Will the Eberspacher EasyStart T Timer 7 day with diagnostic 12v 24v | 221000328800 controller work with my ancient D1LC Compact bought S/H in 2000? Currently I have an 801 controller but its becoming unreliable. I guess I could use an 802 but I want to keep the error display facility. Thanks.

The Eberspacher Easystart T timer will have limited functionality with your earlier generation D1LC Compact heater. 

Timer - OKAY

Error code retrieval - OKAY

Room Temperature control - NO

So it is not possible to simply swap to this from 801 unfortunately - you would still require a controller for temp regulation, you'd need to add an 802 controller, mini-controller etc