EGR is non-negotiable for NOx control on modern diesels—everyone knows that. But it’s also a source of headaches. Sticking valves, cooler cores plugging solid, position sensors that start lying to the ECM. That’s the trifecta. Let’s run through the basics first: what the system does, then where it bites you. The usual failure patterns are predictable—soot packing in the valve throat, coolant seepage into the gas side from a cracked cooler core, and feedback drift where the commanded position and actual position stop agreeing. When that happens, the driver feels it: flat power, a lumpy idle that shakes the cab, or a derate that kills your day. Here’s the thing—don’t fire the parts cannon. Fault codes give you a hint, not a diagnosis. I’ve watched guys throw three valves at a truck and still have the same code. Get into live data. Compare requested EGR position versus actual. Watch the temperature delta across the cooler. Monitor sensor voltage for jitter. That’s where the real story lives. Duty cycle is the big one nobody talks about. Highway miles with active regens? EGR tends to stay cleaner. Low-load, stop-and-go, extended idle? That’s what kills them early. Maintenance wise, pull the crossover pipe and look at the carbon crust—don’t guess. Check coolant condition weekly for signs of oil or combustion gas. Hit sensor connectors with contact cleaner, not compressed air—you’ll just push moisture in. Cummins, CAT, Deere, Perkins—they all respond to the same basic care. Skip it, and you’ll do the same job twice.