Common Generator Faults and Fixes

Generator faults usually show up when the mains fails and the set has to come online. The most common symptoms are the set not starting when cranked, the ATS not transferring the load, and the set shutting down shortly afterwards on a high coolant temperature alarm.

Most of the faults we see on site are not complicated. Battery, fuel, air, cooling and settings. In this article we list the most common symptoms, their likely causes and the first checks. One rule up front: high voltage, rotating parts and hot surfaces are dangerous. Electrical measurements and anything inside the engine are for an authorised technician.

Look at the panel first

Modern controllers log alarms. Before you touch anything, note the alarm on the display and the alarm history. “Low oil pressure” and “fail to start” are very different things. When you call service, this will be the first question.

Symptom 1: Cranks but won’t start

Likely causes:

• No fuel, or fuel not arriving. Empty tank, closed valve, faulty level gauge.
• Air in the fuel system. Very common after a filter change or after running out of fuel.
• Blocked fuel filter. Especially if water and sediment have built up at the bottom of the tank.
• Cold weather. Preheating not working, block heater switched off. Details: starting a generator in winter.
• Weak battery. The starter turns, but not fast enough.

First checks: fuel level, valves, the water separator on the filter. Bleeding is done according to the engine manual.

Symptom 2: Won’t crank at all

The most common cause is the battery. Low voltage, corroded terminals or a loose cable. If the battery charger isn’t working, the battery will be flat within weeks. Other causes: emergency stop button left pressed, panel in “off” mode, faulty starter relay or starter motor. We cover batteries in a separate article: why generator batteries fail.

Symptom 3: Runs but produces no power

• Output breaker open or tripped.
• Panel in “manual” mode, the ATS isn’t taking the load.
• AVR faulty or its fuse blown.
• Alternator excitation lost. Can happen on sets that have stood idle for a long time.
• Loose connection or burnt terminal.

If there is no voltage, or it is very low, the alternator side is checked. These measurements are made on live circuits and need training.

Symptom 4: Shuts down under load

The panel tells you why the set stopped. The most common alarms:

Alarm	Typical causes
High coolant temperature	Blocked radiator, low coolant, loose fan belt, inadequate ventilation, overload
Low oil pressure	Low oil level, wrong oil, sensor fault, internal wear
Overcurrent / overload	Load above set rating, motor starting surge, short circuit
Underfrequency	Insufficient fuel supply, blocked filter, overload, governor setting
Overspeed	Governor or fuel system fault; serious alarm, service needed

High coolant temperature is very common in summer and in enclosed generator rooms. Check whether the room’s air inlet and outlet are big enough, and whether hot air is being drawn back in. Also remember that in hot climates and at high altitude the set cannot deliver its nameplate power; check whether the site’s highest temperature was taken into account when the set was chosen.

Symptom 5: Smoke

• Black smoke: Unburnt fuel. Overload, blocked air filter, injector problem.
• White smoke: Can be normal briefly on a cold start. If it continues, it may be a cold engine, low load or water in the fuel. If coolant is burning, that is a sign of a serious fault.
• Blue smoke: Oil is burning. Long periods at low load (wet stacking), worn piston rings or a turbo leak.

Long running at low load is an underrated problem on site. In engines that run continuously below typically 30% of the set’s rating, oil and fuel build up in the exhaust. The fix: correct sizing and, if needed, periodic running on load. This is why load bank testing exists: what is a load bank test.

Symptom 6: Voltage or frequency fluctuates

If lights flicker or the UPS won’t accept the generator: check the governor setting, AVR setting, fuel supply and the nature of the load (large motor starts, harmonic loads).

Symptom 7: Leaks, vibration, noise

If there is oil, fuel or coolant under the set, don’t run it for long until the source is found. Fuel dripping onto a hot exhaust surface is a fire risk. Coolant loss comes back as a high temperature alarm on the next run.

Unusual vibration usually points to a loose connection, a damaged mount or an unbalanced fan. If you hear metallic knocking, whistling or rubbing, stop the set and call service. Recording the sound on your phone helps with diagnosis.

Common mistakes on site

• Resetting the alarm without reading it. The trace of the fault is wiped, and the same alarm comes back.
• Tracking fuel by level only. The tank is full, but there is water and sediment at the bottom.
• Never testing the set on load. A monthly idle run hides faults.
• Bypassing a protection sensor. The set runs, the engine gets damaged.
• Keeping no spare filters. The set sits idle for days because of a simple filter.

When to call service

Overspeed alarm, low oil pressure, oil in the coolant or water in the oil, metallic noise, a burning smell, anything on the alternator side that needs measuring. With these symptoms, don’t keep trying to restart the set; the damage only grows.

Checklist

• Have the alarm and alarm history been noted?
• Have the fuel level, valves and water separator been checked?
• Is the battery voltage OK and are the terminals clean?
• Are the oil and coolant levels correct?
• Are the radiator and air filter clean?
• Is the room ventilation adequate?
• Is the panel in automatic mode, and is the emergency stop released?

Next step

Most faults are prevented by regular maintenance. For the scope of maintenance, see our scheduled maintenance page; in case of a fault, see our generator service page or contact us.