How to troubleshoot electronic board on WH?

Suddenly this morning DW went to take hot shower and the water was only warm, what is from yesterdays sun.
It is Suburban WH and shows red light on control panel all the time. I bypassed thermo switches for no change.
Removed Circuit Board #06-235658-001 and what next?
Don't see anything burned, so is replacing the board my only way to figure out if it is bad?