Generator Fixed No-Start Now Runs But No Transfer: Second Fault Masking Decision Tree

Why this matters

A common service pattern: the generator was not starting, the tech fixed it, the engine now runs and produces voltage, but the automatic transfer switch (ATS) does not transfer the load. The customer reads this as "the tech only half-fixed it." The reality is usually that the ATS had its own latent fault that was masked by the no-start. Once the engine became reliable, the ATS fault became visible as the dominant symptom. This tree walks the right order, separates the two events for billing and warranty clarity, and gets to a clean close where the load actually transfers under outage conditions.

Section 1: Confirm the engine side is genuinely healthy first

Before chasing the ATS, confirm the generator is producing the conditions the ATS needs to transfer. The ATS will not transfer to a source that does not meet its sensing requirements, and that is not an ATS fault.

Verify at the generator output terminals:

• Voltage on each phase, against the configured nominal.
• Frequency, stable inside the ATS's transfer band.
• Voltage and frequency stability across a 30 to 60 second window. A generator that hunts in voltage or frequency will time out the ATS transfer window.
• No active alarm on the controller that inhibits transfer (low oil pressure warning, high engine temperature warning, low fuel warning, depending on the configuration).

A generator that "runs" but does not produce stable voltage and frequency at the ATS sensing point is not yet fully fixed. The ATS is doing the right thing by not transferring to a marginal source.

Section 2: Confirm the ATS is seeing the generator

The ATS makes the transfer decision based on what it senses at the emergency source terminals. If the sensing is wrong, the ATS will not transfer even with a healthy generator behind it.

Walk:

• Sense wiring from the generator output to the ATS emergency source sense input. Continuity, correct termination, no opens.
• Sense voltage at the ATS emergency source terminals during a generator run. Compare to the generator output terminals; any large drop indicates a problem in the conductor or termination, not the generator.
• ATS configuration for the emergency source nominal voltage and frequency. A configuration mismatch (208 V configured on a 240 V system, or 60 Hz configured on a system with an off-spec governor) will block transfer.
• ATS time delay settings. A long "warm-up" or "stable" delay can give the impression of no transfer when the ATS is simply waiting out its delay.

Section 3: Confirm the transfer command path

The ATS may be ready to transfer but never get the command, or get the command and not execute it.

Verify:

• The generator controller's "ready to load" or "available" signal back to the ATS, if the ATS topology uses one.
• The ATS controller is in Auto, not Manual or Maintenance.
• Any external bypass or maintenance switch is in the run position.
• The transfer mechanism itself is free, not seized, and the operating handle is in the normal position.

A clean test: with the ATS in Auto and the generator running with stable output, simulate a utility loss per the manufacturer's procedure. The ATS should drop the normal source, wait its configured stable delay, and transfer to the emergency source. If it does not, the failure point in the sequence is your finding.

Section 4: Verify the load side

A transfer that initiates and then immediately drops back can look like "no transfer" but is actually a load side problem.

Check:

• Load served by the ATS. If a downstream short or a major load (large motor starting, EV charger, well pump) is on the emergency feed, the generator may sag below the ATS's retransfer threshold and bounce back.
• Any sub-panel breaker fed by the ATS that may be tripped.
• The ATS contactor or breaker itself for closure. A contactor that clicks but does not fully close is at end of life.

Section 5: Why the masking is normal, not a missed diagnosis

A no-start condition prevents the generator from ever producing voltage. The ATS has no opportunity to attempt a transfer, so any latent ATS fault is invisible from outside. Repairing the no-start makes the generator reliable, which then exposes the ATS issue as the dominant symptom on the next test or outage.

This is the same pattern as a microinverter system where a gateway outage masks a single offline unit. The second fault was always there; it became visible only when the first fault was resolved.

The customer-facing framing matters:

• Do not say "we only half-fixed it."
• Do say "with the engine reliable now, we can see there is an ATS-side issue that was hidden before. Here is what it is and what it takes to repair."

Section 6: Documenting the two events and the billing

Document both events in the work order separately:

• The no-start cause, the parts replaced, the test that confirmed the fix, and the date of close.
• The transfer fault as a new finding, its cause, the parts required, and the test that confirms the close.

For billing:

• The original no-start repair is the original work order. Close and invoice as such.
• The transfer fault is a new finding. Open a separate work order or quote, depending on operator policy.
• If the customer is under a service contract, confirm what each event is covered under. Most contracts cover both because both are mechanical faults, but the customer should see the breakdown.

The final test is an end-to-end outage simulation: utility loss simulated at the ATS, generator starts, voltage and frequency stabilize, ATS transfers the load, load is served from the generator, then the utility is restored, ATS retransfers, generator runs its cooldown, generator stops. If any step is missing, the work order is not closed.

A clean close has the generator running on the next exercise cycle, the ATS transferring on simulated outage, the customer told plainly that two issues existed and both are now corrected, and the documentation showing both events independently.

References

• NFPA 110, Standard for Emergency and Standby Power Systems.
• NEC 2023, Article 445 (Generators).
• NEC 2023, Article 700 (Emergency Systems), Article 701 (Legally Required Standby Systems), Article 702 (Optional Standby Systems).
• NEC 2023, Article 705 (Interconnected Electric Power Production Sources).
• UL 1008, Standard for Transfer Switch Equipment.
• UL 2200, Standard for Stationary Engine Generator Assemblies.