I routinely shoot in weather MUCH colder than 40F all the time. 40F isn't even 0C. That's balmy up here.In fact, I've shot thousands of shots in -20C (our average temperature from Nov-March) or colder temps over almost 2 winters without an ERR99 ever. I've even done some limited shooting in -40C without a problem, other than the fact my hands nearly froze to the camera.
I'm impressed because many cars wouldn't even start in that temperature, but my MK3 worked flawlessly!
(I also borrowed a MK3 from the local camera store and shot it for about an hour when it was -20C... no problems.)
Because of our long cold winters I actually do more shooting at below freezing temps than above.
Taken during a 2hr walk on the world's longest skate path at -20C
Taken in January when the windchill was closing in on -45C. We were out for about 2 hrs briefly running inside to warm up every 20 minutes. My assistant gave in to the temps before my camera ever did

You can see the exhaust from a parked/idling car on the right side of the frame.
I don't want to be one of those people who says "since it doesn't happen to me, you must be mistaken". However, if a measly 40F gave the MK3 problems, nobody here in Canada would be able to own one.

But I do know many people up here using the MK3 and it seems to relish our harsh working conditions.
EOS R6 Mark II - Sigma 50/1.4 Art - Sigma 14-24/2.8 Art - Canon EF 70-200/2.8L Mark III - Godox Xpro-C - Godox TT685C x2