Most are located behind the gauge cluster. You have to remove the trim and four screws to get to the little switch either by the spedo cable or lower right corner of the cluster. ( if this a 120,000 mile interval, remove the light!) REMOVE TRIM COVER FROM RIGHT LOWER DASH, YOU WILL SEE A SMALL SWITCH. USING A LONG SCREWDRIVER YOU CAN MOVE SWITCH AND RESET EGR LIGHT.
I'm no mechanic, but from what I have read, the later model engines have hydraulic lash adjusters that can be tested and replaced(Haynes manual).
http://youtu.be/u5BUl9R42t0
geno's garge.com
Inside the dashboard, to the right of the glove box.
Usually passenger side, under the dashboard.
The starter of the 1994 GMC Sierra 1500 pickup is located on the dashboard. It is usually attached by a small wire.
Dashboard are 4x6 rear corner are 4x10
It is in the fuel tank and can be replaced.
Remove the dashboard faceplate, and the rest is pretty simple.
On the bottom side of the dashboard, in close proximity to the steering column,
blown fuse or blown fusible link.
Do you mean the expansion valve for the A/C? That is near the evaporator under the dashboard.