Backup Generators · Service

Generator repair starts with which failure

No crank, cranks but will not fire, fires and dies, runs but will not transfer, runs but drops the load. Five different faults, five different repairs — and knowing which one you have before you call saves you a diagnostic visit.

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In short

What is generator repair?

Generator faults sort into five families, and identifying yours before you call is genuinely useful. It does not crank at all — almost always the battery or the starting circuit. It cranks but never fires — fuel or ignition. It fires and dies — fuel supply or a protective shutdown. It runs but the house stays dark — the transfer switch or its control wiring, not the engine. It runs and then drops the load — the alternator, the regulator, or a supply that cannot hold up under demand.

Triggers

Signs you need generator repair

Nothing happens at all when it should start — no crank, no engine noise, sometimes a single click from the controller
It cranks over and over and never fires, or fires briefly and dies straight away
It starts, runs for a minute or two, then shuts itself down and posts a fault code
The engine is clearly running outside but nothing in the house has power
It carries the load until something large starts — the well pump, the air conditioning — and then everything drops
A fault light has been showing for weeks and the exercise cycle has stopped

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The job

How generator repair actually works

Read the controller before anything is touched

Modern standby units log fault codes, start attempts and run hours, and the log frequently names the fault outright. It also shows whether the machine has been quietly failing its exercise cycle for months — an intermittent fault with a history is a different repair from a sudden one.

Watch for: A technician who starts pulling parts without reading the log, or who clears the codes before recording them. Once cleared, the history is gone and you may be paying for the diagnosis twice.
No crank: test the battery and the starting circuit

By a wide margin the most common standby generator fault. The battery is load-tested rather than measured, because the characteristic failure reads a healthy voltage at rest and collapses under starter draw. Then the charging system, since a failed charger kills batteries silently and will kill the replacement too.

Watch for: A new battery fitted with no check of the charger. If the charger is the fault you will be buying another battery, and the second one will fail exactly the same way.
Cranks but will not fire: fuel and ignition

On a gas or propane standby that means supply pressure, the regulator, and the gas valve. On a petrol portable it is overwhelmingly stale fuel and a gummed carburettor, which is a cleaning job rather than a parts job in most cases. Ignition — plugs, coils — is checked alongside.

Watch for: A portable quoted for a carburettor replacement without an attempt at cleaning it, and without the stale fuel being drained. The fuel that caused the problem is often still in the tank.
Runs but the house stays dark: the switch, not the engine

If the engine is running happily and nothing inside has power, the fault is downstream — the transfer switch, its control wiring, or the sensing that tells it the generator is ready. It is a common and reassuring diagnosis, because it usually means the expensive part of your system is fine.

Watch for: Engine work quoted for a transfer fault. Establish first whether the generator is producing output at its terminals; that single check separates the two halves of the system.
Runs but drops the load: alternator, regulator or supply

Output that collapses when a large motor starts points at the alternator or the voltage regulator, or at a fuel supply that cannot sustain the demand. This is where the repair-or-replace conversation usually happens, because alternator and control board faults are the expensive end of generator repair.

Watch for: A control board replaced speculatively as a first move. Boards are among the priciest parts on the machine and are sometimes fitted on a hunch. Ask what specifically indicated it.

Money

What does generator repair cost?

Diagnosis is usually a call-out fee, often credited against the work. Batteries, plugs, filters and carburettor cleaning are at the modest end; alternators, control boards and gas valves are the expensive end and are where replacement starts competing with repair. See the generator cost guide for ranges.

See the full generator repair cost guide — by material, size and region →

Straight answer

When generator repair is the wrong call

Everyone else ranking for this is paid when you say yes. Here's when you shouldn't.

The machine is old, out of support, and needs a board or an alternator. When the expensive parts are the ones failing on a machine whose parts are getting hard to source, a repair can cost a meaningful share of a replacement and still leave you with everything else at the same age. Ask for both numbers before deciding.
It is a cheap portable needing a carburettor and a coil. On an inexpensive machine, parts plus labour can approach what the whole unit cost. Worth asking the shop directly whether they would repair it if it were theirs.
It has never been serviced and this is the first symptom. A single fault on a neglected machine is rarely the only thing wrong. Book the overdue service rather than a targeted repair, so everything gets checked at once.
The problem is that it never starts itself. If the exercise cycle has not run for months, the useful work is finding out why the self-test stopped — the fault that reveals itself during an outage is often the second symptom, not the first.
You are mid-outage and considering a workaround. Do not improvise a connection to get power back. Run essentials from a portable on cords instead and get the standby system repaired afterwards, because the improvisations people reach for in an outage are the ones that hurt someone.

Straight answer

Can I do this myself?

A standby generator can start itself, without warning, at any moment — that is what automatic means, and the exercise timer will do it too. Before anyone works on one, the controller goes to off, the battery is disconnected, and the fuel is isolated. People are injured by generators that started while a hand was in the enclosure. Portables have a different version of the same rule: never refuel a hot engine, and never run one anywhere partly enclosed while you are working on it.

Reasonable to DIY

Reading the fault code on the controller and writing it down before anyone clears it
Checking the oil level, and looking for obvious rodent damage or nesting inside the enclosure
On a portable: draining stale fuel, changing the oil and air filter, and replacing the spark plug
Confirming the gas supply is actually on, and that the main breaker or the disconnect has not been left off after previous work

Call a licensed pro

Anything involving the transfer switch, which is connected to your service and carries live conductors regardless of what the generator is doing.
Alternator, regulator and control board diagnosis, which needs the manufacturer's procedure and often their software.
Any fuel system work on a gas or propane machine — regulators, valves and pressure testing are licensed gas work.
A machine that keeps tripping a protective shutdown. Repeatedly resetting it risks destroying the engine.

Before you sign

What to check when hiring for generator repair

What is the fault code, and what does it indicate on this machine?

The starting point of any competent diagnosis. A technician who cannot read your controller works by substitution, and substitution is expensive.
If the battery is the fault, will you also test the charging system?

Otherwise the new battery fails the same way and you pay twice.
Before replacing a board or an alternator, what specifically pointed to it?

The expensive parts, occasionally fitted on a hunch. You want the reasoning and the reading, not the conclusion.
Can you quote both the repair and a replacement machine?

On an older unit needing a major part, the comparison is the decision. A dealer who gives you both numbers honestly is worth using again.
Will the repair end with a live transfer test?

The engine running is not the same as the house being powered. Insist it goes through the switch and back.
What warranty covers the parts and your labour, separately?

Usually different terms, and the labour warranty is the one that tends to be short and unstated.

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Related work

Often done at the same time

Generator Maintenance

Generator Transfer Switch Installation

Engine runs, house dark

Whole Home Generator Installation

When replacement wins

Portable Generator Setup

Getting through the outage

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FAQ

Generator Repair — questions people ask

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Why will my standby generator not start?

The battery, in the great majority of cases. These machines sit for months on a trickle charge, and the typical failure reads a normal voltage at rest and then collapses the instant the starter draws on it — so it has to be load-tested, not measured. After that: a charging system that has failed and been quietly killing the battery, a gas supply problem, rodent damage to wiring inside the enclosure, and a low-oil shutdown on a machine nobody has checked in years.

My generator is running but I still have no power. What is wrong?

Usually the transfer switch or its control wiring rather than the generator, and that is relatively good news because the engine is the expensive half. The switch may not be receiving the signal that the generator is ready, the control wiring between the two may have failed, or the switch mechanism itself may not be transferring. The diagnostic step that splits it is checking whether the generator is producing output at its own terminals.

Why does my generator shut down when a large appliance starts?

Something cannot sustain the demand. It may be genuinely undersized for the surge — motors like well pumps and air conditioning compressors draw several times their running current momentarily — or the fuel supply may be adequate at idle and inadequate under load, or the alternator or voltage regulator may be failing. The useful diagnostic is a fuel pressure reading taken while the machine is running and loaded, because a marginal supply looks perfect at rest.

Is it worth repairing an old generator?

It turns on which part has failed. Batteries, plugs, filters, sensors and carburettor work are almost always worth doing regardless of age. Alternators, control boards and major fuel components on an older machine are the point at which the repair can reach a real fraction of a replacement while leaving every other component at its existing age. Ask for both figures, and ask specifically whether parts for your model are still readily available.