Code 15
Onan
Engine Underspeed / Load Too High
Generator shuts down; Code 15 displayed for under-speed condition
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Excessive electrical load causing bogging, Weak governor actuator, Fuel restriction causing low power, Engine mechanical issues (low compression, worn valves)
How to Fix / Troubleshooting
Safety first: Disable auto-start and open the main breaker before troubleshooting.
- Reduce load: Open the generator output breaker and restart. If the unit runs without Code 15, reconnect loads gradually. Identify and remove any large motor loads (AC compressors, pumps) that cause RPM drop.
- Check air and fuel filters: Replace clogged air filter and fuel filter (diesel or gas) with Onan-approved parts. Restricted fuel flow can cause low power and underspeed.
- Inspect governor linkage: Ensure linkage moves freely and the actuator responds quickly to load changes. Lubricate pivots and correct any binding.
- Fuel quality: For diesel units, check for water or algae in the fuel. Drain water separators and treat fuel if contamination is present.
- Engine health: If underspeed persists with light load and clean filters, engine compression and injection timing should be checked by a technician.
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Repair Difficulty
Hard
4/5
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