2012 GMC SIERRA 2500HD

Typical onset: 80,000-150,000 mi

Symptoms: sudden loss of power or no-start, metal shavings in fuel system, fuel in oil (dilution), rough idle before failure

Fix: CP4.2 pump grenades internally, sending metal through entire fuel system. Requires pump replacement, all eight injectors, fuel rails, lines flushed or replaced, tank cleaned or replaced, and often HPFP regulator. 18-25 labor hours depending on contamination severity. Many owners install CP3 conversion kit during repair to prevent recurrence.

Estimated cost: $8,000-15,000

Typical onset: 70,000-120,000 mi

Symptoms: DEF range countdown on dash, reduced engine power mode, check engine light with P20EE or P2BAE codes, DEF heater fault codes in winter

Fix: DEF pump, heater, injector, or level sensor failures trigger limp mode at countdown zero. DEF pump assembly is most common ($1,200 part). Also see DEF tank crystallization from contaminated fluid requiring full tank replacement. 3-6 labor hours for pump, 8-10 for tank.

Estimated cost: $1,800-4,500

Typical onset: 90,000-140,000 mi

Symptoms: transmission fluid in coolant (strawberry milkshake in overflow), coolant in transmission, transmission slipping or harsh shifts, overheating in either system

Fix: Internal cooler in radiator fails, mixing ATF and coolant—catastrophic for Allison 1000. Requires radiator replacement, complete transmission fluid flush (sometimes full rebuild if contamination severe), coolant system flush. Install external cooler to bypass. 6-12 hours labor if trans survives, 20+ if rebuild needed.

Estimated cost: $1,500-3,000 (cooler fix) or $4,500-7,000 (with trans rebuild)

Typical onset: 100,000-160,000 mi

Symptoms: white smoke from exhaust, coolant loss with no external leaks, overheating, rough running or misfires

Fix: EGR cooler cracks internally, dumping coolant into exhaust/intake. Requires EGR cooler, EGR valve often replaced simultaneously, intake cleaning. Some techs delete system where legal. 8-12 labor hours for OEM repair.

Estimated cost: $2,200-3,800

Typical onset: 120,000-180,000 mi

Symptoms: hard start when cold, rough idle, white or black smoke, reduced fuel economy, P0087 low fuel rail pressure codes

Fix: LML injectors fail from high pressure and contamination. Requires injector replacement (typically do all eight for longevity), return line replacement recommended. 10-14 labor hours. Often discovered during CP4 failure inspection.

Estimated cost: $4,500-6,500

Typical onset: 80,000-120,000 mi

Symptoms: clunk when shifting into drive or reverse, vibration at idle in gear, visible sagging of transmission tailshaft

Fix: Rubber mount deteriorates from heat and load, especially with heavy towing. Single mount replacement on two-post lift. 1.5-2.5 labor hours.

Estimated cost: $300-600

Typical onset: 100,000-150,000 mi

Symptoms: hard cold starts below 40°F, extended cranking, white smoke on startup, P0380 glow plug codes

Fix: Glow plug controller or individual plugs fail. Controller is common failure point. If plugs are stuck, can break off requiring head removal. Controller: 2-3 hours. Plugs (all eight): 4-6 hours if they come out cleanly, 18+ hours if head removal needed.

Estimated cost: $800-1,500 (controller/plugs) or $3,500-6,000 (if head removal required)