2006 GMC SIERRA 2500HD

Typical onset: 100,000-180,000 mi

Symptoms: Transmission fluid leaking from radiator area or frame rails, Pink or milky transmission fluid indicating coolant contamination, Overheating transmission, delayed shifts, Catastrophic transmission failure if coolant enters Allison

Fix: Replace transmission cooler lines (often corroded at frame mounting points), flush transmission and cooler completely, replace fluid and filter. If coolant contamination occurred, full Allison rebuild required. Prevention: inspect lines every 50k mi. 3-5 hours labor for lines only, 20-30 hours if transmission rebuild needed.

Estimated cost: $800-1,200 (lines only), $4,500-7,000 (with transmission rebuild)

Typical onset: 150,000-220,000 mi

Symptoms: White smoke from exhaust, sweet smell, Coolant loss with no visible leaks, Overheating, rough idle, misfires, Coolant in oil (milky dipstick) in severe cases

Fix: Remove both heads, resurface or replace if warped, install updated gaskets with ARP studs recommended. Check for cracked heads (common between valves). Includes coolant flush, thermostat, hoses. 12-16 hours labor.

Estimated cost: $2,800-4,200

Typical onset: 180,000-250,000 mi

Symptoms: White or gray smoke at idle or acceleration, Rough idle, cylinder contribution codes (P0201-P0208), Hard starting when engine is hot, Fuel in oil (dilution) detected on oil analysis

Fix: LBZ injectors are more reliable than LB7, but still fail. Replace failed injector(s), return lines, and fuel filter. Balance rates test identifies weak cylinders. 2-3 hours per injector. Many do all eight preventively at high mileage.

Estimated cost: $600-900 per injector, $4,000-6,000 for all eight

Typical onset: 150,000-200,000 mi

Symptoms: Slipping or flaring on 2-3 upshift under load, Long delay going into reverse, Burnt transmission fluid smell, dark fluid, P0735 or P0730 codes (incorrect gear ratio)

Fix: Full Allison rebuild with updated C3 clutches, Alto or Suncoast upgraded frictions recommended for towing. Flush cooler, replace filter, lines. Often combined with torque converter replacement. 18-24 hours labor.

Estimated cost: $3,800-5,500

Typical onset: 80,000-140,000 mi

Symptoms: Service 4WD message on dash, Cannot shift into or out of 4WD modes, Grinding or clicking from transfer case when shifting, 4WD indicator lights flashing or incorrect

Fix: Replace encoder motor on transfer case (NP263 or NP261). Simple bolt-on part, no internal work needed. Common failure point due to corrosion and vibration. 1.5-2 hours labor.

Estimated cost: $350-550

Typical onset: 120,000-180,000 mi

Symptoms: Burning 1+ quart of oil per 1,000 miles, Blue smoke on startup or acceleration, Carbon buildup on spark plugs, Low compression on multiple cylinders

Fix: Requires engine rebuild or replacement. Rings collapse or carbon-lock in grooves. Often combined with head gasket job. Short block replacement common. 24-32 hours labor for full rebuild.

Estimated cost: $4,500-7,000

Typical onset: 60,000-120,000 mi

Symptoms: Fuel odor in cab or under hood, Visible fuel staining on filter housing or frame, Hard starting after sitting overnight, Air in fuel system, rough running

Fix: Replace fuel filter housing assembly or rebuild with new seals and hand primer. O-rings fail, plastic housing cracks in cold climates. Includes new fuel filter. 1.5-2 hours labor.

Estimated cost: $250-450