Raypak RP2100 Service Light

Hi, I have a 15 year old Raypak RP2100 gas heater. The "pressure/high limit" & "service" lights are both red and the unit will not start. Both "power" & "call for heat" lights are green, but the unit will not produce heat. Raypak is of no use whatsoever, so I was hoping that someone can give some troublshooting tips. Thanks in advance.

The service manual is here:
http://www.raypak.com/uploads/6000.52AG.pdf

Unfortunately, I don't have enough experience working with pool heaters to be able to troubleshoot them remotely.