor coil or refrigerant lines are iced, set the system to Off with Fan On for 2–4 hours to thaw before trying Cool again.
- Breakers and disconnect: Check the breakers for the indoor unit and the outdoor unit. Also check the outdoor disconnect. Reset a tripped breaker once. If it trips again, stop and call.
- Outdoor unit: Clear leaves, plants and debris to give at least 2 feet of clearance. You may gently rinse the outside fins with a garden hose at low pressure, with the power off at the disconnect. Don’t open the unit.
- Registers and returns: Make sure supply registers are open and return grilles aren’t blocked by furniture.
- Condensate line: If the indoor unit has a float switch, a full drain pan can shut off cooling. Check for standing water and clear the drain line’s outside opening. Some lines also have an access port, which is a cap on the PVC trap or tee outside the cabinet that you can open by hand without tools. A wet/dry vac at the outside end works well. Don’t open the air handler cabinet to reach the drain; anything inside the cabinet is technician work.
- Panels: Make sure the indoor unit’s access panels are fully seated.
If the outdoor unit hums, buzzes or clicks without starting, turn it off at the thermostat and call a technician. Continuing to try can overheat the compressor.
How a technician will diagnose it
A competent technician will typically:
- Verify the thermostat call. Check for 24 V at Y and O at the outdoor unit. Confirm the thermostat’s O/B configuration.
- Check airflow. Inspect the filter, blower operation and indoor coil condition. Measure static pressure if airflow seems weak.
- Test electrical components.
- Measure the capacitor in microfarads against its rating.
- Check the contactor for voltage drop across the contacts.
- Measure fan and compressor amp draw.
- Check compressor winding resistance.
- Read alerts on communicating systems. Pull alert history from the iComfort/S30/S40 thermostat or the outdoor control’s diagnostic LEDs.
- Connect gauges or probes. Measure suction and liquid pressures, superheat and subcooling, and compare them with Lennox’s charging chart. Most Lennox heat pumps with TXVs use subcooling.
- Test the reversing valve. Compare line temperatures, verify the solenoid gets 24 V in cooling, and check coil resistance.
- Leak-search if the charge is low. Methods include an electronic detector, bubble solution, or nitrogen pressure testing. The leak must be repaired before recharging.
Red flags in a quote:
- “Just topping off” refrigerant with no leak search
- A compressor condemned without amp or winding tests
- A full system replacement recommended before a simple capacitor or contactor has been checked
Symptom, cause and what to do
| Symptom | Likely cause | DIY action | Technician job |
|---|---|---|---|
| Nothing runs indoors or out | Thermostat, breaker, or tripped float switch | Check settings and batteries, reset breaker once, clear condensate line | Trace the 24 V circuit and test the transformer and controls |
| Indoor fan runs, outdoor unit silent | Capacitor, contactor, or no Y signal | Check outdoor disconnect and breaker | Test and replace capacitor or contactor, verify wiring |
| Outdoor unit hums but won’t start | Failed capacitor or seized compressor | Turn system off and call | Test capacitor and compressor windings, replace as needed |
| Outdoor fan not spinning, unit hot | Fan motor or capacitor | Turn off to prevent overheating | Replace fan motor or capacitor |
| Unit runs, house gets warmer | Reversing valve or wrong O/B setting | Confirm thermostat is in Cool | Fix O/B config, test solenoid, replace valve |
| Ice on lines or indoor coil | Low airflow or low refrigerant | Replace filter, thaw with fan only | Check airflow, leak-search, repair and recharge |
| Weak cooling, long run times | Dirty coils or low charge | Replace filter, gently hose outdoor fins | Clean coils, measure superheat and subcooling |
| Alert on iComfort/S30/S40 thermostat | Pressure, inverter or compressor fault | Note the alert number, reset once | Diagnose per model manual, repair board or component |
Lennox heat pump not cooling repair cost
Typical US prices include parts and labor. They vary by region, model and warranty status.
| Repair | Typical cost |
|---|---|
| Thermostat setup or O/B correction | $0–$150 |
| Air filter | $10–$60 (Lennox media filters cost more) |
| Diagnostic or service call | $90–$200 |
| Run capacitor | $150–$400 |
| Contactor | $150–$400 |
| Outdoor coil cleaning | $100–$300 |
| Indoor coil cleaning | $150–$500 |
| Outdoor fan motor | $350–$800 |
| Refrigerant leak search | $200–$600 |
| Refrigerant recharge | $200–$900+, depending on the amount and refrigerant type (R-410A or newer R-454B) |
| Leak repair | $200–$1,500 |
| Evaporator coil replacement | $1,500–$3,000 |
| Reversing valve solenoid coil | $150–$350 |
| Reversing valve replacement | $1,000–$2,500, due to refrigerant recovery and brazing |
| Outdoor or inverter control board (variable-capacity models) | $500–$1,500 |
| Compressor replacement | $1,800–$4,000+ |
Warranty parts reduce costs, but labor often still applies. If the unit is older than 12–15 years and needs a compressor, evaporator coil or major refrigerant work, compare the repair cost with replacement.
Related codes
- Lennox Heat Pump Not Heating: Causes, Fixes & Costs
- Lennox Heat Pump Frozen Coil: Causes & Fixes
- Lennox Heat Pump Outdoor Unit Not Running: Causes & Fixes
- Lennox iComfort/S30 Thermostat Alert Codes Explained
- Lennox Furnace Blower Not Running: Causes & Fixes
Parts & tools for this fix
- Air filter (correct size for your return)
- Lennox Healthy Climate replacement media filter
- Thermostat batteries
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Frequently asked questions
Why is my Lennox heat pump blowing warm air in cooling mode?
The most common reasons are that the outdoor unit isn't running, which is often a failed capacitor or contactor, or that the reversing valve is stuck in heating position. Low refrigerant from a leak is another cause. Check the thermostat mode and the breaker first. The rest needs a technician.
Can I add refrigerant to my Lennox heat pump myself?
No. Handling refrigerant requires EPA Section 608 certification. Low charge also means there is a leak. A technician has to find and repair the leak, then weigh in the correct charge listed on the unit's nameplate.
Should I keep resetting the breaker if the outdoor unit won't start?
Reset it once. If the breaker trips again, or the unit hums without starting, leave it off and call a technician. Repeated resets can damage the compressor or point to an electrical fault that needs testing.
Is a not-cooling heat pump covered by Lennox warranty?
Often, at least for parts. Warranty terms vary by series and installation date, and compressor coverage differs between Merit, Elite and Signature models. Registration status can also change your coverage. Labor is usually not covered unless you bought an extended plan. Before you approve repairs, use Lennox's warranty lookup with your unit's serial number to confirm what applies.