I have a very similar problem with my Napoleon HD40 fireplace to what another user had posted on. My fireplace is using a Dexen 6K module and 6003 3v valve. The pilot ignites fine using the wall switch with either the transformer or 3v battery pack. However the main burner never lights up. I can also bypass the wall switch at the fireplace by shorting the SW1 contacts together . . . giving me the same result. The voltage from TP -> TH/TP is about 480mv after the pilot has been on for a minute or so. If I pull the TH wire from the valve there is no change. So I'm unsure if a) the ignition module isn't sending the TH wire what it needs or b) if the valve itself is defective. How can I tell? What reading should I have at TH on the valve?
I ordered a new ICM and that fixed the problem. In checking the voltages at the valve TH to ground is now 1.92v. Also, the red and green LEDs on the battery relay are now brightly illuminated. With the defective ICM they were very dim. Thanks for all your input. Hopefully this will help others in the future.
In millivolt systems which use a mercury switch the problem is that the switch was designed as 125- 50 VA operation not at 480 mv. The switch contacts need to be cleaned up! Remove the switch from the fireplace. Make a jig with a 20 watt bulb and a 110 volt source and have the bulb turned ON and OFF with a flame source (BBQ or torch). With a voltmeter you will find that the contact resistance is now below 1 ohm when it was some 60 ohms in my system before the aboe "cleaning".
Edit: This solution will work in millivolt system (450 mV-open,~220 mV closed). the current in this case is about 0.450 ma. The switch contacts get contaminated under this regiment. Using a jig to have the switch operate under. 125 volts 40 va range will clean the contacts. This is not the wall switch, it is located inside the fire place and its role is to sense if the pilot is lit. For safety remove ths switch from the fireplace when working with 125 volts.