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My 2016 Volt has 10.5K miles and is just 1 week short of its 1 year birthday from the day I took possession of it. It has been on its original tank of gas which I have never filled it up, and had a little less than 2 gallons of fuel in it.

A few days ago, the Volt popped up a notification about needing to run maintenance. I assumed it was my usual Engine Maintenance that seems to happen every ~6 weeks, so I clicked the Ok button to let it run. However, the engine kept running for my entire 1 hour commute instead of the typical 10-15 min burn, and my battery was still completely full when I arrived at work.

When I left the office, the notification came up again, and this time I closely read the message and realized it was for "Fuel Maintenance," which I had never seen before. I clicked "More Info" and was greeted with: "Fuel Maintenance: Your fuel usage has been so low that the fuel is becoming stale. The engine must run to burn this fuel. Add fresh fuel when fuel lever is half a tank or less. See Owner's Manual for further details." (see image here) I tried everything to force the car to use the battery (changing the driving Mode, power cycling the car, clicking the "Dismiss" button), but the car was in a state similar to Hold where it was going to use the gas engine irregardless. I took it as a mechanism to force me to fill up with new fuel. However, I was left wondering what would happen if it ran out of fuel during my drive. Would it swap over to my (completely full) battery? Or would it actually stall out? I didn't want to risk it, so I pulled into a gas station and filled up for the first time ever. Took a picture to memorialize the moment!

My take away is that the car will trigger this unusual notification and engine always-on mode if the fuel hits a 1 year timer. It must also have an algorithm that resets or modifies this expiration timer when new fuel is added (apparently, if you add at least half a tank of new fuel, the timer resets).

Instead of suddenly putting the car into a permanent engine-mode, I would have preferred to have received a notification about the stale gas, and the option to defer the engine-on mode so that I could dictate how and when the car consumes the old fuel and gets refilled up with new fuel. This happened when I was in a crowded urban environment, so the engine was constantly firing on and off, which I would have preferred to avoid. I get that the Chevy engineers had to design the car for casual consumers who may not understand that fuel has a shelf life, but I'm guessing many Volt owners are very technical and would appreciate having more control.
 

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Page 182 of your manual covers this topic. You did get a warning about the gas that you could have deferred for 24 hours. That would have let you put gas in and control when it ran.
 

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A similar observation was posted a few days ago. You’ve discovered what some other Gen 2 owners are just beginning to approach - the passage of one year using the original tank of gas, triggering an FMM, as covered in the manual. The following is based on my Gen 1 experience (2012 Volt), but it should also apply to the Gen 2 Volt.

FMM occurs when the computer calculates the average age of the gas in the tank to be approximately one year old. You have the option of postponing it for 24 hours.

Once it starts, the Volt runs in Extended Range mode (i.e., uses the ICE) until the average age of gas in the tank is no longer 12 months old. You won’t be able to run on grid power from the battery, even if the battery is fully charged.

You can choose to drive until the gas tank runs dry (at which point you regain the use of the battery, but since the gas engine is no longer available, you’ll operate in Reduced Propulsion mode until you get more gas). This provides the longest interval until the next FMM (~ 12 months). There’s a minimum amount you must add to an empty tank to End the FMM (~1.5 gallons - will likely register on the display as Low instead of a numbered gas range estimate). After you refuel and start the car, the ICE will run briefly to check itself out, and then turn itself off and return you to normal Electric mode.

Once the FMM message has appeared, be sure you Start the FMM (even if you choose to postpone it 24 hours) so the programming can then End it after you take action. The computer may not have been programmed to anticipate someone might postpone the process and then attempt to avoid the FMM entirely by immediately refilling a half empty tank. Perhaps the trigger for recalculating the average age of the fuel and bringing the FMM to an end may require that the fuel door be opened and fuel added AFTER the FMM has been Started. If you postpone the FMM and immediately refill your tank, perhaps the postponed FMM message might pop up again the following day, and you might need to reopen the fueling door, and perhaps even add more gas, to bring it to an end.
 

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I have used about 20 Gal in the last 5 years and most of that is a yearly BIG BURN
for the first few years I just added 2X more gas to the tank and that got me almost another year
Now i just add 2-3 Gals and burn it all off once a year

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This super prius mode is interesting -- the Volt finally doesn't show 250 MPG all the time
still get battery only at low speed and the engine stops at lights. :)
 

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I avoid FMM runs by adding a fresh batch of gas to the car. If you have 2 gallons in your tank, adding 2 more gallons should do the trick. Of course, you will need to use the gas at some point in the future, at your own terms.
 

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If you rarely use gas, keep 1-2 gallons in it "just in case". This way you'll only burn that up if you don't use it at the end of the year.
 
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