Generator Cranks But Does Not Start Cold Versus Warm Decision Tree

Why this matters

A unit that cranks healthy but never fires is the single biggest reason a standby generator earns a useless tag from the homeowner after the storm. The split between cold (under 40 F) and warm (over 60 F) failure is the cleanest first-cut diagnosis in the trade because the engine that fires warm and not cold is almost always a battery, oil viscosity, or fuel-vaporization issue. The reverse points at vapor lock, a hot ignition coil, or a saturated absorber. Walk this split first; it eliminates 70 percent of the parts catalog before you open the enclosure.

Symptom presentation

Confirm the crank itself before chasing fire. Energizes-but-no-crank is a battery or starter solenoid call and exits this tree.

• Crank RPM should sound healthy. A labored crank is low battery, tight engine (oil too thick), or failing starter.
• OEM allows two or three crank cycles before posting Overcrank. Generac Evolution 2.0 cranks 16 seconds on, 7 off, three attempts.
• Cough-and-die is a fuel-delivery starve, not a no-fire - belongs in the fuel branch.

Photograph the active alarm and history. Overcrank latches and prevents auto-restart until cleared.

Quick checks before deep dive

• Battery resting voltage 12.6 V minimum at room temperature.
• Loaded crank voltage above 9.6 V. Sag to 8.5 V with a known-good battery means high starter draw.
• Charger output 13.4-14.2 V at the battery terminals with the unit at rest.
• Oil level on the dipstick. Low oil locks out crank on some controllers and falsely posts overcrank.
• Manual fuel shutoff valve fully open.
• LP tank percentage. Below 20 percent will not vaporize in cold weather.

Cold-fail / warm-pass branch (fires above 60 F, refuses below 40 F)

This is the dominant cold-climate callback in November-February. Three sub-causes in field-probability order:

1. Battery state of charge. A 36-month-plus battery still showing 12.5 V at room temp will collapse to under 9 V when ambient drops to 20 F. Replace, do not load-test. Group 26R AGM is the typical OEM spec; check the OEM service manual for the exact group and CCA rating.
2. LP vapor pressure. Propane vapor pressure at 20 F is approximately 25 psig; at 0 F it drops to about 16 psig. A 100 gallon ASME tank at 30 percent fill in 0 F weather cannot vaporize fast enough to feed a 20 kW engine on first crank-rich enrichment. Customer needs a larger tank or a tank heater (per local code).
3. Cold-start fuel solenoid (LP units). Some Generac LP units use an electrically-heated regulator vaporizer. Probe the regulator heater for 12 V during crank.
4. 5W-30 vs 10W-30 oil. A unit filled with 10W-30 in summer service that sees a 5 F morning will crank too slowly to fire. Drain and refill with the correct cold-weather grade per the OEM service manual.

Warm-fail / cold-pass branch (fires below 40 F, refuses above 80 F)

Less common but it stumps technicians because the cold workaround (just try again) does not apply.

1. Vapor lock on LP vapor draw. Hot enclosure temperature drops vapor delivery; the line between the regulator and the fuel mixer warms past the dew point and the regulator output flashes. Verify with an infrared thermometer on the regulator body and the fuel line during a failed crank. Symptom: cranks, will not fire, fires immediately after the enclosure is opened and the line cools.
2. Ignition coil hot-fail. Coil ohms fine cold, opens or shorts at operating temperature. Tested by allowing the unit to soak in summer sun, attempting start, and probing primary coil resistance immediately. Replace as a pair on twin-cylinder units.
3. Fuel mixer / venturi diaphragm. A failed diaphragm on the Impco or Garretson-style mixer used by Generac and Kohler will leak vacuum when the rubber is hot and pliable, sealing fine when cold and stiff. Swap the mixer assembly.
4. Spark plug fouling. Plugs that are oil-fouled from a previous run will sometimes light cold but never hot. Pull, inspect, gap to OEM spec, replace if bridged.

Universal fail (no fire cold OR warm)

If the unit does not fire at any temperature and the controller is not posting a relevant fault, the engine is missing one of the three fundamentals: fuel, spark, or compression. Quick triage in order:

• Spark test. Pull a plug wire, ground it through a spark tester, crank. Blue spark across the gap means primary ignition is fine. No spark on one cylinder means coil; no spark on either cylinder is a stop-engine relay, a control board fault, or a flywheel key sheared after a low-oil shutdown.
• Fuel at the cylinder. Disconnect the fuel hose at the carburetor or mixer, crank, observe vapor or liquid. Nothing means fuel solenoid not opening, regulator stuck, or upstream shutoff closed. The fuel solenoid should click audibly during crank.
• Compression. A small-engine compression tester reads 90 psi minimum on a twin-cylinder air-cooled. Below 60 psi means valve adjustment or rocker arm failure - common on Kohler 20 kW after 1500 hours.

Confirming diagnosis before quoting

The three confirmations that prevent return trips:

• Controller event log photographed and the last pre-shutdown event noted.
• A live measurement of the failing fundamental (spark, fuel pressure under crank, or compression) - never assume.
• A successful three-cycle re-start at the failure temperature, not at the test-bench temperature.

Remediation by branch

• Cold-fail battery: replace battery, retest at ambient or with a known cold-soak.
• Cold-fail LP vapor: customer upgrades tank or adds a tank heater per local LP code; do not attempt to repair away a fuel-physics limit.
• Cold-fail oil: drain and refill with cold-weather-grade oil per the OEM table.
• Warm-fail vapor lock: route the fuel line away from hot exhaust components; some units take a thermal sleeve kit.
• Warm-fail coil: replace coils as a pair on twin-cylinder, gap plugs to OEM spec.
• Warm-fail mixer: replace mixer assembly; do not rebuild a 10-year-old diaphragm with an aftermarket kit.
• Compression-out: valve adjustment per the service manual; rocker replacement for the Kohler 20 kW.

References

• NFPA 110 Standard for Emergency and Standby Power Systems, 2022 edition, sections on operational testing and routine maintenance.
• NFPA 54 National Fuel Gas Code, 2024 edition, regulator and piping requirements.
• Generac Owners Manual Air-Cooled Standby (current revision), fault definitions and crank cycle timing.
• Kohler RDC2 Controller Operation Manual TP-6804, event history.
• Propane Education and Research Council Vapor Pressure Tables for LPG.