It appears that the C1 ECU parameter, commonly used to improve starting performance, exhibits very high temperature dependency. A C1 setting that works well in summer temperatures doesn't work at all in near freezing and colder conditions.
I have observed on 2 FI WR450Fs that setting C1 to 8 will make the bike extremely rich during the start cycle at temperatures below freezing. If the bike stalls in warm up with C1 set to 8 at these temperatures, it is nearly impossible to restart the bike, due to an extremely rich mixture. Setting C1 to 0 allows the bike to be restarted easily, but that may be the ONLY way to get it to start. Restart attempts at these temperatures with C1 set to 8 will only result in severe flooding and no restart.
This behavior was observed on these bikes with 3 different maps loaded into the comp ECU - stock (all zeros), FMF and the YZ exhaust map. These bikes both had aftermarket exhaust systems and otherwise ran great. They had TPS readings (d:01) of 12 and air pressure readings (d:03) of 87 kPA at 3500-4000 feet when local weather stations were reporting atmospheric pressure readings of ~ 100 kPA. No codes were present on either bike.
I recommend that people fully test the ability to start and restart their bikes in cold weather conditions if they have C1 set to something other than 0. Specifically, in freezing conditions and colder, if you kill the bike during warm up, can it be restarted and can it be restarted when hot.
I make this recommendation in order to prevent someone from being unable to restart their bike should it stall out on the trail in cold weather.
I hope this helps someone. I spent 3 days chasing this issue before I isolated it to the C1 parameter. Luckily mine failed to restart in the garage and not on the trail, in the middle of no where.