The 2002 Diamante is Mitsubishi's top-shelf sedan with the 6G74 3.5L V6, known for smooth power but plagued by automatic transmission failures and catastrophic engine bearing/piston problems that often total the car. These are low-volume vehicles with parts availability challenges.
Automatic Transmission Failure (F4A42 4-Speed)
Common · high severityTypical onset: 80,000-140,000 mi
Symptoms: Harsh shifting or slipping between 2nd-3rd gears, Delayed engagement when shifting from Park to Drive, Transmission overheating, burnt fluid smell, Check engine light with speed sensor or solenoid codes
Fix: The F4A42 transmission suffers from valve body wear, clutch pack failure, and oil cooler contamination. Rebuild runs 12-16 hours labor; used replacements are scarce. External oil cooler often fails first, sending metal shavings into transmission. Replace cooler proactively at 60k if original.
Estimated cost: $2,800-4,500
Catastrophic Engine Bearing and Piston Failure
Occasional · high severityTypical onset: 120,000-180,000 mi
Symptoms: Loud knocking or rattling from lower engine block on cold start, Sudden loss of oil pressure, oil light flickering at idle, Metal shavings or glitter in oil during changes, Smoke from exhaust, loss of compression in one or more cylinders
Fix: The 6G74 V6 develops rod bearing wear, piston slap, and ring land failures, often from inadequate oil changes or long highway runs with marginal oil quality. Requires full engine rebuild or short block replacement: 18-24 hours labor. Many shops won't invest time given vehicle value—common to swap in used engine (10-14 hours) but risk inheriting same problems.
Estimated cost: $3,500-6,500
Head Gasket Failure (Both Banks)
Occasional · high severityTypical onset: 100,000-160,000 mi
Symptoms: White smoke from exhaust, sweet coolant smell, Overheating under load, bubbling in coolant reservoir, Oil looks milky or chocolate-brown on dipstick, Persistent coolant loss with no visible leaks
Fix: V6 head gaskets fail from age and thermal cycling. Job requires pulling both heads, resurfacing, new gaskets, timing belt/water pump while in there. 14-18 hours labor. Often find warped heads requiring machine work ($150-300 per head). Not worth doing unless transmission is known-good given dual risk.
Estimated cost: $2,200-3,800
Transmission and Engine Mount Failures
Common · medium severityTypical onset: 70,000-110,000 mi
Symptoms: Excessive vibration at idle in Drive, smooths out in Neutral, Clunking when shifting from Reverse to Drive, Engine rocks visibly when revving in park, Vibration through steering wheel at highway speeds
Fix: Front and rear engine mounts plus transmission mount deteriorate, allowing powertrain movement. Front mount is worst—hydraulic design leaks fluid. All three mounts: 3-4 hours labor. Do them together; half-measures lead to repeat visits. OEM Mitsubishi parts highly recommended over aftermarket.
Estimated cost: $650-1,100
Fuel Pump and Filter Failures
Occasional · medium severityTypical onset: 90,000-140,000 mi
Symptoms: Hard starting, long crank times especially when hot, Sputtering or loss of power under acceleration, Stalling at idle or when coming to stops, Whining noise from rear seat area (fuel tank)
Fix: In-tank fuel pump weakens, filter (also in tank) clogs. Filter is non-serviceable—requires pump module replacement. 2.5-3.5 hours labor to drop tank, replace assembly. Pumps fail prematurely if owners run tank below 1/4 regularly. Use quality fuel; these engines are sensitive to contamination.
Estimated cost: $550-900
Front Lower Control Arm Bushings and Ball Joints
Common · medium severityTypical onset: 80,000-120,000 mi
Symptoms: Clunking over bumps from front suspension, Steering wanders, requires constant correction, Uneven tire wear on inside edges, Loose or vague steering feel
Fix: Front lower control arms use pressed bushings and integrated ball joints—both wear together. Arms are replaced as assemblies; bushings alone not serviceable. Both sides: 3-4 hours labor plus alignment. Aftermarket arms available but quality is hit-or-miss; OEM preferred for longevity.
Estimated cost: $700-1,200
Alternator Failure and Underhood Wiring Deterioration
Occasional · medium severityTypical onset: 100,000-150,000 mi
Symptoms: Battery light on dash, dim headlights at idle, Electrical accessories cutting out intermittently, No-start, clicking when key turned, Burning smell from engine bay, melted wire insulation
Fix: Alternators fail around 120k; 1.5-2 hours labor. Separate issue: underhood wiring harness insulation degrades from heat (recall addressed some but not all). Causes shorts, especially near exhaust manifolds. Harness repair/replacement: 4-8 hours depending on extent. Inspect wiring during any underhood work.
Estimated cost: $450-1,400
Pass unless under $2,000 with documented transmission and engine health—too many expensive failure modes for a car with minimal parts support.
AI-assisted summary drawn from NHTSA recall data, our labor-times database, and platform knowledge. Not a substitute for a pre-purchase inspection on a specific vehicle.