I am having a an intermittent ignition lock out problem. It is a Bryant Plus 90 evolution model 355 aav with a LP conversion. The stat will call for heat it starts the cycle evacuation fan,ignitor gas valve clicks but more often than not no ignition. Of course it does this several times till it locks out [code 14] It has on occasion lit the low stage burner for a few seconds then it goes out but this does not happen every time. The installer/serviceman has replaced the flame sensor twice the gas valve and checked the low and high stage gas pressure as well as propane inlet pressure. The unit is only 3 years old. It is installed in the attic and the house is very efficient. We have had a problem this year with the condensate line freezing and I would pull the float out so I would have heat. But it hasn't been that cold lately.[20,s-30,s at nite] This seems to occur when we get home and turn the stat up or in the morning when we want a little extra heat. Like I said its intermittent because at times it will work. Of course it worked fine when the service man was here replacing the valve,flame sensor,checking pressure etc..