Find the yellow vent switch was a mission in The Precursor Legacy that Jak and Daxter completed to receive a power cell. Jak discovered an ancient cave (after a hint from Willard) in Snowy Mountain filled with a large number of Precursor machinery that led to another vent switch, unlocking yellow eco vents across the world.
While facing the front of the Lurker fort, turn right and head straight. There will be a small cluster of pine trees concealing the entrance to a cavern filled with various Precursor mechanisms.
The goal is to reach the end here, while not falling into the bottomless area below. The main danger is the ice which makes your movements slippery and makes avoiding the pushers or successfully jumping on to the floating platforms much more difficult. Nonetheless, there are no enemies here and ultimately it is not that long. Towards the end, you will find the yellow vent switch.
Grab the power cell to automatically activate it and unlock yellow eco vents throughout the world. To get back safely, jump back down to the entrance with the pine trees.