Thanks for your quick reply.
my program is large, with a number of local libraries, etc.
I have written a smaller program to try and isolate the problem, but the problem goes away when I make the program smaller.
for example, the large program...
Hello Friends,
my program downloads into my Teensy 4.0 just fine.
and the Teensy Loader says: "Reset OK".
and the program starts, and runs without a problem.
but if I power it off, then when I power it back on, I get the Boot Loader 9 Blinks.
"a...