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.
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.
