THINK TWICE

The LML Duramax's most catastrophic failure is its Bosch CP4.2 high-pressure fuel pump. On the 2011-2016 6.6L diesel the CP4 can self-destruct with little or no warning, and when it does it drives metal debris through the entire fuel system -- injectors, rails, lines, and tank -- so a pump failure becomes a whole-fuel-system replacement running into many thousands of dollars (one complaint describes paying about 6,500 dollars, then failing again shortly after). Owners describe the engine losing power at highway or city speed with no warning (a fuel-filter or reduced-power message, then complete power loss and, with it, power steering and brakes) and refusing to restart; several texts explicitly name the CP4.2 and metal shavings contaminating the fuel and injection system. GM never recalled the CP4, so there is no free fix -- the loss falls on the owner.

Check — Ask whether the truck still runs the original CP4 or has had a CP4 failure/replacement, and whether a disaster-prevention bypass kit is fitted. Metal glitter in a fuel sample or in a cut-open fuel filter is a red flag. Budget for the risk on any unmodified higher-mileage LML, and take any sudden reduced-power or fuel-filter warning seriously.

The LML Duramax's emissions aftertreatment is a chronic running headache. The 2011-2016 6.6L added a full diesel-exhaust-fluid (DEF/SCR) system on top of the diesel particulate filter and EGR, and the complaint texts across these years describe check-engine and "reduced engine power" warnings, "poor engine fluid quality" / DEF-quality messages that drop the truck into a limited-power limp mode, and forced or failed regenerations. These are driveability and cost problems more than crash-safety defects, but they are frequent, they can strand the truck in reduced power, and the repairs (DEF injector, NOx and DEF-quality sensors, EGR, DPF) are not cheap.

Check — Scan for stored emissions/DEF/DPF/EGR codes, ask about reduced-power events, DEF-quality warnings, and regeneration behavior, and check for a "delete"/EGR-DPF tune (illegal, and a smog/registration problem) as well as records of DEF-system, EGR, or DPF work.