After warming up to normal operating temp, it will shut the vehicle off when put in gear.

What make , model an year vehicle ? May not be a fault , may just need to be reset . Has the battery been replaced or unhooked lately ? Look at the owners manual an find driver personal preference setting's for autolocks . Normally they will lock when putting the vehicle into drive , an unlock when putting the gear selector into park .

sometimes the computer gets wiped clean and it has memory on how to idle, memorizes how the transmission shifts when you drive.. when all that is lost it can take a few days to relearn.. I found that I had luck spraying fuel injection cleaner into the throttle body and revving engine enough so it didn't shut off... and then I would drive fairly hard up hills etc.. and usually problem would correct itself

In a perfectly functioning system, when you turn the defrost on it will turn the ac on as well, pulls moisture from air to help defog windows. So that could be normal for the most part.

you could have a leak in the water pump or a freeze plug somewhere.

vehicles and heat don't get along, when they get warmed up stuff starts expanding, as is natural law, so theres things that can happen in a vehicle only when its get to operating temperature which should be around 200 degrees. so in the morning or after work, etc, what was a problem may no longer be a problem until it gets hot again.

not sure if any of this helps, but thought id give ya my 2 cents on something

try unhooking the battery for about 10 minutes, after doing an engine swap on my car the check engine light would come on. there were no problems but unhooking the battery fixed it.

Transmissions can be rebuilt. No need for a new one.

You recycle the old dead battery

Have no idea why you even put that back in

Common sense & your friend is correct

You don't disconnect anything on today's
vehicles, even with the key on

Put in a new battery,start the car, warm it up
& drive it for awhile & maybe you will get lucky
& it will complete some of the drive cycles & run some
monitors ,you know nothing about & it will be fine

If not, your mechanic friend should be about to stop
by with the bosses scan tool & help you out

if the car is warm the fans are designed to continue to run untill temp drops to normal temp and then fans shut off. This is a normal operation of the fan nothing to worry about. Now if they stay running for more than 10 minutes you have a problem.

Either a bad sending temp sending unit (engine) or the cable fell off it

Ok, time for car school. The vehicle is operated by the computer, every electrical concept that is. When the engine is cold, it starts fine because the computer has placed it in "cold start mode", putting more fuel into the engine than normally would until Once the engine is warmed up, a temperature sending unit or temp sensor tells the computer to change to normal run mode. IF said temp sensor is faulty and is not telling the computer to make said change, it stays in cold start mode, still dumping excess fuel in the engine that is now not required, cause it's warmed up. Like keeping the choke closed on older vehicles. Now I think you can answer your own question, when this no start action is happening after warm up and it always starts fine when cold, what's the first thing you check ? Right, the unsuspected temp sensor. Change that and no more start problem. Have a wonderful day. Glad I could be of service.

i know this may sound stupid but if your battery is old or a little weak on this vehicle the idle air control solenoid will act speradicley hunting for were it should be and the vehicle will stall.replace battery