Exhaust Gas Recirculation (EGR)
Routing a measured share of exhaust back into the intake to lower combustion temperature and cut nitrogen oxides. A common failure point on hard-worked diesels.
EGR reduces nitrogen oxides by sending a metered portion of exhaust gas back through the engine. Diluting the intake charge lowers peak combustion temperature, and nitrogen oxides form at high temperature, so a cooler burn produces less of them.
The cost is that the engine is deliberately breathing its own soot. EGR valves and coolers foul over time, and when they do the symptoms are the ones a shop sees constantly: a check engine lamp, a stored trouble code, reduced power, sometimes a coolant loss when a cooler cracks.
Duty cycle drives how fast that happens, and public fleet duty cycles are unhelpful. Long idling and sustained low load keep exhaust temperatures down and deposits up, which is why a refuse packer, a plow or a patrol vehicle can foul an EGR system on a mileage figure that would look premature on a highway truck. Judging these failures against a mileage expectation borrowed from over-the-road fleets is how a shop concludes it has a bad engine rather than a mismatched application.