2001 Cadillac sts fan runs when car is off draining battery

computer is telling it to run, either because its faulty or the coolant temp sensor is bad. some vehicles have up to 3 temp sensors. one for the dash one for the fans and one for the computer so it knows what to set the gas mixture to. does it also idle crappy on a cold start up? if so id would put all my marbles in one bag and say its the sensor. hook up a scanner with data reading and check the ECT sensor. a ACTRON cp9145 will read the data.

Not necessarily. If the water temp is too high the fan will run for a short while (10 - 20 sec) until it falls enough to bring it down within spec regardless of the position of the ignition key ... happens on my CTS-V and Vette once in a while :-).

Like I said if the temp sensor is bad and saying the temp is too high the fans will run until the temp sensor gets into range but since its bad it will never get into range and stay running killing the battery.

AND I'm saying for the fan to run a bit after you shut down does NOT mean you've got a problem. Obviously if it does to the point of running your battery down you DO. Then again, allowing it to do so in the first place means you've got other serious, very serious issues. Have a nice day :-)