How to Fix Onan Generator That Won't Stay Running After Running Out of Gas

The Quick Answer

Your Onan generator's three flashes indicate a low oil pressure shutdown, and since you've confirmed the oil level is correct, you likely have either an oil pressure sensor issue or need to prime the oil system after running it completely out of gas. The most common fix for your specific situation is to reset the system by starting it and allowing it to run for 5-10 seconds, which allows the oil pump to build proper pressure. If that doesn't work, you're looking at either replacing the oil pressure sensor (about $45) or checking for a clogged oil pump pickup screen.

What's Causing This

When your 2018 Winnebago Spirit's generator ran out of gas and shut down completely, the oil system lost pressure when the engine stopped. Onan generators have a safety feature that monitors oil pressure continuously - if pressure drops below about 4-6 PSI, it triggers the three-flash code and shuts down to prevent engine damage. Running out of fuel and subsequent engine shutdown may allow the oil system to lose some residual pressure.

The oil pressure sensor on your model is indeed located in a difficult spot - it's typically mounted on the engine block near the oil filter, and yes, many techs do drop the generator for easier access. However, before assuming it's the sensor, the issue could be that your oil pump simply needs time to build pressure after sitting without oil circulation. Your oil level being between "add" and "full" is actually perfect - overfilling can cause pressure problems too.

Another possibility is that debris from running low on gas has clogged the oil pump pickup screen in the oil pan. This is more common than you'd think when generators run completely dry, as sediment gets stirred up and can block oil flow.

Tools You'll Need

• Multimeter for testing sensor voltage (if needed)
• 3/8" socket set for oil pressure sensor removal
• Oil pressure gauge kit (rental from auto parts store, about $25)
• Fresh engine oil (SAE 15W-40, see manufacturer specs for capacity)
• New oil filter if you need to drop the oil pan
• Basic hand tools (wrenches, screwdrivers)
• Jack and stands if you need to access underneath
• Replacement oil pressure sensor (check service manual for correct part number) as backup

Step-by-Step Fix

Start with the simplest solution first. Start your generator normally (release start switch immediately when it fires up), then let it run for 10-15 seconds to allow the oil pump time to circulate oil and build pressure. Many Onan owners find this solves the three-flash problem after running out of gas.

If that doesn't work, check the oil pressure sensor connection. You can access it from underneath without dropping the generator - consult your service manual for the exact location on your specific model as it varies by installation. Look for a single-wire connector that may have vibrated loose. Clean the connection with electrical contact cleaner and reconnect firmly.

To test if it's actually an oil pressure problem versus a faulty sensor, you can temporarily install a mechanical oil pressure gauge in place of the sensor. Remove the sensor (it's a 3/8" thread), screw in the gauge fitting, and start the generator. You should see 25-40 PSI at idle when warm. If pressure is good but you still get the three flashes, replace the sensor.

If oil pressure is actually low, you'll need to drop the oil, remove the oil pan, and check the pickup screen for debris. This is about a 3-hour job but definitely doable in your driveway. The oil pan is held on with about 12 bolts, and you'll want to clean the pickup screen thoroughly with carburetor cleaner before reassembling with a new pan gasket.

When to Call a Pro

Call a certified Onan technician if the oil pressure gauge shows low pressure and cleaning the pickup screen doesn't solve it - this indicates internal engine wear or oil pump failure, which requires specialized diagnosis. Also call if you're not comfortable working under the generator or if you discover the oil pump itself needs replacement.

However, given your specific situation (recently ran out of gas, proper oil level, three flashes), there's about an 80% chance the extended start procedure or a loose sensor connection will fix your problem. The oil pressure sensor replacement is definitely a DIY job if you're handy with tools, even though access is tight. Budget about 2 hours for sensor replacement if you do need to go that route.

If you decide to call a pro, expect to pay $200-400 for diagnosis and sensor replacement, or $600-900 if the oil pan needs to come off for pickup screen cleaning. Many RV service centers see this exact issue multiple times per season, especially after the spring startup when generators have been sitting all winter.