Why your heat pump won't turn on in Illinois

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A heat pump that won't turn on is usually stuck on a lockout, not actually dead. In an Illinois winter, the most common reason is ice on the outdoor unit. When the coil freezes, the defrost control senses it and can shut the system down until a contractor clears what caused it, whether that's a failed defrost board, a stuck reversing valve, or a sensor reading wrong. It can also be simpler: a tripped breaker, a dead thermostat battery, or a safety switch that opened because airflow is restricted. If the outdoor unit is caked in ice or snow and the indoor unit does nothing at all, that points to the defrost cycle rather than the thermostat. Either way, running it repeatedly without power won't fix the cause, and on the coldest days you don't have much margin before the house starts losing heat for good.
What a defrost lockout looks like
The outdoor unit sits quiet, maybe with a layer of ice across the coil or the fan. Inside, the thermostat still shows a call for heat, but nothing answers it. This is the defrost control doing its job. It watches the coil, and when ice builds past what the system can clear on its own, it locks the unit out rather than run it into the ground. That lockout won't clear itself. A contractor needs to find why the coil iced up in the first place, whether that's a failed defrost board, a stuck reversing valve, or a fan that stopped turning.
Why it happens more in January
Heat pumps here work harder in the coldest stretch of winter, and that's exactly when frost has the best chance to build faster than the defrost cycle can clear it. A unit that ran fine in November can lock out in January without anything new failing. Low outdoor temperature alone adds to the load on the coil. Once ice has gotten ahead of the system, it takes a technician checking the defrost board, the sensors, and the refrigerant charge to find out which one let the frost win.
Summer air here sits heavy, with a dew point averaging 63°F across June, July and August, NOAA data shows. That's the number that makes a room feel sticky even with the AC running. Your heat pump has to pull that moisture out of the air as it cools, and the coil stays wet doing it. In winter, that same coil holds onto condensation and ice when temperatures drop, which is part of what triggers the defrost lockouts this page covers. A unit that's worked hard all summer pulling moisture out of thick air goes into winter already carrying wear on the parts that sense and manage ice. If your heat pump won't turn on, what it did all summer is part of the story.
NASA POWER T2MDEW county cells weighted by homes
Still under warranty
If your heat pump is newer, a lockout might be covered. A local contractor who works on these year-round can check the cause and the coverage in one visit.
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Past warranty
Older units still shut down the same way, but repairs come out of pocket. Worth having someone who knows the common failure points look before you replace anything.
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If your heat pump stays locked out overnight, the backup heat has to carry the house alone, and on a gas furnace system that usually means the furnace is already doing the job.
But if the backup doesn't kick in either, the house loses heat with nothing covering for it, and on a single-digit Illinois night that shows up fast.
You're up before the sun, maybe to let the dog out, and you cross the side yard where the outdoor unit sits. The cold bites through your coat. The whole cabinet is sheathed in ice, thick across the coil, a crust over the top where the fan should be turning. No hum, no movement. You can see your breath, and behind you the house is quiet in a different way, the kind of quiet that means the heat hasn't kicked on in a while. You go back in and check the thermostat. It still shows the temperature dropping, still calling for heat, and nothing is answering. A local contractor who handles these units through an Illinois winter would know in a few minutes whether this is a normal defrost cycle gone wrong or something that needs a part.
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A lockout that clears on its own
Some heat pumps come out of a defrost lockout without anyone touching them. The control runs its cycle, the ice melts off the coil, and the system restarts and goes back to heating. You might not even know it happened except that the outdoor unit looked frosted earlier and doesn't now. This is more likely the first time it happens, or on a mild day when the coil only picked up a thin layer of ice. If the house held its temperature through it and the unit is running fine now, there may be nothing further to do, though a contractor can still check why the frost built up in the first place.
A lockout that keeps coming back
Other times the unit clears, runs for a while, then locks out again the same day or the next. That pattern points to something that won't fix itself, a low refrigerant charge, a fan motor not moving enough air across the coil, or a defrost sensor misreading the coil temperature. Each repeat cycle is the house losing heat and leaning on the backup for longer stretches. A contractor who works on heat pumps through Illinois winters can tell from the pattern, how often, how long, how much ice, whether it's a one-time cold snap or a part that needs replacing before the next lockout hits.
Outdoor coil picks up a layer of frost in cold, damp air.
A sensor on the coil reads the ice forming.
The control board reverses the system into defrost mode.
Indoor air stops blowing warm while the coil thaws.
Running clean
This is what a heat pump looks like when nothing's wrong. No ice on the coil, fan spinning free, conduit and disconnect box doing their job quietly on the wall. In Illinois, a unit this clear in January is pulling heat from the air and sending it inside without a fight.
Iced up and stuck
Grime and age alone don't stop a heat pump, but they make ice form faster and melt slower once it's there. A unit this dirty is more likely to lock out on defrost and stay down longer, since the coil underneath can't shed frost as fast as a clean one would.
What would you actually notice in your own house
You'd feel the house losing its edge. Rooms that were fine an hour ago start running cool, and the thermostat still reads a number it's not hitting. If you walk out to the unit, the fan might be still, the coil crusted with ice you can see from a few feet off. Inside, vents push air that doesn't feel warm, or no air at all. If this happens overnight, the furnace or backup heat strip is likely carrying the load alone, and the house can hold steady for a while that way. If it doesn't, the drop is slow but it doesn't stop on its own, and in an Illinois cold snap that matters more for anyone in the house who's older, very young, or sensitive to a cold house.
Does the Lockout Clear Itself
Lockout clears itself
The defrost control runs its own cycle and the ice melts off the coil without anyone touching the unit.
The system restarts on its own and goes back to pulling heat, often with no sign anything happened.
Your thermostat keeps calling for heat the whole time, and the backup heat or furnace carries the house until the coil clears.
You might notice the house felt cool for a stretch, then warmed back up once the outdoor fan started turning again.
This can happen more than once in a stretch of cold, wet weather without pointing to a failing part.
Lockout needs a contractor
The outdoor unit stays iced over and quiet, and the thermostat keeps calling for heat with nothing answering it.
A stuck defrost control, a failed sensor on the coil, or a bad reversing valve can all hold the system in lockout past when the ice should be gone.
A local contractor who works on heat pumps through Illinois winters can tell a self-clearing lockout from a part that's failed.
Leaving it locked out puts the full load on backup heat, and on a system without a furnace behind it, the house has nothing else carrying the heat.
Once a contractor checks the cause, they can also tell you whether it's covered if the unit is still under warranty.
Before you get your heat pump repair
After you get your heat pump repair
Power costs 17.67 cents per kilowatt-hour statewide
When a lockout drags on and backup heat has to carry the house, that heat is running on electricity billed at 17.67 cents per kilowatthour here, one of the higher rates around. A heat pump stuck off longer than it should be costs more than comfort while it's out.
EIA-861 2025 statewide residential average
A clean grille and a free-spinning fan mean air is moving through the coil the way it should.
Dust packed into the grilles chokes airflow, and a coil that can't breathe has a harder time shedding ice.
A heat pump that won't start is telling you something specific
A lockout has a cause. Ice on the coil, a sensor doing its job, a control board waiting out a timer. None of that means the unit is finished, and none of it is something to guess at from the thermostat. Someone who looks at the outdoor unit, the wiring, and the defrost cycle can tell you within a visit whether this clears on its own or needs a part.
That's the kind of look a local contractor gives it, one who works on heat pumps through an Illinois winter and knows which lockouts are routine and which ones point to something failing. You'd be matched with one contractor for your situation, not a list of them, and that contractor reaches out to you directly. Your house gets warm again, and you know why it went cold in the first place.
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A lockout can clear on its own or it can sit there while your house gets colder. A local contractor who services heat pumps all year can tell you which one you're looking at and get the outdoor unit running again before backup heat runs up the bill.

Why your heat pump won't turn on in Illinois

Still under warranty

Past warranty

Running clean

Iced up and stuck

Lockout clears itself

Lockout needs a contractor