2016 GMC SIERRA 3500HD

Typical onset: 80,000-150,000 mi

Symptoms: sudden loss of power, metal shavings in fuel, hard starting or no-start, fuel in oil, requires towing

Fix: CP4.2 pump grenades internally and sends metal debris through entire fuel system—requires complete fuel system flush, new injectors (all 8), high-pressure fuel lines, fuel rails, tank cleaning, and CP4 replacement. 25-35 hours labor. Some owners pre-emptively install CP3 conversion kit ($2,000-3,000) to avoid this nightmare.

Estimated cost: $8,000-12,000

Typical onset: 60,000-120,000 mi

Symptoms: check engine light with reduced power, DEF quality poor warning, speed limited to 55 mph after countdown, DEF heater failures in cold climates, injector clogging

Fix: DEF tank heater, injector, and pump module fail regularly. Tank often needs replacement due to crystallization. NOx sensors ($300-500 each) also common culprits. Injector replacement 3-5 hours, full tank module 6-8 hours. Diagnosis is critical—throwing parts gets expensive fast.

Estimated cost: $1,200-3,500

Typical onset: 70,000-130,000 mi

Symptoms: transmission fluid puddles under truck, low fluid warnings, transmission slipping or harsh shifts, fluid dripping from cooler lines at radiator

Fix: Allison 1000 cooler lines corrode where they connect to radiator or at hard-line junctions. Replace lines and fittings, flush system if contaminated. 2-4 hours labor depending on 2WD vs 4WD access. Inspect transmission mount while you're under there—they crack frequently on heavy-duty use.

Estimated cost: $400-900

Typical onset: 100,000-180,000 mi

Symptoms: white smoke from exhaust, coolant loss with no visible leaks, overheating, rough idle, milky oil if catastrophic

Fix: EGR cooler cracks internally, dumping coolant into exhaust or intake. Requires cooler replacement, sometimes EGR valve too. If caught late, can hydraulic-lock engine or score cylinder walls. 8-12 hours labor—cab doesn't need to come off but access is tight. Many owners delete EGR systems where legal (off-road use).

Estimated cost: $2,200-4,000

Typical onset: 120,000-200,000 mi

Symptoms: rough idle, white or black smoke, loss of power under load, check engine light with P0299 or injector codes, increased fuel consumption

Fix: LML injectors fail from contamination (often CP4 pump wear even before catastrophic failure) or internal wear. Balance rates go out of spec. Replacement requires 1-2 injectors minimum, often all 8 if contamination suspected. 6-10 hours labor for full set. Use OE Bosch injectors—avoid cheap offshore replacements.

Estimated cost: $3,500-6,500

Typical onset: 100,000-180,000 mi

Symptoms: reduced power especially on acceleration, excessive black smoke, turbo underboost or overboost codes (P0299, P0234), whining or grinding noise from turbo

Fix: Variable geometry turbo vanes stick from carbon buildup (short trips, idling) or actuator motor fails. Turbo can sometimes be cleaned and actuator replaced (4-6 hours), but if shaft play exists or compressor wheel damaged, full turbo replacement needed (6-8 hours). Extended idling kills these turbos—keep RPMs up when towing.

Estimated cost: $1,800-4,500