Generator cranks but will not start; Code 12 displayed indicating low cranking speed

Safety first: Disable automatic start and open the main AC breaker. Wear eye protection when working around batteries.

• Check battery voltage: With the generator off, measure battery voltage at the battery posts. It should be around 12.6 V (for 12 V systems). If below 12.0 V, charge or replace the battery.
• Inspect battery connections: Clean battery posts and cable lugs with a wire brush. Tighten all connections at the battery and at the starter solenoid on the generator. Look for green/white corrosion under insulation.
• Check cable condition: Feel along the length of the positive and negative cables for soft spots or stiffness indicating internal corrosion. Replace any suspect cables with proper gauge Onan-approved cables.
• Listen to cranking sound: If the engine cranks very slowly or just clicks, test the starter motor by measuring voltage at the starter terminal while cranking. If full voltage is present but the starter turns slowly, the starter is likely failing.
• Check for engine seizure: With battery disconnected, try turning the engine by hand at the crankshaft or alternator fan (if accessible). If it will not turn, internal engine damage may be present—contact a technician.