Open the locked temple door was a mission in The Precursor Legacy that Jak and Daxter completed to obtain a power cell. A lone locked precursor door was finally opened by Jak who found a power cell within.
The locked Temple door is located next to the large machine in the back of the area. To get here you can just go to the right (after crossing the first bridge into the jungle) and get on the other side of the river to your left. You will need to be channeling blue eco to open the door. The easiest method is to enter the beam being redirected by the mirror on top of the machine and run over to the door. If the mirror has already been destroyed in Unblock the eco beam, don't fret, you can still jump up into the beam and be fully charged up.
If the mirror already has been broken in "Unblock the eco beam", you can also find a blue eco vent a short distance away. Past the machine, use the grass ledge on the left to jump down to the next area, on your right should be the eco vent. Run into it and run back, if you somehow manage to fall into the canyon below, you can always use the Precursor launcher to yourself right back up.
From here on just run straight up the ramp on the side of the temple to activate the door.