Customer Says It Ran During Last Outage But House Was Dark Translation Decision Tree

Why this matters

"It ran but the house stayed dark" is a translation problem. The customer heard the engine and concluded the generator failed to power the home, but a running engine and a powered house are two separate events linked by the automatic transfer switch (ATS). The engine can run perfectly while the ATS never transfers, or transfers to a panel that is not the one the customer was watching, or the generator started for a scheduled exercise (which deliberately does not power the load) and the customer mistook that for an outage response. Decoding what actually happened keeps you from condemning a healthy generator over an ATS, wiring, or expectation issue.

Symptom presentation

• Engine ran, no transfer occurred, no load energized: ATS failed to transfer, or the generator never reached ready-to-load status.
• Engine ran during a scheduled exercise, not an outage: exercise cycles run the engine without transferring load on most installations; the house staying dark is correct if utility was present.
• Only some circuits stayed dark: a managed/critical-loads panel or load-shed setup, or a tripped branch, not a whole-house failure.
• Engine ran then shut down on a fault, briefly powering then dropping the house: a protective shutdown after transfer.

Quick checks

1. Pull the controller event log: was the start a utility-loss start or a scheduled exercise? Did it reach ready-to-load? Did the ATS command a transfer? Any shutdown fault codes?
2. Confirm what "dark" means: whole house or only certain circuits. A critical-loads subpanel design only powers selected circuits by design.
3. Check the ATS position indicator and any transfer-fault indication. Note whether it sits in Utility or Generator position now.
4. Verify the time correlation: did the run coincide with a real utility outage, or with the programmed exercise day/time?

Isolation tree

Step 1: Was the run an exercise or an outage response? Exercise (matches the programmed schedule, utility was present): the dark house is expected behavior on a standard install; the ATS does not transfer during exercise unless configured for a load test. Explain and confirm exercise settings. Outage response goes to Step 2.

Step 2: Did the controller reach ready-to-load and command a transfer? If the engine started but a fault (low oil pressure, overspeed, high temperature, low/high voltage or frequency) prevented ready-to-load, the ATS correctly held on utility/open and never transferred. Diagnose that engine/alternator fault. If the controller reached ready-to-load and commanded transfer, go to Step 3.

Step 3: Did the ATS actually transfer? Inspect ATS position and transfer history. If it stayed on Utility:

• Check the transfer control signal/wiring between controller and ATS.
• Check the ATS mechanism: a stuck contactor, failed transfer solenoid/motor, or a mechanical bind.
• Check for an open in the transfer-switch control circuit (blown control fuse, loose terminal). If it transferred to Generator but the house was still dark, go to Step 4.

Step 4: Transfer happened but no power at loads.

• Confirm generator output voltage at the ATS line side during the run (from the log or a re-test). No output means an alternator/voltage-regulation fault despite the engine running.
• If output was present at the ATS but loads were dark, suspect the conductors from the ATS to the panel, a tripped main, or a fault in the powered panel. On a managed-loads system, confirm load-shed did not strand the watched circuits.

Step 5: Powered then dropped. If the log shows transfer then a shutdown fault, the generator carried load briefly then protected itself (overload, fuel, cooling, oil). Diagnose the shutdown cause; the customer saw lights flicker on then off.

Confirming diagnosis

• Exercise, not failure confirmed when the run time matches the schedule, utility was present, and no transfer was commanded; behavior is correct.
• Engine fault blocked ready-to-load confirmed by a start followed by a protective fault with no transfer command.
• ATS failure confirmed when ready-to-load and transfer command occurred but the switch stayed on utility, or transferred mechanically but passed no power.
• Downstream/managed-loads confirmed when generator output reached the ATS line side but only certain circuits were dark by design or by a tripped device.

Remediation

• Exercise misunderstanding: confirm/adjust exercise schedule and, if the customer wants a periodic load test, configure or recommend one; document.
• Engine/alternator faults blocking ready-to-load: diagnose and repair the specific shutdown cause (oil, fuel, cooling, voltage regulation) before re-test.
• ATS transfer failure: repair or replace the failed transfer mechanism, solenoid/motor, contactor, or control wiring; verify a commanded transfer end to end.
• Downstream: reset tripped devices, verify ATS-to-panel conductors, and explain managed-loads behavior; reconfigure load priorities if needed.

References

• NEC (NFPA 70) Article 702, Optional Standby Systems
• NEC (NFPA 70) Article 700, Emergency Systems (where applicable)
• NFPA 110, Standard for Emergency and Standby Power Systems
• UL 2200, Stationary Engine Generator Assemblies