When your generator won’t start, it’s more than just an inconvenience — it’s downtime, lost productivity and potential safety risk. Whether it’s a diesel generator, petrol, or gas-powered unit, the causes are often similar: battery failure, old fuel, low oil levels, clogged fuel systems, or ignition issues.
In this guide, we’ll break down the most common reasons why a generator won’t start, why you should never attempt to repair it yourself and what steps to take next. If your generator turns over but won’t start, or starts but won’t stay running, read on.
A dead or weak battery is one of the top reasons a generator won’t start. Without enough charge, the starter motor can’t turn the engine.
Typical signs:
No sound or a weak cranking sound
Dashboard lights flickering or completely off
Generator turns over but won’t start
What could be wrong:
Battery is flat due to long periods of non-use
Corroded terminals
Failed charging system
Important: Battery-related faults can affect other generator components. Replacing a battery incorrectly can damage the control system or cause short circuits. Always leave diagnostics, repairs and replacements to professionals.
If a generator has been sitting unused for months, old fuel can degrade or become contaminated. This affects both petrol and diesel generators.
Symptoms:
Generator starts but won’t stay running
Engine misfires or stalls
Unusual odours from the exhaust
Causes:
Water or condensation in the fuel
Diesel bug or algae build-up
Clogged fuel filters
Important: Fuel system repairs are not safe for DIY. Drainage and flushing require specialist tools and improper handling of fuel poses a serious fire risk.
Even with good quality fuel, blocked lines or dirty filters can stop a generator from starting or running properly.
Warning signs:
Generator turns over but won’t start
Rough idling
Starts but immediately cuts out
Fuel delivery issues can damage fuel pumps and injectors if ignored. These are precision parts that require careful handling during diagnostics and repair.
Modern generators are designed with protection systems. If oil levels or pressure are too low, the engine won’t start or will shut off automatically to prevent internal damage.
What to watch for:
Generator won’t start at all
Alert lights on the control panel
Generator starts but stops after a few seconds
Sometimes, the issue isn’t the oil itself, but a faulty sensor misreporting the pressure or level. Either way, it needs to be addressed by a trained technician.
If you’re running a petrol generator and it won’t start, the spark plug could be worn out, fouled, or not sparking at all.
Look for:
No ignition despite cranking
Engine misfires
Burnt or blackened spark plug tips
Note: Even a seemingly simple spark plug swap can cause damage if gapping or installation isn’t done properly. These faults should be inspected by generator service professionals.
If the starter motor is spinning but the engine won’t fire up, the issue is likely fuel or ignition-related. In diesel generators, air in the fuel system or injector faults are common culprits. In petrol units, ignition failures or air blockages are likely.
Other potential issues:
Dirty air filter
Low compression
Engine sensors preventing start-up
Water ingress in electrics
Attempting to “crank it until it starts” will often make the situation worse — draining the battery, flooding the engine, or damaging internal components.
Sometimes the generator will fire up, run briefly, then stall. This can indicate:
Fuel starvation
Overload or short circuit on the output
Faulty Automatic Voltage Regulator (AVR)
Sensor-triggered safety shutdowns
In all cases, this is not something to ignore or “restart and hope for the best.” The generator needs a full diagnostic to avoid further damage.
Generators are complex machines with high voltage outputs, flammable fuels, and sensitive control electronics. Attempting a DIY repair when your generator won’t start can result in:
Electrical shock
Fire hazard
Permanent damage
Invalidated warranty
Even basic tasks like checking the fuel system or battery involve safety risks. If your diesel generator won’t start, or your generator turns over but won’t start, it’s time to call in a professional.
At Generator Control & Maintenance LTD, we provide expert generator diagnostics and repairs for all major brands and types — diesel, petrol, standby, portable and industrial.
We handle:
On-site fault finding
Load testing and control panel checks
Fuel system cleaning and filter replacement
Battery diagnostics and replacements
Our team ensures your power supply is restored safely and quickly — with no guesswork, no risk, and no downtime.
Whether it’s a generator that starts but won’t stay running, a diesel generator that won’t start, or a unit that just turns over but won’t fire up — we’re ready to help.
We provide full inspection, maintenance and emergency repairs to resolve any generator issue fast, safe and professional.
Contact us now to book a service or request emergency support.
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We only work with a group of tried and trusted suppliers and use the most reliable and effective equipment – which doesn’t always mean the most expensive. And, as controls and systems are continually changing, we keep abreast of all the latest developments to provide you with robust long-term solutions.
