2002 audi a4 avant 1.8t quattro
if you have check engine light on with the p1296 code and your fan is running all the time then read this thread it just may be a solution for your problem.
For what i was able to figure out it is for all audi models from 2002 and up till at least 2005
for me this was a very costly,painful and time consuming issue.
P1296 code came on my car about six monts ago,and since that i did the fallowing to correct the problem;

thermostat(replaced twice)-recomendation by my audi dealer
wather pump
cooant temperature sensor(replaced twice)
radiator fan switch(replaced twice)
maf cleaned and then replaced
coolant flushing(3times)
paid for numerous other diagnoses

it would be good for some time and the engine light would come on with the p1296.
Just over the weekend i was able to conect the dotes with the help of some other people and here is what happened.
When i went to my audi parts specialist to ask for the radiator fan switch i was given a ''colant temperature sensor'' instead of the radiator switch.
They got it all wrong in the audi parts system and not just genuine audi parts(your local dealer) but i allso went in four other local parts stores and they all handed me the same (wrong part)coolant temperature sensor.
Now,how do you put a coolant temp sensor in the place where the fan switch goes?
It is very simple ,becouse it is almost identical body shape,same electrical connection(two pins connection on my model)o ring and it clips on the same way(plastic clip) but it has very different properities;
coolant temp sensor operates at !!! 140f maximum !!! So if you had this guy in the fan switch place, that will couse your car fan to run all the time and your check engine light will come on with the p1296 code
radiator fan switch operates at temps !!!above 200f !!!
I am not saying that this is solution for every p1296 problem so keep that in mind but it is worth looking in to it based on my own painful experience
this switch is located on the bottom of the lower radiator coolant hose (it is not the one you thread in to your radiator-that is i bellive for older audi models)
part costs from around $23. To $35. And is very ease to get to and it is worth a try.
Once again this is all based on my findings,problem related on my 2002 model and what happened to me.
If this helped to you a quick thank you would be appreciated
good luck