Top Nine Reasons Generators Fail to Start

April 1, 2007
Learn the leading causes of “no-start” situations among standby power systems – and what you can do to prevent them

With each passing year, our society's reliance on an unlimited supply of electric power continues to increase. The availability of this continuous power helps businesses maintain operations, avoid revenue loss, and ultimately protect their bottom line. To safeguard facilities from the disastrous consequences of power disruption, more companies are choosing to invest in standby power systems. 

Because investing in such a system can be quite expensive — often at a price tag running into the hundreds of thousands of dollars — companies must understand what it takes to properly operate and maintain their power equipment, as a lack of generator maintenance or operation knowledge will undoubtedly result in generator failure at a time when they need it most.

With today's complex standby power systems, you must understand the basic settings of your systems, the methods and procedures involved in regular maintenance, and the protocol to deal with alarm conditions in the case of system failure. This information will keep you from wasting valuable business time over generator failure due to inaccurate settings or neglected maintenance. Let's take a look at nine of the most common reasons generators fail to start in the first place, and what you can do to try and prevent these from occurring.

  1. Battery failure

    The single most frequent service call for generator failure is related to battery failure. Eighty percent of all battery failure is related to sulfation buildup — the accumulation of lead sulfates on the plates of lead-acid batteries. This buildup occurs when the sulfur molecules in the electrolyte (battery acid) become so deeply discharged that they begin to coat the battery's lead plates. When enough plate area has sulfated, the battery will not be able to provide enough current and will normally need to be replaced. Battery failure is commonly the result of low electrolyte levels — battery plates exposed to air will immediately sulfate.

    Battery cells are shorted when sedimentary trays fill up with lead debris. You can avoid shorted batteries if you replace them regularly. Some experts recommend replacing them every three years.

    Although battery failure can result from open cells, this is not a common occurrence. Open cells are the result of an overcurrent of the battery system. If a generator system has had several battery failures determined to be caused by open cells, the unit may require larger batteries capable of higher cold cranking amps (CCAs).

    Frequently, battery failure is due to the charger breaker being open or tripped — often the result of human error rather than actual charger failure. This usually occurs after service on a generator, or some type of maintenance, where the charger has been turned off and not turned back on again when the service is completed. Always double check a generator system after any service or maintenance to make sure everything is functioning properly.

    Because many battery problems are caused by dirty and loose connections, maintenance is critical. Cable connections need to be regularly cleaned and tightened. Battery charger failures are difficult to prevent, and cannot be accurately predicted. However, monitoring the charge rates from month to month will establish a trend that can help map the potential for failure. A properly functioning battery charger will have a constant charge rate for any given system. An increase of amperage may show signs of a battery or charger malfunction.

  2. Low coolant levels

    The most obvious cause for a low coolant level is either an external or internal leak. Pay close attention to any visible puddles of coolant during weekly inspections of the unit(s). The color of the coolant varies by manufacturer and may look like red-dyed diesel fuel. Inspect oil for any signs of color change or a milky texture and hoses for “crusties” — the sign of coolant seeping and additives drying up at the connection. While many generators are equipped with low coolant level alarms, few have a dedicated alarm indicator for low coolant. Commonly, this alarm will be tied in to a high coolant temp shutdown circuit.

    Internally plugged radiator cores will also cause low coolant level shutdowns. When the generator is under load, the thermostats open completely, and the radiator cannot allow the proper amount of flow through the core. The coolant has to go somewhere, so it purges through the overflow line. As the engine cools off and the thermostat closes, the level drops and activates the low coolant level shutdown.

    This also occurs when float switch-type coolant level sensors are used, and the lines are plumbed to the top and bottom radiator tanks. When the thermostats open, the path of least resistance is through the float switch lines, and the flow causes the float to drop and shutdown the engine. The thermostat will not open enough to cause this during regular weekly running of the generator. It will have to be tested under load to cause the thermostats to fully open. A full load test with an external load bank is the only accurate way to check a cooling system.

  3. Low coolant temp alarms

    Low coolant temp alarms are mainly the result of faulty block heaters. Given the fact that they run 24 hours a day, seven days a week, they're going to fail periodically. A block heater will normally not cause the engine not to run. You may need to allow a generator to run for a few minutes at no load after startup so that the temperature comes up.

    Why have block heaters? A common misconception is that the engine does not need a block heater in warmer climates. However, a block heater does more than help the engine to start in cold weather. Due to the dissimilar metals that the engines are built with, accelerated wear can occur during startup. The pistons, normally made of aluminum, will expand at a faster rate than the iron cylinder liners. This rapid expansion of the pistons can lead to scuffing of the piston skirt. Block heaters relieve most of this scuffing by maintaining the cooling system temperature and keeping the cylinder liners expanded.

    The extreme temperature inside the block heater is what causes the coolant to circulate through the system. (At times, you can hear the coolant boiling inside of the block heater.) The block heater's high temperatures flash off the coolant into small amounts of steam, causing the coolant to evaporate in slight amounts. While it takes a long time to show a difference in the coolant level, recording the amount of coolant added to the system will help establish a trend. (The block heater thermostat is located at the cold side of the heater.) If the engine is continuously using coolant with no signs of leakage, you should take oil samples and analyze them before continuing further troubleshooting efforts. If the block heater temperature gets excessively high, premature block heater failure or extreme engine damage may occur. Some generators use a control panel to operate their block heater.

    Normal walk-through inspections should include checking the cylinder head (or engine thermostat housing) for temperature and verifying that the engine or block heater hoses are warm. The temperature setting should be between 90°F and 100°F — never more than 120°F.

  4. Oil, fuel, or coolant leaks

    Most often, oil leaks are not in fact leaks but the result of “wet stacking” (or “engine slobber”) caused by excessive no-load run time. Diesel engine generators are designed to operate with a load — most effectively in the 70% to 80% range of rated output. When generators operate considerably below the rated output level, the engine can start to over-fuel or “wet stack” and damage the engine.

    Wet stacking — an accumulation of carbon particles, unburned fuel, lube oil, condensed water, and acids in the exhaust system — is caused by low-combustion chamber temperature. When a diesel engine wet stacks, the engine will need to be cleaned up by loading the unit for a few hours and burning off the excess fuel. If a diesel engine generator steadily runs loads considerably below the rated output level, you should pair the generator with an automatic load bank, which will place a false load on the generator system, keeping the engine properly loaded and preventing a “wet stack” condition.

    Another cause of oil leaks is directly related to the crankcase breathers. Most engine crankcase breathers vent directly under the engine. The fumes that come out of the breather contain an oil mist, which can produce a puddle under the engine and coat the generator and radiator with an oil film that collects dirt and debris. Recirculating breathers are available that can separate the oil and return it to the engine; remaining fumes get returned to the engine air intakes.

    The most common coolant leak occurs in the block heater hoses. Extreme temperatures on the outlet make block heaters hard on their hoses. For this reason, you should never use rubber hoses for block heaters; silicon hoses are specifically designed for use with block heaters. Always install isolation ball valves for block heater hose connections.

    Cooling system maintenance will help prevent leaks. Replacement of hoses and coolant every three years is recommended. Generators using newer extended-life coolant should still have hoses replaced and the additive package upgraded every three years. Cooling system antifreeze protection and conditioner should be maintained to the manufacturer's specifications. Radiator core damage is directly related to the coolant that is in the system. Poorly maintained coolant will cause liner pitting and eventually severe engine damage.

    Most fuel leak service calls are due to overfilling of the base tank. This is due to either human error or a failure of a pump system. Check remote tank pump systems and emergency shutoff systems periodically for proper functioning, and flexible fuel lines regularly for cracks and signs of aging.

  5. Controls not in auto

    “Not in auto” messages are the direct result of human error. The obvious reason for “not in auto” situations is because the main control switch was left in the off/reset position. This usually occurs after testing or servicing of a generator. After any service is performed on a unit, always double check the generator system yourself.

    The control switch may have several positions as in “off/reset” and “cool down,” which will cause the generator not to start in the event of a power outage. These positions should give an alarm. “Not in auto” is a generic term for the unit not being shut off, and may not actually be the main control switch. Alarms not reset, breakers open, switchgear not reset, and emergency stop buttons activated are all examples of “not in auto” failures.

    Several generator makes/models are set up to short trip the main circuit breaker during an emergency fault shutdown. When the generator shuts itself down for any reason, someone has to physically reset the control panel to clear the alarm. There may be several things to check and reset after a fault shutdown; however, this should only be done once the cause of the alarm has been identified and corrected.

    Ground-fault sensors are required by certain building codes or engineers during construction, and must be added onto a generator. These sensors look like light switches that are turned off. Ground-fault sensor shutdowns can be difficult to spot; they are not always tied to an audible or visual alarm. It's important that you determine whether or not your standby generator is equipped with one of these sensors — as well as how to reset it. If the generator uses remote switchgear, the switchgear will normally have to be reset. It is important to know your system and what it does during a fault shutdown. Here's a tip: Simulate a fault to see what it takes to get it running again in the event of an emergency.

  6. Air in the fuel system

    This is a common problem with newer generators that are not run on a regular basis. Closer tolerances within the fuel systems to meet today's emission requirements make fuel systems more susceptible to air affecting startup. This is not as common with older generators — many of which may have a leak in a line or check valves that are not properly holding the fuel in the engine.

    Lighter low-sulfur fuel has lower flash temperature, which causes the block heater to flash off some of the fuel within the injectors. One small bubble of air within a unit injector solenoid can cause an injector not to fire at startup. If enough injectors do not fire, the engine will not start. This failure is 100% preventable by periodically running the engine during weekly inspections. The engine does not need to be run until the coolant temp comes to normal. All that is needed is enough time to verify that the engine will start, that the air is cleared from the fuel system, and that the generator comes up to voltage and frequency. This can be completed in less than 5 minutes. Any additional test running would simply burn up fuel and air quality maintenance run time.

  7. Ran out of fuel

    Mechanical fuel level gauges may not always be accurate. Unlike a vehicle that is moving and using a higher percentage of its tank's capacity, a generator tank has no movement, causing the fuel to become stagnant. Mechanical gauges may also stick in a position until vibrations break them free.

    Low level alarms must also be addressed, as they provide the same failure alarm. Some generators are equipped with “low level shutdown” or “critical fuel level shutdown.” These shutdowns are there to prevent the fuel system from drawing in air when running out of fuel is eminent. Bleeding air out of a fuel tank can be an extremely difficult procedure.

    Running out of fuel due to plugged fuel filters can be prevented by maintaining the fuel tanks and periodically checking them for water and contaminants. Water or moisture in fuel can be damaging to diesel engines because the water properties create advanced ignition and accelerated detonation. If you use fuel polishing as an alternative to cleaning your fuel, check with your vendor to see if its fuel is affected by the chemicals; fuel polishing may not be able to remove water. Fuel filter plugging should be expected with the new ultra-low sulfur fuel, which has a very high detergent level and will clean out your lines and whatever else it comes in contact with.

    Engines equipped with electric shutoff solenoids should always have a manual bypass. There are several reasons to have shutoff solenoids; large remote above ground tanks can gravity feed to the engine, overpressuring the seals in the pumps or injectors and causing the fuel to mix with the oil. Solenoids should be DC power activating at the time of initial crank signal and remaining open until after the engine makes a complete stop.

  8. High fuel level alarm

    High fuel level alarms are required by government regulations to prevent the overfilling of a fuel tank. The alarm should activate when the fuel tank reaches between 90% and 95% capacity. This lets the person fueling the tank know when he or she should stop filling.

    Normally, there is nothing wrong with the generator when this alarm activates. On rare occasions, however, the natural thermal expansion of the fuel will cause the alarm to activate. This will usually occur on an extremely hot summer day. High fuel level alarms may or may not clear themselves when the fuel level drops below the set point. You may have to manually reset the alarm when the fuel level drops.

  9. Breaker trip

    First, verify that nobody has accidentally pushed a remote emergency power off switch.

    If a breaker trips after the automatic transfer switch (ATS), the generator will not start. The status of the automatic transfer switch should be checked during a power outage. The ATS should have lights or a display showing the switch position and source availability. If you find a breaker tripped, make sure you can determine the cause of the trip prior to resetting.

    Never try to manually operate an ATS if you don't know how to do so properly. Severe bodily injury or immediate death can occur. The transfer switch is smarter than you think and has a specific reason for being in its current position; attempting manual operation may drag you into a live bus if done incorrectly.

Dembski is the product support sales representative for Peterson Power Systems, San Leandro, Calif.

Sidebar: Maintenance Basics

It's important to remember that a standby generator is a mechanical and electrical device that will require service and parts to maintain proper function. In addition to exercising the generator on a monthly schedule, you should pay close attention to the following list of routine maintenance activities.


  • Radiator fins must be inspected on a monthly basis and cleared of all dirt and debris. Make sure the generator is OFF prior to inspecting by shining a light through the front of the radiator. If the light doesn't shine through the fins, carefully clear the blockage.
  • Antifreeze levels should be checked on a weekly basis.
  • Make sure the block heater is plugged in and warm. Block heaters should be plugged in year-round, as they reduce wear on the generator's engine.
  • Inspect hoses and the water pump for signs of wear, bulges, cracking, and leaks; check the hose clamps for tightness.


  • Change fuel filters every 200 to 250 hours, depending on environmental conditions and how clean the fuel is. At a minimum, change the filter on an annual basis.
  • Note whether or not wet spots appear around the fittings.


  • The charging gauge (or indicator light) should read OK on the battery charger.
  • Make sure the battery and charger connections are tight, and clean any corrosion off the terminals. The battery charger must be turned off before working on the battery or the starter.


  • Inspect the engine for leaks and wear.
  • Check engine belts for wear, cracking, splitting, or looseness.
  • Check oil levels. In addition, the engine oil and oil filter should be changed annually, or every 100 to 250 hours, depending on the environment in which the unit is located.
  • Check the air filter every 100 hours of operation (more frequently in dustier environments) and change the filter at least once per year. Make sure the filter canister does not contain dirt or other debris.

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