What is the maximum altitude this Eberspacher Airtronic heater control unit ECU can operate at and if an altitude kit is required. I have the 802 controller. Thank you very much.

Do you have an Eberspacher Airtronic D4S ( sometimes referred to as D4 PLUS) ?

All standard control units for this heater model, including replacements are okay for heater regular use at up to 1500 mtrs. It is possible to operate the heater for short periods only at up to 2000-2200 mtrs  - higher altitudes or more constant regular use and the heater should be equipped in addition with a High Altitude Kit - see weblink below 

https://www.butlertechnik.com/item/Eberspacher/Airtronic-and-Hydronic-heater-high-altitude-kit-221000332200/6SF