I've been banging my head up against the wall on this. Everything checks out -- the thermopile (TP) gives ~290mV once the pilot has been on for a few minutes (and around 590mV open). The pilot light is strong, and has been cleaned. All three jets (TP, TC, and the burner lighter) are equal, and flame impingement is good on the 2 probes.
The pilot light has no trouble staying on. If I take the front and top logs out and turn the burner on, the stove stays lit. If I turn it off and put those logs back in and repeat the whole lighting, the burner comes on for between 10 and 30 seconds and then goes out.
It's a direct-vent stove, and this is the first time I'm running it (a) off of an outdoor LP tank, and (b) in temperatures this cold (~15°F). Previously, I had been running it off of a 20lb grill LP tank, but I finally ran 1/2" black pipe from the 55gal LP tank that we run our cooking range/oven off of. It's 1/2" the whole way and drops down to a 3/8" flexible appliance hookup for the final 3'. The cooking range works just fine, even with all 4 burners going, and turning the stove on and off doesn't have any visible effect on the pilot light, so I don't think it's a supply issue. The cooking range is about 40' closer than the fireplace.
I'm wondering if the air coming in through the direct vent is so cold that it's causing the TP voltage to drop just enough to stop? Looking around online, some valves seem to work fine with 250mV, and some are picky about having >300mV... But, with or without the logs, I get 290mV so I don't know how that could make a difference.
For now, I'm running it just fine without the logs. I'm just lost as to why the logs would be causing the valve to trip.