A heat pump that won't turn on at all - no fan, no compressor sound, nothing - is almost always one of five things: dead thermostat batteries, a tripped breaker, an open outdoor disconnect, a tripped condensate float switch, or a blown low-voltage fuse on the control board. Work through them in that order before calling anyone.
Check the thermostat first
Start here because it's the fastest fix and the most common culprit. A thermostat running on batteries that have quietly died will show a blank or frozen screen, and the system won't respond to anything. Swap the batteries even if the display looks normal; some models hold a ghost image for a while after the batteries go.
While you're there, confirm the mode is set correctly. Heat mode on a heat pump thermostat sometimes has a separate "emergency heat" setting that behaves differently from normal heat. Make sure you're not accidentally in a mode the system isn't set up to run, and confirm the set temperature is actually above the current room temperature. It sounds obvious. It's also the single most common false alarm in this whole list.
Check the breaker
Heat pumps typically run on two breakers: one for the outdoor unit, one for the indoor air handler or furnace blower. Find your electrical panel and look for a breaker that's tripped to the middle position, not fully off. Switch it fully off, then back on. If it trips again immediately, stop. A breaker that won't hold usually means an electrical fault downstream, and that's a technician job, not a reset-and-hope situation.
Check the outdoor disconnect
Near the outdoor condenser unit, usually mounted on the wall of the house a few feet away, there's a small gray metal box called the disconnect. Pull it open and confirm the pull-out block or switch inside is fully seated and in the "on" position. These get bumped, left open after previous service work, or occasionally pulled by kids or pets. It's a five-second check worth doing every time.
Check for a tripped float switch
If your air handler or furnace has a condensate drain line, it likely has a float switch, a small plastic device that senses when the drain line backs up and shuts the whole system down to prevent water damage. This is a safety feature, not a malfunction, and it's doing exactly what it's supposed to do. A clogged condensate line is the usual cause. You can sometimes clear it yourself with a wet/dry vacuum at the outdoor termination point, but if you're not comfortable with that, a technician can clear it in a single visit and it's one of the cheaper service calls on this list.
Check the low-voltage fuse
Most control boards have a small automotive-style fuse, often rated around 3 or 5 amps, protecting the low-voltage circuit that talks to your thermostat. If it's blown, the thermostat display goes dark and nothing runs, similar to the dead-battery symptom above. Replacing this fuse is something a comfortable DIYer can do with the power off, but a fuse that blows again right after replacement means something is shorting the low-voltage circuit, and that needs a technician to trace down. Don't keep replacing fuses hoping the next one holds.
What needs a technician
If you've worked through all five checks above and the system is still dead, the problem has moved past what's safe or practical to diagnose yourself. A few components commonly cause a fully dead system and all require a licensed technician:
- A failed contactor, the relay that switches power to the outdoor unit. These wear out from arcing over years of cycling and simply stop closing.
- A bad capacitor, which can prevent the compressor or fan motor from starting even when power is reaching the unit.
- A failed control board, which can fail silently with no obvious symptom besides the system doing nothing.
- A failed compressor, the most expensive failure on this list and one that sometimes shows early warning signs like humming without starting.
Any of these require opening an electrical cabinet with live components, and a capacitor holds a charge even with the power off. That's not a safe DIY repair. It's also worth noting that if your system is a heat pump rather than straight AC, the same dead-system checks apply either way. The reversing valve and defrost components that make heat pumps unique don't factor into a totally dead system. Those matter more once the system is running but misbehaving. If your issue is actually a cooling problem on a standard AC rather than a heat pump, see AC not turning on for the AC-specific version of this same checklist.
Common questions
Why is my heat pump completely dead with no fan or compressor sound?
The most common causes, in order of how often they happen, are a tripped breaker, a dead thermostat battery, an open outdoor disconnect, and a tripped condensate float switch. All four are safe to check yourself in under ten minutes. If none of those explain it, the problem has moved to a component that needs a licensed technician.
Can a clogged condensate line really shut off my whole heat pump?
Yes. The float switch that monitors the condensate line is a safety device, and it's designed to cut power to the system the moment the line backs up, to prevent water damage to your ceiling or walls. It's working correctly when it does this. Clear the clog and the system should turn back on.
Is it safe to reset a tripped breaker myself?
Yes, once, if it's a heat pump breaker and not a different circuit. Flip it fully off, then back on. If it holds, you're fine. If it trips again immediately or within a few minutes, stop resetting it and call a technician. A breaker that won't hold is protecting you from an electrical fault, not being oversensitive.
How much does a technician charge just to diagnose a dead heat pump?
Most Arizona HVAC companies charge $75 to $150 for a standard diagnostic visit, sometimes up to $200 with larger companies. That fee is usually applied toward the repair cost if you move forward with the same company. Ask before they come out whether it works that way.
Get a technician out today
If you've checked the thermostat, breaker, disconnect, and float switch and the system is still dead, stop troubleshooting and get a licensed contractor out. Use the quote form below to compare a couple of options near you, including Tempe and Scottsdale. Look up any contractor's license at azroc.gov before they arrive.