The 4WD switch is an electronic switch or button located on the dash or the center console near the shift lever and its purpose is to initiate the change from two wheel drive to four wheel drive on vehicles equipped with a four wheel drive system. As is the case for most electrical components, the most common reason you would replace this part is simply because it stops working altogether. This usually doesn't mean the vehicle is disabled. You should still be able to drive the vehicle in whatever drive it was in before and you now just won't be able to change it until you replace the switch. It's best to have a mechanic diagnose the problem to be sure it's just the switch and not something more problematic like the transfer case, axles, or the transmission.
Up until the last few years, to engage 4WD, there would be a small lever next to the shifter and the shifting occurred manually and needed to be done when the vehicle was stopped. Vehicles nowadays can change on the fly while driving with just the push of a button and you don't even feel the difference. The buttons or switches can be different depending on the vehicle. Some of them have push buttons, some of them have knobs you would rotate, and still others have a flip switch, like a light switch in your house.
If your vehicle has 4WD and it's no longer able to make the change from two wheel drive to four wheel drive, and either you or your mechanic has determined that the switch is what needs to be changed, check out our website. Car Parts Discount has a selection of four wheel drive switches available for a variety of cars and trucks at the lowest possible prices.