Commercial Generator Troubleshooting: Common Problems, Causes and When to Call for Service

When a commercial generator has a problem, the symptoms can often provide important clues about the cause.

A generator that won't start may have a battery, fuel, or control issue. A generator that starts but stops unexpectedly could be experiencing overheating, low oil pressure or a fuel-delivery problem. And if the generator is running but your facility still has no emergency power, the problem may not be the generator at all—it could involve the automatic transfer switch (ATS), switchgear, breakers or controls.

For facility managers, the goal of generator troubleshooting isn't necessarily to make the repair yourself. It's to recognize what the system is telling you, gather useful information and know when to bring in a qualified emergency power technician.

Here are some of the most common commercial generator problems and what they may mean.

1. The Generator Won't Start

"Why won't my generator start?" is one of the most common emergency power troubleshooting questions.

There are several possible causes.

Weak or Dead Starting Battery

Battery problems are among the first things to consider when a generator fails to crank. Batteries can deteriorate with age, temperature, corrosion, loose connections or problems with the charging system.

What facility personnel can check:

• Is the generator controller powered?
• Does the display indicate low battery voltage?
• Are there visible signs of corrosion or damage?
• Has the battery or charger recently generated an alarm?
• When was the battery last tested or replaced?

Avoid disconnecting, jumping or servicing generator batteries unless you are trained and authorized to do so.

Generator Is Not in AUTO

A generator that was placed in OFF or MANUAL mode during maintenance may not automatically start when utility power fails. Check the controller from the normal operating position and confirm that the generator appears to be in the appropriate automatic mode.

If you're unsure why the operating mode changed, investigate before simply resetting it.

Fuel Supply Problem

A generator needs an adequate supply of usable fuel to start and continue operating. Depending on the system, problems can include:

• Low fuel level
• Closed fuel valve
• Contaminated fuel
• Fuel restriction due to clogged filter(s)
• Air in the fuel system
• Fuel pump problems
• Old or degraded diesel fuel

For diesel generators, fuel quality can become especially important at facilities where stored fuel remains in the tank for extended periods.

Active Fault or Shutdown Condition

Modern generator controllers monitor numerous operating conditions. A shutdown may prevent the generator from starting until the underlying condition is addressed.

Rather than repeatedly clearing an alarm, record the exact fault code and when it occurred. That information can significantly help a technician diagnose the problem.

2. The Generator Starts but Shuts Down

Sometimes a generator starts normally and then shuts down after a few seconds or minutes. That usually means the system detected an operating condition that required a shutdown or the engine could not continue running properly.

Possible causes include:

• High coolant temperature
• Low oil pressure
• Fuel-delivery problems
• Cooling-system problems
• Sensor or control faults
• Overspeed or under speed conditions
• Excessive electrical load

What Should a Facility Manager Do?

Record:

• The alarm or fault code
• How long the generator ran
• Whether the facility load transferred
• Generator temperature or pressure readings visible from the controller
• Any unusual sounds, smells, smoke or leaks
• What was happening immediately before the shutdown

Do not repeatedly restart a generator that continues to shut down. The shutdown may be protecting the equipment from a more serious problem.

3. The Generator Is Running, but the Building Has No Power

This is an important distinction:

A running generator does not automatically mean the emergency power system is working.

If the generator starts but the required facility loads remain without power, the problem could be elsewhere in the emergency power system.

Potential causes include:

• ATS failure to transfer
• Open or tripped breaker
• Stator issue
• Switchgear issue
• Control or communication failure
• Generator output problem
• Incorrect operating mode
• Problems within downstream electrical distribution

This is why emergency power troubleshooting should often go beyond the generator itself. The generator, automatic transfer switch, controls, switchgear, fuel system and electrical distribution all have to work together for emergency power to reach the intended loads.

Prime Power takes a system-level approach to troubleshooting because a problem that appears to be a "generator failure" may actually originate somewhere else in the emergency power system.

4. The Generator Is Overheating

Generator overheating can lead to shutdowns and may indicate a problem that needs immediate attention.

Potential causes include:

• Low coolant
• Coolant leaks
• Blocked radiator or ventilation
• Failed fan or damaged belt
• Dirty cooling-system components
• High ambient temperatures
• Excessive generator load
• Cooling-system component failure

Facility personnel can look for visible leaks, blocked ventilation openings and controller alarms without opening guarded or energized equipment.

If a generator shuts down because of high temperature, allow qualified personnel to determine the cause before returning the unit to service.

Simply adding coolant without identifying why the level is low may leave the underlying problem unresolved.

5. The Generator Has an Oil, Fuel or Coolant Leak

Leaks should never be treated as normal simply because the generator continues to operate. A leak can indicate a deteriorated hose, fitting, gasket, seal, tank, radiator or other component.

Look around the equipment for:

• Puddles or wet areas
• New stains
• Strong fuel odors
• Visible dripping
• Fluid around hoses or fittings
• Changes in coolant or oil level

Document where the leak appears to originate but avoid reaching into operating equipment or removing guards and panels to investigate.

Fuel leaks should be treated seriously because they can create both reliability and safety concerns.

6. Generator Voltage or Frequency Is Unstable

A commercial generator needs to provide power within appropriate voltage and frequency ranges for the equipment it supports.

Symptoms of unstable power may include:

• Voltage or frequency alarms
• Equipment dropping offline
• Lights flickering
• Breakers tripping
• UPS alarms
• Motors operating abnormally
• Generator speed fluctuating

Possible causes range from engine-governing and voltage-regulation issues to load problems, controls or electrical faults. This type of troubleshooting typically requires trained technicians and appropriate test equipment.

Facility personnel should document when the problem occurs and what loads were operating at the time.

7. The Generator Displays an Alarm or Fault Code

An alarm is information—not an inconvenience to be cleared. Modern generator controls can provide useful diagnostic information about conditions such as:

• Low battery voltage
• High coolant temperature
• Low oil pressure
• Overspeed
• Overcrank
• Low fuel
• Charger failure
• Emergency stop
• Communication problems
• Generator output conditions

When an alarm occurs, record the exact message or fault code before anyone resets the controller.

If possible, also note:

• Date and time
• Whether utility power had failed
• Whether the generator was exercising or carrying building load
• How long the unit had been operating
• Whether the alarm has happened before

A recurring alarm should be investigated even when the generator appears to return to normal operation.

8. The Generator Exercises Normally but Fails During an Outage

This can be one of the most frustrating emergency power problems for a facility team. The generator may start and run successfully during its routine exercise but struggle when required to support an actual facility load.

Why?

Running is not the same thing as carrying load. Problems that may only become evident under load include:

• Cooling deficiencies
• Fuel-delivery restrictions
• Voltage or frequency instability
• Weak engine performance
• Breaker problems
• ATS problems
• Excessive or unexpected load
• Electrical-system deficiencies

This is one reason weekly and monthly operational testing and appropriate load testing are important parts of emergency power reliability.

A generator that starts every week can still have problems waiting to appear when the facility actually needs it.

Commercial Generator Troubleshooting Guide

Symptom	Possible Causes	Recommended Next Step
Generator won't crank	Battery, charger, starter, controller or shutdown condition	Review controller and visible indicators; request service if unresolved
Generator cranks but won't start	Fuel supply, controls or engine-related issue	Record alarms and verify basic operating status
Generator starts then stops	Temperature, oil pressure, fuel, sensor or load issue	Record fault and have the cause diagnosed
Generator runs but facility has no power	ATS, breaker, switchgear, controls or generator output	Evaluate the complete emergency power system
Generator overheats	Coolant, airflow, radiator, fan, belt or excessive load	Stop operation if required and investigate
Generator leaks	Hose, seal, fitting, radiator, fuel or lubrication system	Document the leak and request service
Voltage/frequency is unstable	Governor, voltage regulation, controls, load or electrical problem	Qualified electrical diagnosis recommended
Alarm keeps returning	Mechanical, electrical, sensor or control issue	Diagnose the underlying cause instead of repeatedly resetting

What Facility Managers Can Safely Check

Facility personnel can provide valuable troubleshooting information without performing internal mechanical or electrical repairs.

From normal operating positions, consider checking:

• Generator controller status
• AUTO/OFF/MANUAL position
• Visible alarm or fault codes
• Fuel level
• Visible leaks
• Unusual noises
• Unusual odors or smoke
• External equipment condition
• ATS status indicators
• Recent maintenance and test records

Good documentation can shorten the troubleshooting process.

Photos of controller screens, alarm codes, equipment identification and visible conditions can also be useful when contacting your service provider, provided they can be taken safely without opening energized equipment or removing protective guards.

When to Stop Troubleshooting and Call for Generator Service

Facility personnel should contact a qualified emergency power technician when:

• The generator will not start
• The generator repeatedly shuts down
• An alarm continues to return
• The generator is overheating
• Fuel, coolant or oil is leaking
• Smoke or a burning odor is present
• Voltage or frequency is unstable
• The generator runs but emergency loads remain without power
• The ATS does not transfer
• Breakers repeatedly trip
• There is evidence of damaged electrical equipment
• A problem involves energized electrical components
• The cause of the problem is unclear

Do not bypass safety devices, defeat interlocks, repeatedly reset shutdowns or open energized electrical equipment in an attempt to restore service.

Emergency power systems can involve hazardous voltage, moving equipment, hot surfaces, pressurized systems and combustible fuel.

The Best Generator Troubleshooting Happens Before an Outage

Troubleshooting during an outage is reactive.

Preventive maintenance, inspection and testing give facility teams the opportunity to find many of the same problems before emergency power is actually needed. Battery deterioration, fuel problems, recurring alarms, small leaks, transfer-switch issues and inconsistent operating data can all provide early warnings of a developing failure.

Maintenance records are especially valuable. One unusual reading may not tell the entire story, but a pattern across multiple inspections or tests can reveal deteriorating performance.

For facilities where uptime is critical, emergency power reliability should be evaluated as a complete system—not simply as a generator engine that starts.

Troubleshooting the Complete Emergency Power System

When utility power fails, multiple systems have to work in sequence.

The generator must start. The engine must reach proper operating conditions. Generator output must be available. Controls must communicate correctly. The ATS must recognize the emergency source and transfer the required load. Switchgear and distribution equipment must then deliver that power where it is needed.

A failure anywhere in that sequence can leave a facility without emergency power.

Prime Power provides generator troubleshooting, preventative maintenance, emergency service, ATS support, switchgear service, load bank testing, fuel-system support and other emergency power services for commercial and mission-critical facilities backed by a full engineering department (PE, ME, EE).

If your generator won't start, repeatedly alarms, shuts down unexpectedly or runs without supplying power to the facility, the problem deserves more than another reset.

Have the complete emergency power system evaluated and identify the cause before the next outage puts it to the test.

Frequently Asked Questions

Why won't my commercial generator start?

Common causes include a weak starting battery, fuel problems, an incorrect controller mode, active shutdown faults or starting-system problems. Check the controller for alarms and confirm basic operating status. If the cause isn't obvious, have the generator inspected by a qualified technician.

Why does my generator start and then shut off?

A generator may shut down because of high coolant temperature, low oil pressure, fuel problems, excessive load or a sensor or control fault. Record the shutdown code rather than repeatedly restarting the unit.

Why is my generator running but my building still has no power?

The generator may not be the source of the problem. An ATS, breaker, switchgear, control-system or electrical-distribution issue can prevent emergency power from reaching the required loads.

Should I reset a generator alarm?

Record the alarm or fault code first. A one-time reset may be part of an approved operating procedure, but recurring alarms should be diagnosed rather than repeatedly cleared.

Can a generator pass its weekly exercise and still fail during an outage?

Yes. An unloaded or lightly loaded exercise does not necessarily reproduce the conditions the generator will experience while supporting facility loads. Problems with fuel delivery, cooling, generator output, ATS operation or other parts of the emergency power system may only become apparent under load.

When should I call a commercial generator technician?

Call for service when the generator won't start, repeatedly shuts down, overheats, leaks, produces unstable power, shows recurring alarms or runs without supplying the required facility loads. Issues involving energized equipment should always be handled by qualified personnel.