TL;DR

A generator that won't crank points to the battery or starter. A generator that cranks but won't fire points to fuel. Work in that order: battery voltage, starter circuit, fuel level, fuel valve, then injectors.

What you might see

no crank or slow crank on start command
engine cranks but does not fire
starter relay clicks but motor does not engage
EMCP control panel shows start fail alarm

Likely causes

Tap to mark as ruled out

Dead or sulfated battery unable to provide sufficient cranking current
Fuel level low or main fuel shutoff valve closed
Starter motor solenoid or drive gear failed
EMCP control system in a fault or shutdown lock-out state

Required tools

Multimeter (12V or 24V battery range)
Battery load tester
Clamp ammeter for starter circuit current

Safety first

Diesel generators produce carbon monoxide. Never run a generator in an enclosed space without adequate ventilation.
The starter circuit draws hundreds of amps. Do not work on starter cables with the battery connected. Disconnect the negative battery terminal first.

Procedure

7 steps · ~30 min

1
Read the EMCP (Electronic Modular Control Panel) display for any active fault codes or shutdown reason. Record every code before resetting.[1]
2
Check the battery voltage with a multimeter: 12V or 24V battery system should read at least 12.4V or 24.8V at rest. Below 11.5V or 23V means the battery is deeply discharged.
3
Check the battery cable connections at both the battery and the starter for corrosion and tightness. A loose or corroded cable causes a voltage drop that prevents cranking.
4
If the battery is good, attempt a manual crank and listen. A single click from the starter relay with no crank means the starter solenoid is energized but the motor is not turning. Tap the starter housing while a second person attempts to crank: if it cranks once, the starter is failing and needs replacement.
5
Check the fuel level in the day tank and the main tank. Verify the main fuel shutoff valve is fully open and that any solenoid-operated fuel valve is energized.[1]
6
Check the fuel filters for restriction. A clogged primary fuel filter prevents fuel from reaching the injection pump.
7
If no fault codes, battery is good, fuel is present, and the starter cranks but the engine doesn't fire, consult a CAT service technician. The issue is likely air in the fuel system, low compression, or an injection pump fault requiring specialized tools.

Sources

Caterpillar C-Series Generator Set Operation and Maintenance Manual, Caterpillar Inc.
Caterpillar C-Series generator set operation and maintenance manual, starting system and EMCP troubleshooting (general)
View source

More guides for Caterpillar C-Series (C9/C18/C32)

How to diagnose black smoke from a CAT C-Series diesel generator exhaust

Black smoke means unburned fuel: too much fuel or not enough air. Check the air filter first (quickest fix), then inspect the injectors. Blue or white smoke has a different cause and needs separate investigation.

~30 min7 steps

How to diagnose engine overheating on a CAT C-Series diesel generator

Generator overheating is almost always low coolant, a blocked radiator, or a failed water pump. Check coolant level first, then radiator cleanliness, then thermostat and water pump operation.

~45 min7 steps

How to diagnose excessive vibration on a CAT C-Series diesel generator

Generator vibration above normal is usually an engine misfire on one cylinder or a coupling alignment problem between the engine and the alternator. Check for misfire with the EMCP, then inspect the coupling.

~45 min6 steps

How to diagnose low oil pressure on a CAT C-Series diesel generator

Low oil pressure shuts down the generator to protect the engine. Check oil level first, then oil pressure sensor reading, then filter condition. Do not run the engine with low oil pressure confirmed.