Lennox Heat Pump Won't Turn On: Causes, Fixes & Costs
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Lennox heat pump won't turn on: what it means
“Won’t turn on” is a symptom, not a Lennox fault code. It means the heat pump does not respond when the thermostat calls for heat or cooling. Neither the indoor blower nor the outdoor unit starts, and the thermostat may be blank.
This page covers a system that is completely unresponsive. If the indoor fan runs but the outdoor unit doesn’t, see the related articles on the compressor and outdoor fan instead.
How it shows up depends on your system:
- Conventional 24 V systems: There is usually no code. The thermostat may simply go blank, or it may show a call while nothing happens.
- Communicating iComfort systems: These often post an alert on the thermostat, such as a lost-communication or low-voltage alert. Write the alert number down before you reset anything.
On many Lennox outdoor boards, a pressure-switch lockout holds until power is cycled. Some systems also include a short-cycle delay of about 5 minutes. After a reset or power outage, waiting a few minutes is normal and is not a fault.
Lennox heat pump won't turn on: common causes, ranked by probability
- Thermostat issue. The thermostat may be set to Off, the wrong mode or the wrong setpoint. It may also have dead batteries or a schedule hold.
- Tripped breaker or switch off. This could be the indoor air handler breaker, the outdoor unit breaker, or the service switch that looks like a light switch near the indoor unit.
- Condensate float switch tripped. A clogged drain lets water rise in the pan or line. The float switch then cuts 24 V power and shuts the whole system down. This is very common in summer and with attic air handlers.
- Blown 3 A or 5 A low-voltage fuse on the indoor control board. This is usually caused by a shorted thermostat wire, often from chafed outdoor wiring or a damaged line.
- Failed 24 V transformer in the air handler or furnace.
- Loose blower or access door. On units fitted with a door interlock switch, a loose door cuts power to the board.
- Communication or wiring fault on iComfort systems, or a broken thermostat wire.
- Control board failure. This can affect the indoor board or, on variable-speed models, the outdoor inverter board. It is often caused by a power surge.
How to fix Lennox heat pump won't turn on: safe checks first
- Thermostat: Set it to Heat or Cool with a setpoint at least 3°F past room temperature. Make sure the fan is on Auto. Replace the batteries if the screen is dim or blank.
- Breakers: Check the breakers for both the air handler (or furnace) and the outdoor unit. Reset any tripped breaker once. If it trips again, stop and see the Breaker Keeps Tripping article.
- Switches: Confirm the indoor service switch is on and the outdoor disconnect is seated.
- Condensate drain: Look for water in the secondary pan or an overflow at the float switch. Clear the drain line opening with a wet/dry vacuum from the outside end. Power often returns once the float drops.
- Air filter: Replace it if it’s dirty. A dirty filter can contribute to freeze-ups and drain overflow.
- Cabinet panels: Make sure the blower access panel is fully closed and seated. Do not open the cabinet.
- Wait and reset once: After restoring power, wait 5 to 10 minutes for the delay timer. Cycle power once for a possible lockout. If the system still doesn’t run, call a professional.
How a technician will diagnose it
A technician will usually work through these steps:
- Check for line voltage at both the indoor and outdoor units.
- Read the indoor board’s diagnostic LED, or the iComfort alert history.
- Measure the transformer’s 24 V secondary output and check the board fuse. A blown fuse means they will look for the short, often in the outdoor thermostat cable, before replacing it.
- Test the float switch and door switch for continuity.
- Jumper R to G, Y or W at the board to rule out the thermostat versus the wiring.
- On inverter models, read the outdoor board codes and check communication.
A good quote names the failed component and explains the root cause, such as a short or a surge.
Symptom, cause and what to do
| Symptom | Likely cause | DIY action | Technician job |
|---|---|---|---|
| Thermostat blank | Dead batteries or loss of 24 V | Replace the batteries | Check the fuse and transformer |
| Everything dead, water in the drain pan | Float switch tripped | Clear the drain line | Clean the drain and pan, test the float switch |
| Everything dead, breaker tripped | Electrical fault | Reset the breaker once | Find the short or failed component |
| Dead right after a storm | Blown fuse or failed board or transformer | Reset the breaker once | Test and replace the board or transformer |
| Fuse blows repeatedly | Shorted thermostat wire | None | Trace and repair the wiring |
| iComfort alert showing | Communication or low-voltage fault | Note the code, cycle power once | Diagnose the communication wiring or board |
| Indoor runs, outdoor dead | Outdoor-side fault | Check the outdoor breaker | See the compressor and outdoor fan articles |
Lennox heat pump won't turn on repair cost
| Repair | Typical US cost |
|---|---|
| Diagnostic visit | $80–$200 |
| Clear the condensate drain or replace the float switch | $100–$250 |
| Replace the low-voltage fuse and repair the shorted wire | $120–$350 |
| Replace the 24 V transformer | $150–$350 |
| Replace the door switch | $100–$250 |
| Replace the thermostat (installed) | $150–$400 standard; $300–$700+ for iComfort |
| Replace the indoor control board | $350–$800 |
| Replace the outdoor inverter board (variable-speed models) | $700–$1,500+ |
Related codes
- Lennox Heat Pump Breaker Keeps Tripping: Causes & Fixes
- Lennox Heat Pump Compressor Not Running: Causes & Fixes
- Lennox Heat Pump Outdoor Fan Not Spinning: Causes & Fixes
- Lennox Heat Pump Not Heating: Causes, Fixes & Costs
- Lennox Heat Pump Not Cooling: Causes, Fixes & Costs
- Lennox Furnace Won’t Turn On: Causes, Fixes & Costs
- Lennox Furnace Code E115: Low 24 VAC Causes & Fixes
Parts & tools for this fix
- Furnace/air handler air filter
- AA/AAA thermostat batteries
- Heat pump compatible programmable thermostat
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Frequently asked questions
Why is my Lennox heat pump completely dead, with no indoor fan and no outdoor unit?
When both the indoor and outdoor units are dead, the fault is usually upstream of both: the air handler or furnace breaker, the indoor service switch, a blown low-voltage fuse on the indoor board, a failed 24 V transformer, or a tripped condensate float switch. Check the breakers, the switch and the float switch yourself. A technician should handle the fuse and transformer.
The indoor fan runs but the outdoor unit won't turn on. Is that the same problem?
No. If the indoor blower works but the outdoor unit is silent, the problem is on the outdoor side, such as the outdoor breaker, the disconnect, the contactor, a capacitor or the compressor. Our separate articles on a Lennox heat pump compressor not running and outdoor fan not spinning cover this in detail.
Does a Lennox heat pump reset itself after a lockout?
It depends on the board. Many Lennox outdoor controls allow a few automatic retries after a pressure-switch trip. They then hold a hard lockout until power is cycled or the thermostat call is removed. Communicating models may need the alert cleared at the thermostat. Reset once only. If it locks out again, call a technician.
Can a blank thermostat stop the whole heat pump from turning on?
Yes. Battery-powered thermostats stop calling for heat or cooling when their batteries die. A thermostat powered by the C-wire goes blank if the system loses 24 V, which often points to a blown fuse or a tripped float switch. If new batteries don't fix a blank screen, the system's low-voltage power is the likely cause.