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Generator Maintenance
Checklist

The complete commercial generator maintenance checklist — daily, weekly, monthly, and annual. Keep your generator running at peak performance and avoid costly breakdowns.

47Checklist items
5Maintenance intervals
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Complete Generator Maintenance Checklist

Click each item to mark it complete. Your progress is saved in this browser. When finished, use the print button to save or print a PDF for your records. The schedule table below marks which tasks NFPA 110 requires and which follow manufacturer schedules. Your engine manual and your authority having jurisdiction (AHJ) have the final word. Why each check matters →

Daily
Daily Inspection
Prime power or a set running through an outage: every 24 hours of operation
0 / 9 ▼
Fuel System
Check fuel level — confirm the tank still holds the runtime your site needs
A tank lower than the gauge said is a known cause of failed runs. Order fuel before you reach your refill point, not at it.
Fuel
Inspect fuel lines and connections for leaks, cracks, or loose fittings
Any fuel odor or visible wetness should be investigated immediately.
Critical
Check for water or sediment in fuel — inspect sight glass or drain petcock
Water-contaminated fuel causes injector damage and no-start conditions.
Fuel
Fluids & Engine
Check engine oil level — top up if below the minimum mark
Check on level ground with the engine stopped. After a run, wait about 10 minutes for the oil to settle (Cummins). Never overfill.
Critical
Check coolant level in the overflow reservoir
Only check when engine is cold. Never open a hot radiator cap.
Safety
Inspect for oil or coolant leaks around the engine block, hose connections, and fittings
Note any new stains on the floor beneath the unit.
Critical
Electrical & Safety
Check battery charger — confirm "float" or "ready" status on the indicator
Weak or undercharged starting batteries are the most common cause of standby power failures (Cummins). A faulted charger lets the battery run down quietly.
Critical
Verify the controller is in AUTO and shows no active alarms or fault codes
"Not in auto" is a top no-start cause (EC&M). Document and investigate any fault code before clearing it.
Safety
Inspect surrounding area — ensure no debris, flammables, or obstructions within 3 feet
Maintain clear access to all sides of the generator at all times.
Safety
Weekly
Weekly Checks
Every 7 days — the NFPA 110 weekly inspection
0 / 8 ▼
Test Run
Inspect the whole system and, if a weekly exerciser is fitted, watch it start and run
NFPA 110 calls for a weekly inspection, not a weekly run. A no-load start proves the engine cranks, not that it carries load, and long no-load running causes wet stacking, so keep it short (Cummins). The loaded monthly exercise is what prevents wet stacking.
Critical
Record operating voltage, frequency (Hz), and oil pressure whenever the set runs
Log readings in your maintenance record. Deviations from spec indicate developing issues.
Listen for unusual noises — knocking, rattling, or excessive exhaust smoke
Black smoke = overfueling or air restriction. White smoke = coolant leak or cold start. Blue smoke = oil burning.
Safety
Mechanical
Inspect drive belts for wear, cracking, or improper tension
A failed belt will shut down cooling and charging. Check deflection against manufacturer specs.
Check the air filter service indicator — inspect for dirt or restriction
Cat's standby schedule checks the indicator weekly. A restricted intake cuts power, raises exhaust temperature and can make the turbo slobber oil.
Fuel
Inspect exhaust system for leaks at joints, flanges, and flexible sections
Inspect with the set running (Cummins). Exhaust leaks indoors or near air intakes are a carbon monoxide hazard: repair them at once.
Safety
Electrical
Inspect batteries — electrolyte level or voltage, and terminals clean and tight with no white/blue corrosion
NFPA 110 requires weekly battery inspection, plus monthly specific gravity testing and recording for lead-acid cells. Loose or corroded terminals add resistance that makes starting harder (Cummins).
Inspect the automatic transfer switch (ATS) — in AUTO, normal source indicated, no alarms
NFPA 110's weekly inspection includes transfer switches. Electrically operating the switch is the monthly test below.
Critical
Monthly
Monthly Maintenance
Every month — NFPA 110 loaded exercise and transfer test
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Load & Transfer Testing
Exercise the generator for at least 30 minutes at 30% or more of nameplate kW, or at the maker's minimum exhaust temperature (NFPA 110)
Building load counts. You only need a load bank when the building can't get you there (see the annual test). This loaded run, not a weekly no-load start, is what prevents wet stacking.
Critical
Operate each transfer switch — electrically transfer from normal to emergency and back (NFPA 110)
Start the test from the ATS test switch. Transferring the real building load also counts toward the 30% exercise. Put the controller back in AUTO afterwards.
Critical
Log the run — load (kW or % of nameplate), run time, exhaust temperature, voltage, amperage, frequency and coolant temperature
NFPA 110 requires a permanent record of inspections, tests and repairs. Trends over months catch problems before they become failures.
Fuel System
Drain and inspect primary fuel filter/water separator — replace if contaminated
Water in diesel promotes microbial growth (diesel bug) which clogs injectors and filters.
Fuel
Check fuel quality — inspect for dark color, cloudiness, or unusual odor indicating degradation
Stored diesel degrades within months. NFPA 110 also requires a lab fuel quality test at least annually (see Annual). Consider fuel polishing for stored reserves.
Fuel
Top up the tank after tests — keep it at the level your required runtime needs
Monthly runs and load bank tests draw the tank down. A keep-full program sets the fill frequency from your test schedule and consumption history.
Fuel
Cooling System
Clean the radiator core and fins with low-pressure air or water, against the normal airflow
Cummins' method. A blocked core can't shed the heat of a loaded engine through a long outage.
Physical Inspection
Inspect all electrical connections and cables for loose terminals, wear, or heat damage
Thermal imaging during operation is best practice for identifying high-resistance connections.
Safety
Check enclosure, base, and mounting bolts — tighten any that are loose
Vibration loosens hardware over time. Check anti-vibration mounts for wear and deterioration.
Every 6 Months
Semi-Annual Service
Every 6 months, or sooner by hour meter on sets that run a lot
0 / 8 ▼
Oil & Filters
Check the hour meter against the oil-change interval — change oil and filter early if hours come due
Standby oil is changed at least yearly (see Annual). Heavy running brings it due sooner: every 250, 500 or 1,000 hours by sump size on Cat 3500B sets. Whichever comes first wins.
Critical
Replace fuel filter(s) at the maker's service interval — primary and secondary if equipped
Clogged fuel filters restrict flow and cause hard starting, low power, and injector damage.
Fuel
Replace the air filter element when the service indicator calls for it — inspect housing gaskets and seals
Cat services the element yearly on standby sets. Dusty or construction sites need it sooner. Follow your manual on cleaning versus replacing.
Battery & Electrical
Load-test the starting batteries — confirm they can deliver cranking current, not just voltage
Resting voltage doesn't show whether a battery can deliver starting power (Cummins). A load tester or a controller battery test does.
Critical
Check and tighten all electrical connections — including control wiring and power cables
Torque to manufacturer specifications. Use a calibrated torque wrench on main lugs.
Cooling & Belts
Inspect and replace drive belts if showing wear, glazing, or cracking
Replace belts in sets. Mixing old and new belts causes uneven loading and premature failure.
Test the coolant — freeze protection and condition — and replace it at the interval for its type
Cat's standby schedule samples coolant yearly and replaces conventional heavy-duty coolant every 3 years, extended-life coolant every 6. Never mix coolant types.
Inspect hoses, clamps, and flexible couplings — replace if brittle, cracked, or swollen
Squeeze hoses — they should be firm but pliable. Spongy or rock-hard hoses are near failure.
Annual
Annual Major Service
Every 12 months, or sooner by service hours — full service
0 / 13 ▼
Engine Service
Change engine oil and oil filter, and send an oil sample for analysis
Oil ages in the crankcase even when the set barely runs, so Cat's standby schedule changes it every year. Drain it warm. Rising wear metals in the analysis point to bearing and wear-surface problems.
Critical
Check and adjust valve clearances at the interval in your engine manual
Out-of-spec valve lash costs compression and performance. Some engines call for it less often than yearly.
Perform compression test on all cylinders — document and compare to spec
Compare cylinders with each other and with the maker's figures, and trend results year to year to catch gradual ring and valve wear.
Inspect and test injectors — check spray pattern, opening pressure, and flow rate
Worn injectors waste fuel and cause smoke. Rebuild or replace injectors that are out of spec.
Fuel
Inspect turbocharger (if equipped) — check for shaft play, oil leaks, and housing cracks
Check shaft play against the maker's limits. Oil at the turbo or exhaust can also come from long low-load running or a plugged air filter (Cat).
Load Testing
Annual load bank test — if monthly runs didn't reach 30% of nameplate or the minimum exhaust temperature
NFPA 110 (2010 and later): 50% of nameplate for 30 minutes, then 75% for 60 minutes, 1.5 hours total. Level 1 systems also need a test at least every 36 months (see the schedule below).
Critical
Fuel Tank & System
Inspect the fuel tank for water, sludge and microbial growth, and make sure the vent is clear
Diesel bug grows where condensation collects. A blocked vent pulls a vacuum that can starve the engine at higher loads.
Fuel
Send a fuel sample to a lab — water, sediment, microbial growth and stability
NFPA 110 requires a fuel quality test at least annually, using tests approved by ASTM standards. It catches contamination before it plugs filters.
Fuel
Electrical & Controls
Check battery age — replace starting batteries on the maker's schedule, and any defective battery at once
Cat's standby schedule replaces batteries every 3 years; Cat dealer Cleveland Brothers says every 2–3 years. NFPA 110 requires defective batteries to be replaced immediately.
Critical
Service the automatic transfer switch — check connections, look for overheating and contact erosion, clean out dust
NFPA 110's transfer switch maintenance program covers these, plus replacing contacts when required. Leave the switch in AUTO with downstream breakers closed.
Critical
Test all safety shutdowns — low oil pressure, high temperature, over/under speed, and overcrank
Use a calibrated test set. Never bypass safeties for testing — use proper simulation procedures.
Safety
Perform megohmmeter (insulation resistance) test on windings — document results
Winding insulation degradation is invisible to the eye. Annual megohm testing catches problems before failure.
Update and file a complete service record — include all readings, parts replaced, and technician sign-off
NFPA 110 requires a permanent record: date, who did the work, problems found and the corrective action. Cummins notes the log also matters for warranty claims.

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Daily (9 items)0%
Weekly (8 items)0%
Monthly (9 items)0%
Semi-Annual (8 items)0%
Annual (13 items)0%

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Why it matters

Why generator maintenance is non-negotiable for commercial operations

A generator that fails during a power outage means lost production, spoiled inventory, failed inspections and, in hospitals and other life-safety sites, risk to people. The engine itself is rarely the problem. Cummins names weak or undercharged starting batteries as the most common cause of standby power failures. Controls left out of auto, cooling faults, fuel problems and a transfer switch nobody tested make up much of the rest, and every one of them shows up on a routine check. Our diesel generator maintenance guide explains why each check matters.

NFPA 110 (Standard for Emergency and Standby Power Systems) sets the minimum testing and maintenance for emergency power systems where it applies. Your local code or AHJ decides whether it applies to your site, and which edition. CMS, for example, requires hospitals to meet it (42 CFR 482.15(e)). Sites outside its scope can still use it as a benchmark. NFPA 110 also says the program must be based on the manufacturer's recommendations and instruction manuals, so most items above follow manufacturer schedules (Cat, Cummins), not the standard itself.

⚠ Batteries fail most often. Don't let fuel be next

Cummins names weak or undercharged batteries as the most common cause of standby failures. Fuel problems are a major cause too: a tank lower than the gauge said, air in the fuel system of a set that rarely runs, or contaminated fuel that plugs filters once the engine draws hard. This checklist covers fuel at every interval, and a scheduled on-site fuel delivery plan keeps the tank at the level your runtime needs.

Daily

Prime power and long outages

A standby set gets its walk-around weekly. Once a set runs for real (prime power, or an outage past a day or two), check fuel, oil, coolant, the charger and alarms every day.

Weekly

Inspection, not a load test

NFPA 110 calls for a weekly inspection of the whole system, transfer switches and batteries included. A short no-load exerciser start proves the engine cranks, not that it can carry load.

Monthly

Loaded exercise required

NFPA 110: at least 30 minutes at 30% of nameplate kW or the maker's minimum exhaust temperature, plus operating each transfer switch. Building load is fine. A load bank is only needed when it falls short.

Annual

Full professional service

Oil and filters, the fuel quality test, battery age, ATS maintenance and shutdown tests. Cat's manual leaves everything past the weekly checks to an authorized service technician or dealer.

Common mistakes

The 5 most common generator maintenance mistakes

Mistake 01

Running at low load

Running diesel generators below 30% load causes wet stacking — unburned fuel accumulates in the exhaust system, leading to carbon buildup, injector fouling, and premature engine wear. Keep no-load runs short and do the monthly exercise under real load. Kohler says some modern common-rail engines can be exercised unloaded if they get an annual load bank test, but not engines with aftertreatment that dislikes low temperatures. Check with your engine maker first.

Mistake 02

Neglecting the fuel tank

Stored diesel degrades within months and picks up water and microbial growth (diesel bug). NFPA 110 requires a fuel quality test at least annually, and periodic fuel polishing keeps stored reserves usable.

Mistake 03

Never actually transferring

A weekly exerciser proves the engine starts, not that the switch moves. NFPA 110 calls for operating every transfer switch monthly, and EC&M lists "not in auto" and a tripped breaker after the ATS among the top reasons generators fail to start.

Mistake 04

Ignoring battery age

Cat's standby schedule replaces batteries every 3 years, and Cat dealers say every 2–3. In between, load-test them: a battery can show normal resting voltage and still fail to crank.

Mistake 05

No fuel delivery plan

The most sophisticated maintenance program fails if there's no fuel when you need it. Establish a scheduled on-site fuel delivery relationship with a 24/7 provider before an emergency arises.

Maintenance log reference

Generator maintenance task quick reference

Use this summary table for quick scheduling reference or to build your own maintenance calendar. NFPA 110 required means the standard's routine maintenance chapter calls for it, where your AHJ enforces NFPA 110. Per manufacturer means follow your engine's maintenance manual (Cat standby figures shown). Recommended is good practice beyond both.

Generator maintenance schedule — all intervals at a glance
TaskIntervalWho performsBasis
Fuel, oil & coolant level checkDaily (prime / long outage)OperatorRecommended
Controller in AUTO, alarm checkDaily (prime / long outage)OperatorRecommended
System inspection, incl. transfer switches & batteriesWeeklyOperatorNFPA 110 required
No-load exerciser start (if fitted)Weekly — keep it shortOperatorOptional
Air filter service indicatorWeeklyOperatorPer manufacturer
Loaded exercise: 30 min at ≥30% nameplate kW or min. exhaust tempMonthlyOperator / technicianNFPA 110 required
Transfer switch operation (normal → emergency → normal)MonthlyOperator / technicianNFPA 110 required
Lead-acid battery specific gravityMonthlyTechnicianNFPA 110 required
Fuel filter / water separator drainMonthlyTechnicianRecommended
Battery load test6 monthsTechnicianRecommended
Fuel filter replacementService intervalTechnicianPer manufacturer
Oil & filter changeYearly, or by hours if soonerTechnicianPer manufacturer
Coolant test / replacementTest yearly; replace 3 yr (conventional) or 6 yr (extended-life)TechnicianPer manufacturer
Supplemental load bank test: 50% for 30 min, then 75% for 60 minAnnual, if monthly runs fall short of 30%TechnicianNFPA 110 required
Fuel quality test (ASTM methods)AnnualLabNFPA 110 required
Transfer switch maintenance (connections, contacts, cleaning)Annual (NFPA 110 sets no interval)TechnicianNFPA 110 required
Compression testAnnualTechnicianRecommended
Injector inspection/testAnnualTechnicianRecommended
Winding insulation testAnnualTechnicianRecommended
Battery replacementAbout every 3 years (Cat); 2–3 years (dealers)TechnicianPer manufacturer
Level 1 duration test at ≥30% loadAt least every 36 monthsTechnicianNFPA 110 required (Level 1)
Written record of every inspection, test and repairEvery timeOperator / technicianNFPA 110 required

NFPA 110 sorts systems into Level 1 (where failure could cost lives or cause serious injury) and Level 2 (less critical). The weekly inspection, monthly loaded exercise, monthly transfer switch test, annual supplemental load test (when monthly runs fall short) and annual fuel quality test apply to both. Level 1 systems add a test at least once every 36 months, run for the system's class duration at 30% load or more, which may stop after 4 hours. At either level, the program must be overseen by a properly instructed individual, and every inspection, test and repair goes in a permanent record.

If your generator supports a critical operation and you don't yet have a scheduled fuel delivery plan in place, that's the most important single action you can take today. Emergency fuel delivery runs 24/7/365, and customers with site details already on file are dispatched first. Call us at 800-720-4546 to set up a delivery schedule for your site.