I bought a used 2018 F150. THe auto start stp appears to hae been disabled, but NOT by any of the common techniques- there is no test plug in the trailer conector, the plug under the dash near the brake pedal is plugged and there is no eliminator module connected to the disable swtitch.

This would leave only a FORSCAN modification if I understand this.

Here is the clue-

When enging started the icon on the dash indicates the auto start stop is DISABLED (the icon has a slash through it) BUT the light on the switch is NOT illuminated and the vehicle does NOT auto stop. If you push the switch, it lights up and icon disappers from the dash and a message stating auto start stop DEACTIVATED appears. Is this what happens when FORSCAN is used to idsable auto start stop?

No, to be clear, I LIKE this mod, as I don't like the auto start stop, I would just like to know what mod has been done to accomplish it because I am also experiencing random "Drive Mode not available" messages on the dash as well and am wondering if they may be a correlation.

Finally, the truck as a new battery that tests 12.2 volts when cold and has been sitting over night.

 

Unless the battery is brand new and fully charged, the Battery Management System may be turning AS/S off. I have a 3 year old AGM battery in good condition, but I make a lot of short trips, and my AS/S (when I try it occasionally) is often overridden for battery charging. It only works after I've made a long drive or connected my battery tender. No matter to me, I normally have it turned off anyway. There's a screen on the instrument panel that shows AS/S status and why it's active or not active.

 

12.2 volts is nearly dead... FYI @12.4 volts its in desperate need of a charge... should be close to 13 volts fully charged.
I highly suggest putting a trickle charge on it and reset the BMS.

 

There are a handful of things that could stop the a.s.s. from functioning.
​​​​​​For example if it's cold and the truck is working to heat the vehicle, that could kill the a.s.s from working.

You mention yer batt was low. Should be around 12.6/12.8. like mentioned above, that could kill it as well.

 

I also would guess battery, it needs to be fully charged. Be sure to connect ground to frame, not battery.

 

Another thing that will disable the a.s.s. if the sensor on the negative terminal on the battery is disconnected. This prevents the BMS from being able to see the battery state and thus doesn't enable it.

 

Stop Start is disabled due to a low state of charge of the battery. Get a charge or maintainer to keep the battery full.

 

I disabled the start/stop on my 2018 F150 by unplugging the BMS at the negative. Yours may look plugged in but not actually be snapped all of the way.
I have never had any codes or messages pop up. I've had it this way for the last 130K miles without issue. First battery lasted 6 years.
The best thing about this is that the battery gets a full charge and is not subjected to how the BMS wants to be smarter than the alternator.

 

Yes the computer thinks the battery has a low state if charge because it is not getting a voltage reading.
Another good thing is the voltage will always be in the 14v region and not be recused by the sensor.
High voltage means Stop stat is not defeated.

 

What does the instrument cluster say on the start/stop status screen? Does it say disabled for charging? That will tell you, otherwise we are all sitting here guessing.