Standby Generator Starts on Test But Fails on an Actual Outage: Decision Tree

Why this matters

The worst possible failure mode for a standby generator is one that hides during weekly self-test and only shows up during a real utility outage, exactly when the customer needs it. The reason this happens is that the exercise cycle and a real outage exercise different circuits. The weekly test commands the engine to run but typically does not transfer load or rely on the utility-loss sensing and transfer-switch operation that a real outage requires. So a generator can pass every Tuesday test for months while a fault in the automatic transfer switch (ATS), the utility-voltage sensing, or the start signal sits undetected. Diagnosing this means deliberately recreating the outage path, not just watching another self-test pass. Getting it right is the difference between a generator that protects the customer and a very expensive lawn ornament.

Symptom presentation

The unit runs flawlessly during its scheduled exercise. During an actual power loss it either never starts, starts but never transfers load, or transfers but the engine quits under load. The customer reports the lights stayed off in the last storm despite a generator that "ran fine on its test the week before." The controller history may show successful exercise cycles and either no outage event logged at all (the start signal never came) or a logged start that failed to carry load.

Quick checks

Read the controller event/exercise history and look for the last real utility-loss event versus exercise events. Confirm the exercise mode: is it set to no-load (engine only) or load-transfer exercise? A no-load exercise never proves the ATS. Simulate a real outage safely by opening the utility supply to the ATS (turn off the main utility breaker feeding the transfer switch) and watch the full sequence: utility-loss sense, start signal, crank, run warm-up, transfer to generator, load pickup. Note exactly where it stalls. Verify the ATS is in AUTO, not OFF or MANUAL, and that any controller dip switches or settings for utility sensing are correct.

Isolation tree

Drop the utility to the ATS and observe. Did the engine receive a start signal and crank? No crank: branch to the start path, the utility-loss sensing or the start signal from the ATS to the controller is not getting through, check the ATS utility-sense wiring, the engine-start (two-wire start) connection, and the controller AUTO state. Cranks but will not start on the outage when it starts fine on exercise: branch to fuel/sensing differences, on a no-load exercise the unit may run on minimal demand, while an outage start under cold or full conditions exposes a weak battery, fuel-pressure, or choke issue, recheck battery under crank load and fuel supply. Starts and runs but never transfers: branch to the transfer mechanism, the ATS contactor or its transfer coil/control is failing to switch from utility to generator, inspect the transfer solenoid, mechanical linkage, and the controller transfer command. Transfers then drops load: branch to a load or undervoltage/frequency fault, the engine cannot hold the real load it never saw on a no-load test, check governor, fuel volume, and the controller under-frequency/under-voltage shutdown log.

Confirming diagnosis

The diagnosis is confirmed by watching the full simulated-outage sequence and identifying the exact stage that fails, which a no-load exercise can never reveal. A start-path fault shows no crank on utility loss but a normal crank on a manual start, isolating the sense/start signal. A transfer fault shows a running engine that never energizes the load bus despite a transfer command. A load-handling fault shows successful transfer then a controlled shutdown logged as under-frequency, under-voltage, or overload, distinguishing it from a no-start. Repeating the simulated outage after the fix and seeing a clean end-to-end transfer confirms the repair.

Remediation

Set the exercise to a load-transfer exercise where the equipment allows it, so future tests actually prove the ATS and load path. Repair the utility-sense or two-wire start wiring and confirm the ATS is in AUTO. Replace a weak battery and correct fuel supply (pressure and volume) that only fails under a real cold or loaded start. Service or replace a failing ATS transfer contactor, coil, or linkage. Address governor, fuel-volume, or sizing issues behind a load-drop, and clear the controller fault. Always finish by recreating a full simulated outage and verifying the complete sequence start-to-load.

References

• NFPA 110 Standard for Emergency and Standby Power Systems, transfer-switch and testing provisions
• NEC 2023 Article 702 Optional Standby Systems and 700/701 where applicable, including transfer equipment requirements
• Generator manufacturer controller and ATS service manual, exercise and transfer sequence (cite the installed model)
• NFPA 110 Chapter 8 routine maintenance and operational testing requirements