Generator Runs Then Shuts Off Around 30 Seconds Decision Tree

Why this matters

A standby unit that cranks, fires, and runs cleanly for 20-45 seconds before shutting itself off is the most common warm-weather no-power callback. The owner already saw it work, so they assume a small fix. In reality the controller posted a fault during the warm-up walk-in and latched a shutdown to protect the engine or the load. The shutdown class points straight at the failed subsystem if you read the controller log before swapping parts. The same symptom from Generac, Kohler, Cummins, and Briggs is a fuel issue about 60 percent of the time, an alternator/voltage fault about 20 percent, an oil/coolant sensor fault about 15 percent, and an exercise misconfiguration the rest. Walk it in that order.

Symptom presentation

Confirm the actual behavior before you commit a diagnosis path.

• Did it crank, or did it just energize the controller and never crank? Energizes-but-no-crank is not in this tree.
• Did it run under load or only with the ATS still on utility? A unit that runs in exercise but shuts down under load points at voltage or load-shed; a unit that shuts down with no load points at engine protection.
• How many seconds from fire-up to shutdown? Under 10 seconds is usually RPM/AC-sense; 20-45 seconds is fuel; 60-120 seconds with smoke is oil/coolant; over 5 minutes is a thermal or runtime fault.

Read the active alarm and history. Generac Evolution 2.0 shows alarms on the front panel; Kohler RDC2 under Event History; Cummins PowerCommand under Faults. Photograph the screen before you clear anything.

Quick checks before you open the enclosure

• Battery resting voltage 12.6 V or higher. A 12.1 V battery will crank but droop under the AVR field-flash load and trip undervoltage around 20 seconds. Replace, do not recharge, any battery older than 36 months reading under 12.4 V.
• Battery voltage during crank above 9.6 V. An 8.5 V sag with a good battery means high starter draw.
• LP tank at 25 percent or higher and service valve fully open. A tank below 20 percent in cold weather cannot vaporize fast enough to feed a 22 kW load.
• Air filter clean; rodent screen clear.
• Controller event log photographed. The last event before shutdown is the diagnosis.

Isolation tree by controller fault code

Overcrank / Failed to Start latched after a run: Engine ran, then the controller posted Overcrank when it tried to auto-restart. This is a fuel-starvation pattern. Go to the fuel branch.

Low Fuel Pressure / Fuel Pressure Switch: Fuel regulator output is dropping under run load. Static pressure at the inlet of the regulator must be 11-14 inches WC for natural gas, 11-13 inches WC for LP vapor, measured with the engine running. If static is good but it drops below 7 inches WC during run, the regulator is undersized or failing. Verify pipe sizing per the OEM installation manual; a 22 kW Generac wants 1 inch black iron at minimum past 25 feet of run.

Undervoltage / Voltage Sensing Loss: AVR is not seeing stator output, or the stator is not building voltage. With the engine running just before shutdown, measure stator AC output at the AVR sense leads. If under 200 VAC line-to-line, the rotor is not exciting - check brushes, slip rings, and field flash voltage.

Overvoltage: AVR overshooting. Often a failing AVR or a loose stator sense lead. Replace AVR after confirming sense leads are tight and the correct gauge.

Low Oil Pressure (after 30 seconds): Mechanical or sensor. See the dedicated low-oil-shutdown tree before assuming the pump is bad.

High Engine Temperature (rare at 30 seconds): Almost always a coolant sensor short, not a real overtemp - the block cannot heat that fast from cold start. Ohm the sensor cold against the OEM spec table.

No active fault, but it shut down: Walk-in exercise completed normally and you are reading the wrong symptom. Confirm with the owner that this happens during a real outage, not during a Wednesday exercise.

Fuel branch - the 60 percent case

Most 30-second shutdowns are fuel-starvation under load. The unit fires on residual line pressure, runs while pressure drops, then the controller catches low fuel pressure or stalls and reads it as overcrank.

1. Verify tank/meter capacity. A 22 kW air-cooled at 50 percent load consumes around 3.5 gallons LP per hour or 248 cubic feet of natural gas per hour. Cold-weather vaporization is the limit on small LP tanks.
2. Verify pipe sizing per NFPA 54 sizing tables. A 200 foot natural-gas run to a 22 kW load needs 1 1/4 inch pipe, not 1 inch.
3. Measure static gas pressure at the unit inlet, engine off, all other gas appliances off. Must meet OEM minimum.
4. Measure operating pressure at the unit inlet under rated load. A drop of more than 1 inch WC from static to running means the regulator or the supply line is undersized.
5. Inspect the fuel solenoid. Some Generac air-cooled units use a normally-closed solenoid that loses pull-in voltage when the battery droops mid-run.

Voltage / AVR branch - the 20 percent case

When the engine sounds healthy through shutdown and the controller posts an under- or overvoltage fault, the alternator is the culprit.

• Field-flash voltage from battery to rotor must be present at startup. Probe the field leads at the AVR.
• Brush condition on units with brushes (Generac Guardian, older Kohler): brush length under 1/4 inch or arcing on the slip rings means dropout under load. Replace brushes and dress slip rings with fine emery, never sandpaper.
• Stator output at the AVR sense leads with the engine running just before shutdown: under 200 VAC line-to-line and dropping means rotor collapse - brushes or a shorted rotor winding.
• Replace AVR only after sense, brushes, and rotor are verified. A new AVR will not fix a failed rotor.

Confirming diagnosis before you quote

Do not quote a part until the controller log, a fuel pressure reading under run, and an AC output measurement at shutdown all agree. A clamp meter on the field lead and a manometer on the gas inlet are mandatory tools for this tree.

Remediation by branch

• Fuel starvation: correct pipe sizing per NFPA 54, install or replace the regulator with the OEM-spec part, retest under full rated load for a 10 minute run.
• Brush wear: replace brushes as a matched pair, dress slip rings, retest field voltage and AC output.
• AVR failure: replace AVR after verifying the rest of the alternator subsystem.
• Battery: replace with a Group 26R or 35 AGM per OEM spec, never refurbish.
• Low oil sensor short: replace sensor, retest.

After any repair, force three consecutive walk-in test starts and one transfer-to-load test under the owner's actual load. A unit that passes one start and fails the next still has the original fault.

References

• NFPA 110 Standard for Emergency and Standby Power Systems, 2022 edition, Chapter 8 routine maintenance and operational testing.
• NFPA 54 National Fuel Gas Code, 2024 edition, gas piping sizing tables.
• Generac Owners Manual A0001234567 Air-Cooled Standby Generators (current revision), fault code definitions.
• Kohler Operation and Installation Manual TP-6804 (RDC2 Controller), event history and shutdown classes.
• Propane Education and Research Council Tank Sizing Guide for Standby Generators.