No Power / Won't Turn On: A Cross-Trade Decision Tree
Why this matters
"It is completely dead" is one of the most common calls and one of the easiest to over-think. The instinct is to suspect the expensive component first; the discipline is to start at the wall and work inward. Most dead units are dead for a cheap, simple reason - a tripped breaker, a blown fuse, an open safety switch, a loose connection - found in the first few minutes if you follow the power from source to load. This source-to-load method works on any electrically powered equipment across every trade.
Safety first
You are tracing live power. Test before you touch every conductor, and confirm your meter works on a known live source first so a dead meter does not fool you into calling a live wire safe. Use lockout/tagout before you open enclosures or work on connections. Never bypass a safety switch or interlock to "see if it runs" - those devices are open for a reason, and defeating one can energize a hazard or start equipment that should stay off.
Start here: confirm there is power at the source
Do not assume the building is feeding the unit. Begin at the supply and move toward the device, checking each handoff.
- If the breaker or disconnect is tripped or off: reset it once and watch. If it holds, the trip may have been a one-time event - but note it; a breaker that tripped did so for a reason. If it trips again immediately, stop and go to a short-circuit diagnosis; do not keep resetting into a fault.
- If a fuse is blown: a fuse is a deliberate weak link that opens on over-current. Replace it once with the correct rating. If it blows again, there is a fault downstream - find it, do not up-size the fuse.
- If the breaker is on and the fuse is good but no voltage reaches the unit: the break is between the panel and the device. Check the disconnect, the wiring, and every connection along the run.
Walk the chain: source to load
Think of it as a relay race. Power passes from the panel, through a disconnect, through any switches and safeties, through the controls, to the device. Find the baton drop.
- Supply present at the panel/source? Confirm voltage leaving the breaker or disconnect.
- Power reaching the unit's terminals? Measure at the device input. Voltage here but a dead unit means the fault is inside the unit or its controls. No voltage here means the fault is upstream.
- Controls calling for operation? A thermostat, switch, sensor, or control board has to command the device. A dead unit may simply never be getting the start command. Confirm the control is closed and sending its signal.
- Safeties and interlocks closed? A tripped high-limit, a door interlock, a float switch, a pressure cutout, or an overload will keep a perfectly good unit dead. These open to protect; an open safety usually means a real condition exists. Find why it opened, do not just jumper it.
If power reaches the unit but it stays dead
You have crossed from supply to device. Now the fault is local:
- If a control board or module shows no signs of life: check its own supply (low-voltage transformer, control fuse) before condemning the board. A blown low-voltage fuse or a failed transformer leaves the whole control side dark.
- If the device hums or clicks but will not start: it is getting power but cannot run. That is no longer a no-power problem - jump to electrical-vs-mechanical isolation (hand-turn the load, check the start component).
- If a connection is loose, burnt, or corroded: a bad connection is an open circuit. Loose terminals are a top cause of "dead" equipment and a fire risk. De-energize, then re-make it clean and tight.
Common culprits, cheapest first
| Cause | How it presents | Check |
|---|---|---|
| Tripped breaker / off disconnect | Whole unit dead | Reset once, see if it holds |
| Blown fuse | Dead, often after a surge or fault | Replace correct rating, see if it reblows |
| Open safety / interlock | Dead despite good power | Find why it opened |
| Failed control / no command | Power present, no start | Confirm control signal |
| Loose / burnt connection | Intermittent or fully dead | De-energize, inspect, re-make |
| Blown low-voltage fuse / transformer | Controls dark | Check control-circuit supply |
| Failed device | Power and command present, nothing | Last, after all above |
Work the table top to bottom. The expensive failed-component answer is at the bottom for a reason - it is the least common cause of a dead unit.
A breaker or fuse that keeps blowing is telling you something
If protection reopens the moment you restore it, you do not have a no-power problem - you have a fault drawing too much current. Stop resetting. Move to a short-circuit or overload diagnosis. Repeatedly resetting into a dead short is how you turn a fault into a fire.
Confirm and note what you found
When the unit comes back, confirm it stays on under normal operation, and write down the actual cause. "Reset the breaker" without finding why it tripped is an unfinished job. If a safety opened, the reason it opened is the real repair.
References
- OSHA electrical safety (29 CFR 1910 Subpart S) and lockout/tagout (1910.147) - verify de-energized before opening enclosures.
- NFPA guidance on over-current devices and not defeating circuit protection.
- Manufacturer wiring diagrams and control-sequence documentation for the specific unit.
- See related: Electrical vs Mechanical Fault Isolation, Trips Immediately vs Trips Under Load.