The display is very faint? I cannot find a way to darken it?

change the battery

Generally for LCD calculators there is no adjustment of the screen that you can do.
If it's dim and battery powered, try replacing the batteries. If it's solar powered, try keeping it in strong light

Turn the calculator on. Hold down the "C" key (just above the ON key) and press + to darken the screen. If you go too far, use the C and - keys to lighten the screen.

It may be the backlighting has been readjusted by someone other yourself. Whenever you go on battery, I'm sure you've seen the screen darken a bit. This is the backlighting being turned down to save battery life. There is usually a blue function "fn" key that allows the user to brighten or darken the display. Look at the blue icons in the F1-F12 keys. The one to brighten the display looks light a sun and the one to darken the display looks a full moon eclipsed. Press and hold the fn key and tap the desired function key to brighten or darken the display.

try to disable the toner saving mode...

Turn the calculator on. Press and release the 2ND key. Press and hold the up-arrow key to darken the screen/increase the contrast.

Obviously, to go the other way you use the down-arrow key instead of the up-arrow.

the monitor may be going into power save mode which it should come out of when you press a key or move the mouse but it sounds like you have tube inverter issues. Your screen has light tubes behind it to illuminate the screen, if the tubes or the inverter develop a fault, then a very dark screen can result as the tubes are turned off. There may also be an ambient light sensor which will darken the screen in low ambient light. If the sensor is faulty, darkened screens will result. If the inverter is failing, you may be able to replace it, contact viewsonic. if the tubes are failing, replace the panel or the entire monitor. If the ambient light sensor is faulty, replace the panel or entire monitor as the sensor is part of the display panel.

go into the settings and turn the brightness up when the unit is plugged in and when it is running on battery mode (when it is plugged in it automatically goes into behicle mode and dims the display)

Most probably the batteries since I just had the same exact problem and replaced them. Works like a charm now. Takes two batteries. A76R W82.

you need to take the phone and have the LCD replaced