you don't see it at first but your code where you create the String object is in a massive switch/case.
in C++ you cannot declare local variable in a case statement unless you create a {} block for the case.
so try with
switch (tempToRead) {
case 3: [b][color=red]{[/color][/b]
...
[color=green]String heatConv0 [/color]= String(temp0Int); // construct string representation
heatConv0.toCharArray(TemperatureF0char, 5); // string representation is 4 chars plus null terminator, copy 4 characters, ignore null terminator
tempToRead = 4;
break;
[b][color=red]}[/color][/b]
need to do this every time this will be in a switch statement
side note - try to remove those String construction just for the sake of getting the toCharArray after... use C functions for this or you'll get possibly at some point possibly some memory errors if this is called often