How to Fix Fault Code 15 on Onan Generator? (Error Solved!)

onan generator code 15

Fault code 15 on the Onan generator is one of those fault codes that can have a high number of potential causes, some of which you will not be able to fix on your own.

But those that you could be able to fix have a common theme, they are related to the generator engine’s supply of both air and fuel.

When you are out and away from the beaten tracks, it is important to be able to rely on your generator to provide you with the electricity, so keeping it in a proper and working condition is important.

So, how to fix fault code 15 on Onan generator? To solve the error code 15 on Onan generator, just check and follow the steps below:

–           Step #1: Check the load and altitude settings

–           Step #2: Check air filter

–           Step #3: Check exhaust

–           Step #4: Check fuel lines, pump, and filter

–           Step #5: Check fuel filter

–           Step #6: Check the fuel pump

–           Step #7: Check governor spring position

–           Step #8: Check carburetor butterfly

–           Step #9: Check spark plugs

What is Fault Code 15 on Onan Generator

Fault code 15 happens when the generator is unable to provide AC with a frequency above 54Hz.

This can happen in three general situations, first, the generator engine is unable to provide the amount of power that is asked from it, second, the generator is not receiving a proper air/fuel ratio, and third, there are faults in the generator’s alternator.

The first two situations are something that you can fox yourself, but the third requires a professional shop.

So let’s move on with troubleshooting.

Reading the Fault Code on Onan Generator

Before you can fix any problem, you must first establish that you have it, and Onan generators have a simple way of informing you about it.

The STOP/PRIME button has an indicator light that will blink depending on the fault code it is displaying.

In case that you are not certain which fault code it is, it’s very easy to retrieve it.

  • First, you need to press the STOP/PRIME button three times inside five seconds.
  • This will initiate the indicator light to start blinking three times over and over again with a short pause between.
  • While it is blinking, you will need to press the STOP/PRIME button once and hold it for about a second.
  • When you release it, the light will start blinking in pattern for a two-digit fault code. It will flash two groups of blinks separated by a pause, after them will come to a long pause and the flashing pattern will repeat.

The indicator will first flash a group of blink for the first digit of the code, make a pause, and then flash for the second digit.

If the indicator flashes once, pause, and then five times, you have a fault code 15 on your hands.

If it flashes some other pattern it is showing some other code altogether, and you should search in our blog information on how to fix that fault code.

Read also:

>> How to Fix Fault Code 12 on Onan Generator?

>> How to Fix Fault Code 14 on Onan Generator?

>> How to Fix Fault Code 13 on Onan Generator?

Troubleshooting Fault Code 15

Check first for the simplest solutions

The fault code 15 appears most often when the generator’s engine is not able to rev high enough to produce the alternating current of more than 54Hz.

If you understand what issues with the internal combustion engines could cause such behavior, you should have a bit of understanding of which things you should be checked right away.

So, the first thing is to check the ambient temperature, all gasoline models of Onan generators can experience the vapor lock if the ambient temperature is above 120 degrees Fahrenheit.

Keep in mind that forecast may say that temperature at your location is lower, but it is measured in a shade, so the temperature to which your generator is subjected can be considerably higher.

Temperature can be an issue on the other side of the scale, as the propane models of Onan generators are sensitive to temperatures below 40 degrees Fahrenheit, as the propane will not be able to vaporize at a sufficient rate for the engine to run at required RPMs if the propane bottle is less than half full.

If it’s neither too cold nor too hot for your generator to work properly, you will have to start troubleshooting the problem.

Step #1 Check the load and altitude settings

The first potential cause you should be checking for is the combination of load and altitude settings.

If there is excessive load on your generator it will slow down, which in turn will drop the AC frequency, so turning off one airconditioner may sort out the problem.

This is particularly in the case that you are at a higher altitude, Onan generators just don’t like it, and at altitudes above 5,000 feet, they may lose as much s 15 percent of the maximum power.

Loss of power can be even more exacerbated by the wrong altitude settings.

Step #2 Check air filter

The second suspect when the generator is unable to produce high enough frequency is the engine’s inability to rev properly.

So you should start with checking for the potential problems that can “choke” the engine.

On all Onan generators, the air filter is easily accessible once you remove the access panel, and you should check it for dirt and any physical damage.

In case that it doesn’t look good, or is dirty, you should replace it and your fault code should be fixed.

Step #3 Check exhaust

If the problem is not the inability of the engine to draw fresh air, it could be the inability to exhaust the fumes, so you should inspect all parts of the exhaust for any blockage or damage.

On the majority of Onan generators, the exhaust pipe is a relatively short piece of pipe bent 90 degrees up.

That bend is a perfect place for dirt and debris to accumulate, and a perfect place to start the inspection.

Step #4 Check fuel lines, pump, and filter

In case that the air pat of the air/fuel ratio is not the problem, it could be the fuel.

The easiest is to check whether there is some fuel leak, as you should notice the liquid and wet fuel lines in case of gas or diesel generators.

If it is a propane generator, you should smell the characteristic smell of leaking propane, whichever is the case, replacing fuel lines should fix it.

If your generator is an LPG-powered one, you should also check the propane bottle regulator and fittings for leaks.

“The simplest way is to shit down the generator if it isn’t already and mix a 50-50 solution of water and a detergent that is safe fr hands, dish soap, hand soap, or shampoo are all fine.”

Next, it is best to use a brush or a sprayer bottle, and brush/spray this soapy solution on the bottle’s valve, regulator, and fittings.

If there is a leak, a bubble will appear and start growing at the spot of the leak, it will grow at least an inch in diameter before bursting, so freely ignore smaller bubbles that may appear as they are created by brushing/spraying.

Step #5 Check fuel filter

In case the fuel lines are OK and there are no leaks, the case may be that the fuel filter, whether it is a gas or diesel generator, is dirty and obstructing the flow.

Step #6 Check the fuel pump

The easiest way to check whether the fuel pump is working properly after a shutdown of a carbureted engine is to drain the carburetor bowl.

You will need to loosen the nut that is all the way on the bottom, below the altitude adjustment, and the main mixture adjustment knobs, while holding some container that is gasoline safe under the drain port that is behind the nut.

If the fuel pump is working properly, there should leak exactly an ounce of fuel, if the fuel pump is not working properly, there will be maybe a few drops of fuel or a little bit more, but well less than half an ounce.

So you will need to replace it.

In the case of diesel and LPG generators, you will need to use a voltmeter to measure the voltage on fuel pump connectors while cranking the engine.

If the voltage is stable and slightly under 12V, it is OK, otherwise it needs to be replaced.

You should keep in mind that fuel lines could be blocked, as well as carburetor needle jet, especially if you have recently replaced the fuel pump.

Because when taking off the old one a small piece of the rubber fuel line can be pulled off by the pump connector’s barbs, and it can find its way into the carburetor.

If you are unsure how to check fuel lines for blockage, it’s better to take it to a repair shop, it is a bit complicated as you need an alternative fuel source.

Step #7 Check governor spring position

On the right side of the carburetor there is a slotted and toothed bar to which a spring is attached at an angle, this is the governor’s spring and it should be attached to the 5th slot between teeth when counting from the right.

Step #8 Check carburetor butterfly or throttle body for binding

Another possible cause of the lack of fuel in the engine is if the carburetor butterfly is not working properly and is not opening fully.

Step #9 Check spark plugs

Last possible cause that you could check and diagnose yourself is the potential misfiring of spark plugs.

You should check them for carbon buildup and their gap, and if needed replace them.

On engines with fuel injection, the equivalent issue is the binding of the throttle body.

You should also check all the linkages on either carburetor or the throttle body whether they are working properly and there is no slack, damage, or debris around them.

Causes of the Fault Code 15 You Can’t Fix

In case that the error code 15 persists after this troubleshooting, the problem is almost certainly something you will not be able to repair and requires a visit to a shop.

Slip rings, generator brushes, windings, rotors, stators, etc. are complex for both diagnosing and repairing and require some tools that can be found only in shops.

So, it’s better to leave fixing those issues to professionals.

Prevent Fault Code 15 from Reoccurring

The main and most common causes of the fault code 15 are related either to lack of air, fuel, or the ability of a generator to provide enough power for all appliances it is asked to.

So preventing error code 15 from reoccurring comes down to providing the generator with proper air and fuel supply, and not loading it more than it can take, especially at high altitudes where its output is decreased due to a lower concentration of oxygen in the air.

In other words, you should change the air filter every 150 hours of running, or every 100 hours if you camp in particularly dusty places.

Replace spark plugs and fuel filter every 500 hours.

This way, your generator will work smoothly, and your camping trips will not become an unpleasant experience.

I FIXED MY ONAN GENERATOR MYSELF! >> Check out the video below:


Mike Gilmour

Hi, I'm Mike, co-founder, and editor of RV and Playa. My passion is traveling (with my RV) and enjoying the day at the beach (Playa)! Well, I originally created this blog as a way to share what I've learned by experimenting with the RV lifestyle, and I want to help others develop in life through new skills and opportunities.

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