Genset Runs Then Shuts Off On Load vs No Load Decision Tree
Why this matters
An air-cooled standby that starts, runs the warm-up cycle, and shuts down on the load transfer presents a different fault tree than a unit that runs at no-load for 20 minutes and then quits during the weekly exercise. The on-load shutdown is almost always a fueling, governor, or AVR fault triggered by the step increase in demand. The no-load shutdown is almost always a sensor, oil pressure, or controller fault that takes time to drift outside the trip threshold. Treating the two as the same call leads to repeated battery and starter parts swaps that solve neither and burns the customer's confidence in the warranty.
Symptom presentation
Capture the exact sequence from the controller event log. Generac Evolution 2 and 2.1 controllers, Kohler RDC2 controllers, and Briggs InteliLite controllers all log the trip code with a timestamp. The most common codes that map to this complaint family are RPM Sense Loss, Low Oil Pressure, Overcrank, Overspeed, Undervoltage, Overvoltage, Stepper Overcurrent, Wiring Error, and Low Voltage Drop. The code combined with the on-load versus no-load distinction is the diagnostic anchor.
Confirm with the homeowner: did the shutdown happen at exactly the ATS transfer moment or 60 seconds later, during the warm-up cycle, or sometime after the load was carried? Did the lights in the house bounce once and quit, or did the unit run for a quarter hour and then quit? Each timing detail eliminates branches of the tree.
Quick checks at the controller
Pull the event log. Note the last three trip codes and their timestamps. Confirm the controller firmware revision is current; Generac field bulletins published through 2024 cover specific firmware fixes for nuisance Low Oil Pressure and RPM Sense Loss codes on Evolution 2.1 units, and the fix is a firmware update, not a parts swap. Check the battery state of charge with a load tester, not just a multimeter; a battery that reads 12.4 volts at rest can collapse to 9 volts on cranking and trip Undervoltage on the controller before the engine builds RPM.
Isolation tree
Start with the on-load versus no-load distinction.
On-load shutdown branch: the engine is making power until the load transfers, then trips. The candidates are fuel supply pressure, governor response, AVR sense, and load size. Pull a fuel pressure reading at the regulator outlet during a manual exercise run with a load applied. Natural gas labels at the genset inlet for most residential standbys want 5 to 7 inches water column under load; propane wants 11 to 14 inches water column under load. A meter that reads 7 inches at no-load and drops to 3 inches when the load transfers fails the fueling check; the fix is upstream regulator sizing, meter sizing, or line sizing, not the genset. Confirm the connected load with a clamp meter on the load side of the ATS; a 22 kW unit attempting to carry 20 kW with poor power factor is operating at its trip threshold and any startup surge on a compressor or pump will dip the AVR sense and trip Undervoltage.
If fueling is in spec and the load is within the unit's nameplate rating, move to the AVR. Read the AVR sense voltage at the controller terminals during the transfer; the sense should hold at 240 VAC plus or minus 10 percent through the transfer. A momentary drop below 200 VAC trips the Undervoltage protection. AVR replacement on Generac Evolution units is a known-issue part on units with high run-hour totals; verify firmware first to avoid swapping a good AVR.
No-load shutdown branch: the engine runs for a period under the exercise cycle with no transfer and then quits. The candidates are oil pressure sender, temperature sender, governor, and air filter. Read the oil pressure gauge or the controller's oil pressure value during the run; a unit that drifts from 40 psi at startup to 18 psi at 15 minutes is at the Low Oil Pressure threshold and the trip is real, not nuisance. Check engine oil level and oil specification; an air-cooled standby running 10W-30 in summer heat past the OEM recommendation is a known cause of pressure drift.
If oil pressure holds in spec, read the cylinder head temperature or the engine block temperature on the controller. An air-cooled standby with a blocked enclosure inlet or a clogged cooling fin pack will climb above the High Temperature threshold and trip after 15 to 25 minutes. Inspect the enclosure inlet and the cooling fins; squirrel nest debris is a known issue.
If oil and temperature are in spec, move to the governor. A stepper motor that is sticking will let RPM drift below 60 Hz target by 1 to 2 Hz over a long run; the Underspeed protection trips. Stepper Overcurrent codes in the log point directly to this branch.
Confirming the call
Document the trip code, the timestamp, the on-load versus no-load classification, the fuel pressure reading under the operating state that triggered the trip, the load size on the ATS load side, and the controller firmware revision. The defensible diagnosis is the four data points plus the firmware revision. Schedule a confirmation run after any fix; an air-cooled standby should carry the maximum customer load for 30 minutes without a controller code as the operational sign-off.
Remediation by branch
Fuel supply low under load: size the gas line per the NFPA 54 fuel gas code tables to deliver the required cubic feet per hour at the OEM inlet pressure spec. NFPA 110 Type 10 emergency power systems require fuel supply to support full nameplate load. Verify meter sizing with the utility if natural gas is the fuel; a residential meter rated for 250 cubic feet per hour will starve a 22 kW genset under load.
AVR sense fault: replace the AVR with the OEM part for the model and firmware revision. Confirm firmware is current before the swap.
Low oil pressure real: change oil to the OEM spec viscosity for the operating ambient. If the pressure still drifts, the pressure sender or the oil pump is the next step.
High temperature: clear the enclosure inlets, clean the cooling fins, and inspect the cooling fan. Verify clearance distances to obstructions match the OEM installation guide.
Stepper governor: replace the stepper motor or the throttle body assembly per the OEM service bulletin.
NFPA 110 Standard for Emergency and Standby Power Systems requires that any Level 1 system pass a documented monthly exercise under load. A unit that nuisance-trips on the monthly exercise must be tagged out and repaired before it is returned to service; running a known-faulty Level 1 emergency power system into a critical load is a code violation under NFPA 110 Chapter 8 and a potential life-safety exposure. Healthcare, life safety, and standby on critical occupancies are not the place to defer this work.
References
- NFPA 110, Standard for Emergency and Standby Power Systems, 2022 edition.
- NFPA 54, National Fuel Gas Code, 2021 edition, fuel pipe sizing tables.
- NEC NFPA 70 Article 445, Generators, 2023 edition.
- Generac Power Systems, Evolution 2.1 Controller Diagnostic Repair Manual, current revision.
- Kohler Power, RDC2 Controller Service Manual, current revision.