Typically millivolt systems have a few components that need to be checked (I know, I'm troubleshooting my versa 405B). First, check the millivolt output of your generator. It needs to be between 250 and 750 mV, the higher the better since most gas valves have a minimum of 250mV needed to open the main valve. Mine is running about 400mV (brand new pilot assembly). Then, throw the switch and see if the voltage drops significantly. If it does not, there is something open in your circuit (or too much resistance). The next line of check would be either to jumper each individual component and see if you can tell where the main voltage drop is. timer switch, pressure switch, thermostat, on/off switch and 2 temp limit switches (one usually on the manifold on the opposite side of the input/output and one mounted near the input header near a pressure relief valve). If you can isolate which component is causing the unit not to light, first, clean the contacts. The other approach is to disconnect something in the loop (so you are only measuring one way around the system) and check the ohm values of each component and feed wire. I found that I had very high resistance in one of my limit switches until I wiggled the wire connector and made better contact. I also had bad contact within my pressure switch. I took that apart and cleaned the contacts and now it reads .1 ohm when closed rather than 60+ ohms. Once you eliminate all excess resistance, check the mV again when the system is activated. It will need to hang above a couple hundred mV under load to operate the main valve. Good luck