Generator Exercise Cycle Failed But Runs On Power Loss Decision Tree

Why this matters

A unit that completes its real-outage transfer cleanly but never starts on its weekly exercise is the most overlooked failure mode in standby service. NFPA 110 requires monthly exercise for emergency and legally required systems; residential units are weekly by OEM default and the homeowner only notices when the neighbor's unit fires and theirs does not. By the time it matters in an outage, the battery is unmaintained, the engine has not warmed, and the fuel system has not flushed condensation. The diagnosis lever is controller scheduling, time-of-day, and the ATS exercise relay - not the engine or fuel system. Walk the controller side first.

Symptom presentation

• Owner says the unit "never runs on its own." Pull the event history. Generac Evolution shows it under Maintenance Log; Kohler RDC2 under Event History; Cummins under Operator History.
• Did the controller attempt to start (crank record) or did the event not fire at all? Crank-attempted-and-failed is a fuel or start problem; never-attempted is a scheduling or relay problem.
• Has the homeowner manually started recently? A manual start clears fault latches; the next scheduled exercise may succeed by accident, hiding the problem.
• What date/time does the controller think it is? Power-loss resets revert some models to factory defaults.

Quick checks at the controller

• Date and time: verify against actual. Daylight-saving transitions trip controllers without a battery-backed RTC.
• Exercise schedule: day, time, duration, transfer-or-not. Generac Evolution defaults to weekly without load transfer.
• Exercise mode: Quiet Test vs Run Test. Some models offer a low-RPM exercise. A homeowner who switched to Quiet Test may want to switch back.
• Controller backup battery (coin cell on some models): replace if over 5 years old.
• Active alarms block scheduled exercises (overcrank, low oil, undervoltage). Clear the fault and force an exercise.

Isolation tree

Branch A - controller never attempted the exercise:

1. Schedule was wiped or wrong. Reset per the OEM manual. Verify by setting an exercise for 5 minutes from now and watching.
2. Daylight saving caused a missed event. Some controllers re-schedule against the saved time-of-week and miss the seasonal shift. Update firmware if a fix is published, otherwise educate the owner.
3. The schedule includes a "skip if recently run" rule. If the homeowner ran the unit manually 48 hours ago and the controller is set to skip exercise within 7 days of a real run, the next exercise will be delayed. This is correct behavior; not a fault.
4. Controller AC sense lost: the controller needs to see utility AC to know to fire on exercise (not on outage). A failed AC sense fuse or sense wire from the ATS leaves the controller in "outage" mode permanently - it will not exercise because it thinks it is already running. Verify by reading the AC sense voltage at the controller terminals.

Branch B - exercise event posted, engine cranked, did not start:

This is a start failure that only manifests on exercise because the homeowner does not see real-outage starts often enough to notice the pattern. Branch into the cold-vs-warm no-start tree. The battery state of charge is the leading suspect: a 36-month-plus battery that the maintainer kept above 13.0 V passes a manual start by surface charge and fails the unscheduled exercise after the surface charge dissipates.

Branch C - exercise event posted, engine ran, no ATS transfer occurred:

Correct behavior if the schedule is "no-load exercise." If the owner expected a transfer test, the schedule needs the "exercise with transfer" option enabled. Note: NFPA 110 transfer-test requirements apply to commercial Level 1 systems on monthly transfer test; residential exercise without transfer is the OEM default and is acceptable for non-NFPA installations.

Branch D - exercise event posted, engine ran, ATS transferred, then unit ran abnormally:

The exercise itself worked - the abnormality during the run is its own diagnosis. Cross to the runs-then-shuts-off tree or the low-oil tree.

Battery degradation - the silent killer of exercise

A standby battery on a healthy maintainer reads 13.4-14.2 V at the terminals. The same battery may have degraded internal resistance to the point where the cranking current droops it below 9.0 V under starter load. This unit will pass a manual front-panel start by accident if you press the button right after the maintainer brought the surface charge up, but it will fail the 2:00 AM exercise when the battery is at its lowest state. The fix is battery replacement on calendar, not on test result.

OEM service intervals for the residential air-cooled units typically call for battery replacement at 36 months regardless of measured condition. Compliance with the interval matters because the battery is the most common single cause of an unattended exercise failure that becomes an outage no-start.

ATS exercise contact - rarer but real

On some installations the ATS has its own exercise relay (typically the older Generac RTSY series) that triggers on a controller signal. A failed relay leaves the engine willing to exercise but the ATS unwilling to allow the start signal. Verified by jumping the exercise relay coil terminals briefly and watching the engine attempt to start. If it cranks with the jumper but not without, the relay is the fault. Replace.

Confirming the diagnosis

Three live confirmations:

• Force an exercise from the front panel and watch it complete (engine starts, runs the configured duration, shuts down clean).
• Verify the controller log shows the forced exercise was recorded correctly.
• Schedule the next normal exercise for a time when the technician can verify remotely (Generac Mobile Link, Kohler OnCue) or the homeowner can confirm.

If the forced exercise from the panel works but the scheduled exercise does not, the fault is in the schedule storage or the RTC - replace the controller battery, reset the schedule, and verify next-week.

Remediation summary

• RTC or schedule wipe: reset schedule, replace controller backup battery, document the firmware revision in the service record.
• Battery degradation: replace battery at calendar interval, verify maintainer is putting out healthy voltage at the battery terminals.
• AC sense wiring: repair or replace sense wire, retest.
• ATS exercise relay: replace, retest with full exercise + transfer cycle.
• Exercise mode misconfiguration: confirm with owner whether Quiet Test or Run Test is desired, configure accordingly.

After any of these repairs, the next two scheduled exercises must complete cleanly before the call is closed. A unit that runs once after the repair and fails the following week is unfixed.

References

• NFPA 110 Standard for Emergency and Standby Power Systems, 2022 edition, Chapter 8 on operational testing and exercise requirements.
• Generac Evolution 2.0 Controller Owners Manual (current revision), exercise scheduling and event history.
• Kohler RDC2 Controller Operation Manual TP-6804.
• Cummins PowerCommand 1.1 Controller Service Manual, exercise schedule programming.
• IEEE 446 Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications.