Onan Fault Code 33 [Cummins Generator Overheat]

A Onan generator may report a code 33, overheat.

You may need to replace:

  • sensor
  • radiator
  • fan belt
  • water pump
  • Coolant
  • Oil


Check the coolant and oil levels.

Owners report this issue happens after 1000 hours.

Note that it will shutdown itself if it gets too hot.


Fault code 33 status led will blink 3 times, pause, then 3more.

Here is a video of how one person fixed overheating:



  1. Low coolant level: Insufficient coolant can lead to overheating. Check the coolant level and add more if necessary.
  2. Blocked or damaged radiator: A blocked or damaged radiator can impede the flow of coolant and lead to overheating. Inspect the radiator for damage or debris, and clean or repair as needed.
  3. Faulty thermostat: A malfunctioning thermostat may not regulate the engine temperature properly, leading to overheating. Replace if needed.
  4. Faulty water pump: A damaged or malfunctioning water pump may not circulate coolant effectively, causing the engine to overheat. Inspect the water pump and replace it if necessary.
  5. Leaks in the cooling system: Leaks can cause the coolant level to drop and lead to overheating. Inspect the entire cooling system for leaks, including hoses, connections, and seals, and repair as needed.