getting errors 2 and 36 on startup

I keep getting errors during startup of out PE2400 II. initially during warm up I will get error 2- combustion temperature over 1100°C, but the unit is nowhere near that warm. After one or two attempts the i can get the unit to run, but eventually it will fail with an error 36, (or 2) again a combustion overtemp. Has anyone had this issue. I have replaced the heater and thermocouple, and ohmed out the TC leads to the board with less than an ohm on each lead.