Cummins ISX15
The X15's immediate predecessor and one of the most common engines in the used market. The 2013 and later builds are materially better than the 2010 to 2012 ones, which is the single most useful thing to know when shopping.
Output
400-600 hp
1450-2050 lb-ft peak torque
Fuel per mile
$0.623
at 6.5 mpg and $4.049 a gallon
Fuel a year
$74,751
over 120,000 miles
In-frame overhaul
$20k-$30k
rough, and shop rates move it
In the exhaust
DPF and SCR
Selective catalytic reduction adds a second fluid, diesel exhaust fluid, dosed into the exhaust to convert nitrogen oxides. It let manufacturers tune the engine for economy again, so SCR engines are generally BETTER on fuel than the DPF-only generation before them.
Known issues
EGR cooler and valve failures, DEF head and dosing problems, and camshaft wear on higher-mileage examples. Early build years attract most of the complaints.
Verdict
Very buyable at 2013 and later. Below that, budget for aftertreatment work you did not plan and check whether the EGR cooler has already been done.