Here's a couple of exerpts from our records. I'm not sure these will help... it's all I was able to find from a search on "PT_LEN1":
PT_LEN1 is the array to store the incremental length of the path based on the nodes.
...Now if these are not the problems. It is very likely that the problem is resulting due to 2 issues:Make sure that the Last Patch in the details of the Heat Flux is turned on.The time you are inputting is causing an issue. Basically the end time (5sec) may be too off. The idea is to round it off to the next integer so if the path is 0.015m long and velocity is 0.005. It would take 3sec. So if the model takes 2.7 sec (let's say) then make it 3sec. Also make sure that the step end time is long enough in your analysis settings.