what causes the fan to keep going on and off on a c15 cat.

is your ac on whats your enigine temp during this

 

i had this problem. i had a leak inside the fan hub that only leaked when engine running. i rebuilt fan hub with a kitmaster gold top rebuild kit. once i had it apart i could see the plastic part that had o-rings on it that had all but fallen apart.

 

ground fan solenoid maybe. maybe

 

Is this a 2007 or newer truck? We have seen lots of issues with the power steer temp sender, or more likely in the wiring, it is tied into the a/c and an open in the circuit will turn the fan on. We normaly bypass this at the fan on pressure switch at the a/c copressor, just unplug the wiring to the p/s res and plug the connector from the main loomed harness into the a/c fan switch (orange pressure switch).

 

sounds like a ground wire.

 

p/s is power steering , res=reservour. 2005 would not have a temp sensor there, starts on 2007 and up.
Your 05 could have a bad/intermittant open in the harness from the a/c switches at the front left corner of the rad to the connector plug at the engine. when the fan is not running (eng. running) just wiggle the harness from the a/c switches back to the plug/connector at the front of the engine, if the fan kicks in test it again to be sure, we replace lots of these as the coolant level wiring is some times in this harness.