I have a 2003 Dodge Ram 1500 and the mechanic has discovered that the heater core is clogged. The truck has 25,000 miles on it and the coolant level is in the normal range. The coolant is from the factory. What could have caused the clog?

Tap water contains a lot of minerals that buildup and can block radiators and heater cores. You only want to use a 50/50 mixture of the recommended antifreeze and distilled water. Positively do not use tap, well, or soft water.