Power Present but Nothing Happens Decision Tree
Why this matters
You confirm power is at the equipment, you hit the call, and nothing moves. This is one of the most common and most misdiagnosed faults across every trade, because "it has power but does nothing" has a dozen causes and people guess instead of trace. The discipline is to follow the signal from the supply to the load, checking the cheap and common things before the expensive board, and to do it without putting hands on energized parts you have not tested. This tree gives you that ordered path so you replace the right thing once.
Start here: confirm power safely, then confirm the demand
First, separate "no power" from "power but no action," and do it safely. Verify with a tester, not an assumption, and keep your hands off energized terminals you have not proven. If you find a burning smell, scorched terminals, or any combustion or shock hazard while checking, stop and switch to the relevant safety path before continuing.
- Is supply voltage actually present at the equipment under load conditions, measured at the right point with a known-good meter? A breaker that looks on can be tripped internally; a connection can be open. Prove it.
- Is the equipment actually being told to run? No-action sometimes means no-demand: a thermostat, a switch, a float, a timer, or a remote signal never asked for operation. Confirm the call exists before you blame the machine.
If power is present and a real demand exists, work the chain.
Walk the chain: supply to load
Trace in order, simplest and most common first.
- Controls and inputs. Is the device set to operate at all - the right mode, not in a lockout, not paused, not in a setback or off position? Many "dead" units are simply off or in a state that suppresses output.
- Safety and limit devices. Most equipment will refuse to run if a safety control is open: a high-limit, a pressure switch, a float, a door interlock, an overload. These are doing their job. Check whether one is open and, if so, why - that open switch is often the real fault upstream.
- The control circuit. Many systems run a low-voltage control circuit that energizes the load. If the control voltage is missing, the load never gets the command. Verify control voltage is present where the load expects it.
- The switching component. A relay, contactor, or solid-state switch sits between the command and the load. Confirm it is receiving its command and actually closing. A coil that energizes but contacts that do not pass power is a classic dead-but-powered cause.
- The load itself. Only after the command reaches the load do you suspect the motor, element, or device. An open winding, a seized mechanism, or a failed component shows here. A capacitor-start motor that hums but will not turn points at the capacitor or a mechanical bind, not the supply.
- The control board, last. Boards are expensive and commonly blamed first and wrongly. Reach the board only after you have proven the inputs are good, the safeties are closed, the command is present, and the switching and load are accounted for.
Use the no-action matrix
| What you find | Likely cause | Next step |
|---|---|---|
| No voltage at equipment under load | Supply/breaker/connection open | Restore and verify supply |
| Power present, no demand signal | Control/thermostat/float not calling | Confirm and correct the call |
| A safety/limit device is open | Safety tripped (often a real fault upstream) | Find why it opened |
| Control voltage missing at load | Control-circuit fault | Trace control circuit |
| Switch device commanded but not passing power | Failed relay/contactor | Test and replace switch |
| Command reaches load, load dead | Open winding, seized, failed component | Test the load |
| All inputs good, switching good, load good | Control board | Replace board last |
The order matters: each row you confirm tells you whether to stop or keep walking. Skipping to the board and finding it fine wastes a part and the visit.
Close it with root cause
When you find the failed item, do not stop at the swap. A tripped safety or a failed switch usually has a reason - ask why it tripped or failed before you call it done, or you will be back. A control that did its job correctly is pointing you at the real fault, not asking to be replaced.
Bank this: power present but no action is a trace job, not a guess job. Prove the supply, confirm the demand, then follow command-to-load through the safeties and the switch before you ever blame the board.
References
- Manufacturer wiring diagrams and sequence-of-operation documentation for the equipment class
- NFPA 70 and NFPA 70E for safe electrical verification and work practice
- Basic meter-use and circuit-tracing practice (digital multimeter, non-contact voltage tester)
- See related: The Five Whys on a Service Call, and Lockout Before You Work