For clarity, I assume you're meaning RAK-15/4 and that this is the manual you're using:
http://www.electrothermal.com/adminimages/M7064_RAK15_4_iss_3(2).pdf
The full statement of issues there is that a red light is triggered by a boil-dry condition, voltage issue, or that "If fault condition indicator remains on after these actions, or any other lamp sequence is evident that is not covered by tables I, II & III above, then HW&R service/ maintenance is required."
I'd first ensure that you're absolutely positive that the voltage is remaining OK while the heater is in operation and that you do not have a low volume of water in the unit.
CHECK. I'd then explore to make sure that the pressure relief valve is working fine, and would further go on to test the thermal control circuit that would normally protect the unit from overheating.
CHECK.
This TM will likely give more useful details, including parts and repair processes:
http://www.liberatedmanuals.com/TM-10-7310-241-12-and-P.pdf
Edit: I realize that the TM I linked to appears to be mostly about the RAK-15/2 unit and an equivalent. Still, I believe the principals of operation and its assembly are very similar and should have a lot of commonality. Perhaps not close enough to identify exact replacement parts if needed, but close enough to help you figure out how to troubleshoot.