Clogged radiator symptoms and how to fix them. Here’s a tip before you price out a new radiator—most clogs are repairable. It usually comes down to two things: crud blocking the fins on the outside, or rust and scale buildup inside choking off coolant flow. The main signs are pretty clear—temperatures creeping up under load, coolant dumping into the overflow tank, or cold spots on the radiator after it should be warm. This guide shows you how to confirm the problem step by step. Shine a light through the fins to see if they’re packed with debris, check what your coolant looks like, and rule out the thermostat, water pump, cap, and fan system before you pull the trigger. Clean external debris with low-pressure water or air, use a flushing product to break up internal deposits, and only replace the radiator if the flow doesn’t come back after cleaning.