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Checking the Oil in your RV’s Onan Generator

| Updated Oct 24, 2013

If your Roadtrek is equipped with an Onan 2800/2500 there are a few things you should know how to do. One of them is checking the oil level. Many Onans have a low oil cut-off sensor to prevent them from running if the oil is low.

So if your Onan will not start or it dies while running the oil level should be second on your list to check (first make sure you have more than 1/4 tank of gas) .  Unfortunately for many owners, that means crawling under your Roadtrek.  Find a parking spot where you can overhang a downhill to make things easier.  Or you can use leveling blocks to raise the back of the Roadtrek for more clearance (use your emergency brake and chocks).

Before you crawl underneath get a flashlight and a rag or a few paper towels.  A tablecloth, tarp, or a patio mat placed on the  ground will make things a bit more pleasant.  Once you get underneath this is what you will see:

Onan 2800 is located under the center rear in newer Roadtreks.
Onan 2800 located under the center rear

If you have an older Roadtrek with a compartment mounted Onan,   you won't have to do any crawling.

Open the Onan access panel door by pulling up and then out on the latches.  Set the panel aside and be careful to get it securely re-installed.


Behind the panel you will see (click on photo to enlarge):

Here is behind the Onan Access Panel
Here is what is behind the Onan Access Panel

Note:  If you have Sprinter you will have the Onan 2500 which is the propane version.  It will not have a carburetor like the gas powered 2800 shown  in this photo.  Consider printing a copy of this photo and carrying it in your Roadtrek.  If you are physically unable to check your oil, the young guy in the next campsite might be recruited for the task!  With a photo you can shown him what to do.

Dipstick is next to the fuel filter
Note: Circuit Breaker is Off, should be On

Next locate the orange dipstick.  It might be a bit dirty, but it is next to the fuel filter, roughly in the center.  Unscrew it and wipe it off with a rag or paper towel.  Then insert the dipstick into the filler hole but DO NOT screw it in.  This is different from all the bigger Onan generators (sometimes RV dealers get it wrong – one reason for low oil).


Oil should be in the XXXX area
Oil should be in the XXXX area

Now pull out the dipstick and examine the oil level with your flashlight.  The oil level should be in the hatch marked area between the F (full) at the top and the L (low) at the bottom.  Add oil if necessary.  Since the Onan only takes one quart of oil, it will not need much if it is a little low.  Several people have used plastic squeeze bottles with a spout (think those red and yellow ketchup and mustard bottles you see in some restaurants)  for adding oil a squirt at a time.  A couple of squirts may be all you need.  Add some and check the level until you have the oil level in the correct range.

Internal Onan Control Panel
Internal Onan Control Panel

With the level adjusted, screw the dipstick back in.  You can test to see if adding oil fixed your problem by pushing on the Start/Stop switch below the air filter.  Does it run?  Great!  Push stop and you are ready to button her up.   If not, your problem is elsewhere and requires more extensive troubleshooting.   We have some troubleshooting tables on our website.

Now re-install the Onan access door.  Remember the latches are at the bottom (you will struggle a long time if you try to install it with the latches at the top – don't ask how we know).  Take extra care to make sure you get the door securely latched because more than one Roadtrek owner has lost an Onan access panel on the road.   Should you loose the panel, do not run the generator  more than a couple of minutes without covering the opening or it will not cool properly.  And since a replacement access panel costs nearly $100, you really don't want to loose it.

Note:  If you have a 2013 Roadtrek you may have the newest version of the Onan 2500/2800.  Consult your manual, many things have changed.

Mike Wendland

Published on 2013-10-24

Mike Wendland is a multiple Emmy-award-winning Journalist, Podcaster, YouTuber, and Blogger, who has traveled with his wife, Jennifer, all over North America in an RV, sharing adventures and reviewing RV, Camping, Outdoor, Travel and Tech Gear for the past 12 years. They are leading industry experts in RV living and have written 18 travel books.

11 Responses to “Checking the Oil in your RV’s Onan Generator”

June 23, 2014at10:12 pm, Jan Jorgensen said:

Mine is under the van, how can I check it?

June 23, 2014at1:09 am, Bridges Crawford said:

I need checking the baring covers on trailer tires how to…..and why in the world my battery won’t charge when I plug it in at night….and what kind of toilet paper chemical works best….and how to back up without taking 5 million attempts…and on n on n on….like an on call direct line advisor?!? …

November 16, 2013at3:15 pm, Paul Taylor said:

This post was actually helpful, just what I was kind of looking for on fuel generators- thanks!

November 01, 2013at12:17 pm, Jim Langley said:

That’s super helpful, Lynn & Roger. Thanks so much. I’ll visit your other page, read more of your tips and pickup some oil to keep on hand now that I know the types that will work. I really appreciate the help. Best,

October 31, 2013at1:28 pm, Lynn & Roger Brucker said:

Jim, if all you are doing is adding a little oil to top it off, it hardly matters what kind. At least that is what we’ve been told – we are not experts. I think we just have some 10W-30 for that. For changing the oil, Onan does recommend using an oil that has the additives in it for small air-cooled engines. There is one with Onan’s name on it. Briggs & Stratton has one too It would be the type you would use in your lawn mower. Synthetic is fine, I think I remember hearing that the Onan oil was half & half. We have a little more Onan info here:

October 31, 2013at12:41 pm, Jim Langley said:

Great tips, thank you Roger and Lynn. Could you please tell what brand/type of oil is put in the Onan generator? I would like to buy some and keep it on hand just in case. Our Onan stopped working soon after we picked up our 2013 Agile SS. It turned out to be a loose ground wire and an issue with it getting enough propane. Those are fixed but if we have the right oil, thanks to your tips, we should be able to fix it if it shuts off due to low oil. Thank you, Jim & Deb

October 24, 2013at11:45 am, Bill Sprague said:

A great how-too and invaluable for those of us coming over from travel trailers.



October 24, 2013at11:09 am, Nancy said:

Good information for problem solving. When our 2800 generator would only run while holding the button then quit, and then would not start at all, we found our oil not to be low oil, but that our low oil cut-off sensor malfunctioning. The float inside of the sensor was not working properly. A new sensor unit cost around $58, and a new gasket for the oil pan $11.
We had no labor cost since John was able to drop the generator and do the repairs himself 🙂 Dodge 2001 P

October 24, 2013at10:10 pm, Nancy said:

Correction: Ours is a 2500 generator not a 2800.

October 24, 2013at10:28 pm, Lynn & Roger Brucker said:

The 2500 is nearly identical to the 2800. It just uses propane as the fuel so it does not have a carburetor. It produces slightly less power as well, hence the name 2500 (for 2500 watts) vs 2800 (for 2800 watts). Onan does not make a diesel generator in this size.

You were very lucky that your problem was the oil level sensor – there are some very expensive things that cause the generator to only run with the start button is held down. Apparently the oil sensor fails often enough that at some point Onan just quit using them (in the 2500/2800) anyway). It is possible to disconnect them when they fail, and just be careful about checking the oil level regularly. It was great John was able to do the work – that would have been an expensive labor job.

October 24, 2013at8:40 am, Dave Miller said:

Great article! it’s articles like this that helps people better understand their rigs and have the best RV experience. We just got back from a 3600 mile trip to Maine and had an incredible trip with no problems. Thank you, Bigfoot Dave

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