I manage a large fleet with a number of Cat C7 engines. Sensors, Injectors and HEUI pumps are a constant problem, along with an occasional dropped exhaust valve. The 3126 and C7 are closely related, nearly identical engines.
The C7 has a design life of 18,000 hours, most of mine are lucky to do half of that. I have had Cummins QSB and ISB engines go over 75,000 hours, Detroit Diesel 2 strokes go 65, 000 hours. Must be because our duty cycle is so high? The engine moves the truck (56,000 lbs) and also powers a Roots blower that is 210 HP (not both at the same time).
My Cat dealer hates working on them, they make much more money working on off road equipment. When I drive one in the yard you can see mechanics running out the back door.