Actually, if the power light flashes, yes it's showing overheat, OR the sensors may not be plugged in. If BOTH power and ready lights flash, its the pressure switch.
You need to take the two temp sensors off the heater and plug them in to the control box.
Assuming they are both good, red light should stop flashing and jet pumps can be turned on.
If your heater
has the pressure switch (two sensors and a third small grey wire coming out of the end of the heater) you will need to shunt the wires from the pressure switch. There is an actual part number for a pressure switch bypass, but I usually just cut the wire, strip the red and black and crimp them together.
Failing that - simply hot wire the pumps over to incoming 220 - one at a time, power off, of course.
HTH