An EGR cooler is a component of the Exhaust Gas Recirculation (EGR) system that is designed to lower combustion temperatures by diluting the air/fuel mixture with a small amount of inert exhaust gas. The EGR cooler is a water-to-air heat exchanger that sits between the turbo and the EGR valve, and it is responsible for cooling the exhaust gas before it is recirculated back into the engine. The EGR cooler has a hollow tube or series of tubes for the hot exhaust gas to pass through, and surrounding these tubes are several coolant passages. Heat is transferred through the walls separating the exhaust and coolant, delivering cooler, inert exhaust gases ready to lower peak combustion temperatures. The flow of exhaust gases through the EGR cooler is controlled by the vehicles computer via the EGR valve.

The EGR cooler is an important component of the EGR system, and its failure can lead to coolant loss, overheating, and engine damage, among other failures. Symptoms of EGR cooler failure include odd-looking exhaust, engine overheating, and low coolant levels. EGR coolers are a known weak point on most diesel engines, and they can exhibit cracking or warping thanks to repeated cycling of extreme temperatures.

In summary, an EGR cooler is a component of the EGR system that cools the exhaust gas before it is recirculated back into the engine, lowering combustion temperatures and reducing pollutant formations.