12V to 6.7.
Every Generation.

The single most common 6.7L Cummins turbo failure. The EGR system routes exhaust gas back through the intake, and soot from that exhaust accumulates on the VGT vane assembly over time. When the vanes stick, the turbo loses variable geometry function -- boost comes on wrong, the exhaust brake stops working, and fault codes appear. Using the exhaust brake regularly is the most effective preventive measure -- it exercises the vanes and keeps them from cementing in place. At rebuild the vane assembly, unison ring, and nozzle ring are cleaned and inspected.

The electronic VGT actuator controls vane position and is a separate failure point from the turbo itself. A failed actuator presents as fault codes, boost irregularities, or engine derate. Calibration is required after any actuator replacement using Cummins INSITE, the Holset E-Tool, or compatible diagnostic equipment. The actuator on ISX applications comes in 12V and 24V variants -- installing the wrong voltage actuator means an immediately dead unit. Confirm your system voltage before sourcing a replacement.

The 6.7L Cummins EGR cooler is a known failure point. When an EGR cooler fails it can introduce coolant into the oil system, and that contaminated oil goes through the turbo bearings. Any 6.7L that has had an EGR cooler failure should have the turbo inspected before returning to service -- coolant-contaminated oil destroys turbo bearings quickly and silently. On older 5.9L trucks, extended oil change intervals and sludge buildup in the oil passages are the equivalent concern.

Intake-side foreign object damage contacts the compressor wheel at operating speed and causes immediate imbalance. On the 5.9L, heavily tuned trucks running the HX35W or HE351CW beyond their designed flow range experience compressor surge -- a cyclical stall condition that hammers the thrust bearing and fatigues the compressor wheel. A surging turbo makes a distinctive fluttering sound under boost. The wheel is inspected at teardown and documented before any rebuild quote is issued.

Shutting down a hot diesel without a cooldown period allows residual oil in the bearing housing to cook with no oil flow to carry the heat away. This is particularly relevant to work trucks and tow rigs that get pushed hard and then shut down immediately. A 2-3 minute idle after sustained high-load operation significantly extends bearing life. On turbocharged diesels used for towing, a turbo timer or manual cooldown discipline is one of the lowest-cost reliability habits available.

The HE351CW introduced an integral electronic wastegate solenoid -- a first for the 5.9L ISB. The solenoid controls boost pressure and can fail electrically or mechanically. A failed solenoid presents as overboost or underboost depending on whether it sticks open or closed. It is distinct from the VGT actuator on the 6.7L but serves a similar boost control function. The solenoid is inspected at every HE351CW rebuild and replaced if out of spec.