Generator repair starts with which failure

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.

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

Seeing something not on this list? Browse backup generators problems by symptom →

How generator repair actually works

1. 1

   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 forA 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.

2. 2

   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 forA 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.

3. 3

   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 forA 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.

4. 4

   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 forEngine 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.

5. 5

   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 forA 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.

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 →

If the number is the problem, here are the ways people pay for it →