Troubleshooting Cummins Onan 4000 Starts But Won't Continue Running on 2021 Thor Class C
Updated 09/29/2025 | Published 09/08/2025
Question:
I have a 4000 on my 2021 Thor class c just like video it will start but not keep running help
asked by: Pete C
Expert Reply:
Hi Pete; there are quite a few reasons this could be happening with your Cummins Onan QG 4000i # CM42ZR. I would start by checking the oil level. One of the most common problems is too much oil or too little. If you just filled it and it is sitting right at full you might want to suck out a little of the oil and see if that fixes it.
You may also have stale fuel if it has been sitting for a while, a clogged fuel filter, dirty carburetor, or a dirty air filter restricting the air flow. If you're able to rule all of these out then it might be more related to parts like a dirty/bad spark plug or even a bad fuel pump.
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Robin H.
9/29/2025
Hi Pete, did checking the oil level or any of the other suggestions help get your generator running smoothly? I'm curious to know because your experience could help others facing similar issues.