A fading LCD on your Samsung Propel can make the device difficult to use even if you just want to make a phone call. The phone's sliding design adds one main point of failure for the LCD, but the LCD could be fading in and out for a variety of reasons.
While it is rare for a software error to cause the LCD to fade in and out, it is the easiest cause to check as it doesn't require that you disassemble the device. First, flip the device over, remove the battery cover and remove the battery. After a few seconds, put the battery back in and turn on the device. This simple software reset will often clear up any software issues that could be causing the LCD to flicker. Additionally, the Propel has a feature that will return its settings to their factory defaults. While this feature, found at the bottom of the "Settings" menu, will rarely correct a flickering LCD, it is best to give it a try before you start taking the Propel apart.
LCD Flex Cable and Module
The most obvious reason for a flickering LCD is an issue with the actual LCD module or the flex cable that runs between the LCD control board and the device's motherboard. To get the Propel apart, you will need a #00 Phillips screwdriver and a pry tool. First remove the back cover, then the motherboard and finally the hinge assembly. Check the flex cable to ensure that it isn't ripped or crimped and replace it if necessary. Then pry out the LCD module and replace it as well. Note that by opening the Propel you will void any warranty on the device and always run the risk of damaging something else inside the phone.
If water got into your device, it could be causing the LCD to fade in and out. Even after the water dries up, the resulting corrosion could prevent electricity from flowing correctly inside the device's motherboard. When you have to Propel open, check the motherboard, LCD control board and LCD cables for signs of corrosion, which will look like a green or brown growth on any metal components. If you find this damage, you can attempt to clean it with a stiff brush and some Isopropyl alcohol; however, while cleaning the corrosion can correct the problem, it can also make it worse.
If replacing the LCD and the flex cable doesn't fix the problem, the issue is almost certainly with the hardware inside the Propel. There isn't any way to diagnose or repair these types of problems, so your only option is to have the device replaced. If your device is under warranty and you haven't voided it by dissassembling the phone, you can contact your wireless carrier to have it replaced. If your Propel is out of warranty, your only recourse is to purchase another device.