How to Fix Fault Code 12 on Onan Generator? (Error Solved!)


The fault code 12 on Onan generator, in the simplest terms, means that over-voltage, a state when the generator is producing too high voltage or the control board is erroneously detecting a too high voltage.

This is not among the most common fault codes, and if you are inexperienced in the maintenance of Onan generators, it can be an anxiety-inducing event.

Fault code 12 can be caused by few potential issues, finding the correct one is the first step to fixing it.

When you are out there, dry camping, the only source of electricity for your RV and the appliances in it, will be your generator.

Thus, it is very important that it is in proper working order and supplies power reliably to your motorhome.

So, how to fix fault code 12 on Onan generator? To solve the error code 12 on Onan generator, just check and follow the steps below:

  • Step #1: Trip all breakers
  • Step #2: Check the loads
  • Step #3: Check the generator connector
  • Step #4: Check RV’s main breakers
  • Step #5: Check branch breakers

What is Fault Code 12 on Onan Generator

When you take your RV away from the conveniences of the campgrounds and shore power, the main source of electricity for it is the generator it has.

Any type of malfunction of the generator can simply ruin your RV trip.

The main appeal of RVing is the ability to take all the comforts of modern civilization out in the wild, and the crucial device that makes it possible is an electric generator.

When your generator starts displaying an error code, it can be a mildly frightening experience, Onan generators are built with very robust protection from malfunctioning, and most of the issues that can arise you can fix by yourself.

The fault code 12 signifies that the control board is detecting that generator is either producing voltage between 138-150V continuously for longer than 3 minutes or above 150V when it will instantaneously shut down.

This is a fine distinction, as there are several reasons why this situation can happen, sometimes it can happen even if your generator is producing a proper voltage in the neighborhood of 120V.

But, I’ll get to that in a moment, for now, let’s move to troubleshoot the cause of the fault code 12.

Read also: How to Fix Fault Code 14 on Onan Generator (Always Works!)

Reading the Fault Code on Onan Generator

Before you can fix an issue, or even start troubleshooting for its cause, you first need to make sure that you have it.

The Onan generators have a very simple system of displaying fault codes, though it can be sometimes a little bit confusing, especially when the generator grinds to halt without any obvious reasons.

“The STOP/PRIME button has a convenient indicator light that will continuously light when there are no problems but will start flashing when some problem exists”

It will continue blinking a specific code related to a detected issue for five minutes, after which it will stop.

But, in case that you didn’t have enough time to read it, there is a way to again retrieve the error code.

  • You will first need to access the generator, by opening its hatch on the side or back of the RV, depending on its model.
  • Next, you need to find the STOP/PRIME button. Depending on the model of your generator, it is either in the lower right corner of the generator, on older models, or in the upper left corner, on newer models.
  • Next, you will need to press it three times. From the first press, you have five seconds for the other two, so there is no reason to frantically mash the button.
  • The indicator light will start repeatedly flashing three blinks followed by a pause. This flashing pattern means that you have a fault code 3, which means that the generator has stopped because of a two-digit fault code.
  • Now, you will need to press the STOP/PRIME button once, and hold it for about a second.
  • As soon as you depress the button it will change the blinking pattern. Flashes will be split into two groups, the first one will signal the first digit and the second one will signal the second digit of the fault code. After a long pause, the pattern will repeat, and keep repeating for five minutes, or until you press STOP/PRIME button twice quickly.

If the indicator light flashes once, pauses, and then flashes twice followed by a longer pause, you are dealing with the fault code 12.

You should have in mind that the fault code 21 is not assigned, so there are very few chances of confusing them.

If you think that you are dealing with code 21, you have mixed up the short and long pause between flashes, it is actually the fault code 12 you are dealing with.

In case that the flashing pattern is different altogether, then you have a different fault code on your hands.

In that case, your best course of action is to either consult the operator’s manual on how to deal with it, if you have one, or to search our blog for a guide on how to deal with that particular fault code.

Troubleshooting fault code 12

There are several possible causes of the fault code 12, in other words, to push your generator to produce a significantly higher voltage than 120V.

Some of them you can fix yourself, but some require some special tools to both diagnose and repair.

There is also a chance that your generator is producing the proper voltage, but the control board is faulty and detects it wrongly, in which case you will need to have it replaced.

Before you start doing anything, you should be aware that some fixes may be against either Cummins’ or Onan’s warranty policy, or your RV dealership’s warranty policy.

So, before you do anything, you should consult your RV dealer and check whether it would void your warranty if your RV and generator are still in the warranty period.

Fixing any problem will require from you some level of skills fixing machines and working with various tools, so if you are uncertain of your skills, it’s better to take the RV to the dealership or some other repair shop.

Check first for the simplest solutions

The easiest thing to check and fix is one that you would never associate with the voltage, it’s the fuel.

“You should check the fuel level and top it off if needed.”

If everything the generator has more than ¼ of fuel, then you will need to start troubleshooting the problems, for which it is a good idea that you have a multimeter.

Step #1: Trip all breakers

You should first turn off all breakers, the two on the generator, main RV breaker, and all RV branch breakers.

Once you have done it, you should locate the AC terminal block TB2, which location depends on the model, but most commonly is in the rear upper left corner of the generator, under the top service panel.

Here you will need to measure voltages across terminals TB2-1 and TB2-3, and across TB2-2 and TB2-3; while the generator is working.

In case that you measure no voltage across one of these two terminal combinations, or on both voltage is equally increased, the problem is with the generator itself.

Possibly rotor or stator are damaged, or brushes are misaligned, slip rings could be damaged, winding, and few more issues; common for them is that you will have to take to your RV dealership or Cummins’ repair shop to have it fixed.

But if both readings show close to 120V you should move to the next step.

Step #2: Check the loads

Next, you should turn ON both breakers and measure the voltage across their terminals.

The 25A breaker should read around 120V AC, while the 5A breaker should read 13.4V to 13.8V DC.

If the 25A breaker is reading high, very likely there is something wrong with the electric system right after the generator, and you should move to the next step.

But, if the 5V breaker reads more than 13.8V DC, most likely the battery charger booster, if your RV has it inside the inverter, is the problem.

A temporary solution for this is to decrease the battery charge rate to 30% if it’s adjustable, but for a long-term solution, you should see the RV dealership.

Step #3: Check the generator connector

RVs either have an automatic transfer switch or the generator has to be manually hooked up to RV.

In the latter case, you will need to test voltage at the generator’s side of the cable, if it is increased then there is a short in cables between the generator and RV.

Step #4: Check RV’s main breakers

If the voltage measured in the previous step is OK, or you have an automatic transfer switch, you should turn on the main breaker in RV and measure the voltage across it.

In case that it is higher than 130V, it is either a faulty transfer switch or carburetor, but you should continue with troubleshooting to eliminate one of these two.

Step #5: Check branch breakers

You should start turning on one by one the branch breakers in your RV and measuring the voltage across them.

In case they all show increased voltage, you should remove the air filter on the generator and spray a small amount of carburetor cleaning spray in it.

If this leads to the generator working properly for more than half an hour, the carburetor needs to be replaced.

If it doesn’t help, the most likely culprit is the dirty fuel filter, which you should replace, or the faulty control board, which needs to be replaced.

Prevent Fault Code 12 from Reoccurring

Unfortunately, there isn’t much you can do to prevent the fault code 12 from happening in most cases.

Keeping critters away from your RV can prevent them from nibbling on the wiring between the generator and RV while using quality fuel will slow down the fouling of the fuel filter.

All other potential causes are related to the lifespan of various parts and the quality of their built materials.

My Onan Generator Starts But Won’t Stay Running – REPAIR AND TROUBLESHOOTING GUIDE >> 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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