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This is an older unit so HERE is a manual to help you identify the switch and has some troubleshooting procedures that might be helpful.The sail switch is located at the back of the furnace next to the discharge blower wheel. You will have to pull the furnace out of the camper, remove the Right side cover and combustion fan cover and you should see it. Not too hard of a job. Check to make sure that the discharge wheel/fan is not clogging up with lint have seen this on the older units, would be a good idea to clean this area... Hope this helps, IF you have anymore questions or concerns please feel free to contact me...
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RV Mechanic
10 Years RV experience... Cummins North West, Country Coach, Monaco Coach, Vendor certs.