The 2001 Diamante's 3.5L V6 is mechanically solid when maintained, but this generation suffers from catastrophic automatic transmission failures and oil consumption issues that can grenade the engine if ignored. These are expensive, low-volume luxury sedans where parts availability is declining.
Automatic Transmission Failure (F4A51 4-Speed)
Common · high severityTypical onset: 80,000-140,000 mi
Symptoms: Harsh or delayed 2-3 upshift, slipping under load, Transmission overheating, burnt fluid smell, Complete loss of forward gears or stuck in limp mode, Whining or grinding noises from bell housing area
Fix: The F4A51 transmission fails due to oil cooler contamination, clutch pack wear, and valve body issues. Rebuild requires 12-16 hours labor; used units are risky. External oil cooler replacement is mandatory during any trans work. Remanufactured unit swap is 8-10 hours.
Estimated cost: $2,800-4,500
Severe Oil Consumption / Piston Ring Failure
Common · high severityTypical onset: 120,000-180,000 mi
Symptoms: Burning 1+ quart every 500-800 miles, Blue smoke on startup or acceleration, Fouled spark plugs, misfires on multiple cylinders, Low compression across multiple cylinders
Fix: The 6G74 V6 develops oil control ring carbon buildup and eventual ring land failure. Requires engine removal, full teardown, piston/ring replacement, cylinder honing, and head gasket replacement. 24-32 hours labor. Short block swap is sometimes more economical at 18-22 hours.
Estimated cost: $4,500-7,000
Transmission Oil Cooler Line Corrosion
Common · high severityTypical onset: 60,000-120,000 mi
Symptoms: Transmission fluid leaking near radiator or under engine, Milky or contaminated transmission fluid (coolant mixing), Sudden transmission failure after coolant intrusion, Pink coolant residue visible on cooler lines
Fix: Steel cooler lines rust through, especially at crimped fittings. Coolant intrusion destroys the transmission in hours. Must replace all cooler lines (4-6 hours), flush trans and cooling system, and install external cooler. If contamination occurred, full trans rebuild mandatory.
Estimated cost: $600-1,200 (lines only); $3,500-5,000 (with trans damage)
Failing Engine and Transmission Mounts
Occasional · medium severityTypical onset: 90,000-150,000 mi
Symptoms: Excessive vibration at idle, especially in Drive, Clunking when shifting from Park to Drive, Engine rocks visibly during acceleration, Transmission tunnel heat and noise increase
Fix: Hydraulic mounts collapse, causing drivetrain misalignment and accelerated wear on CV axles and exhaust hangers. Front mount requires subframe access (3 hours), transmission mount is 2 hours. Replace all three mounts simultaneously for best results.
Estimated cost: $650-1,100
Crankshaft Position Sensor Failure
Occasional · high severityTypical onset: 80,000-140,000 mi
Symptoms: No-start condition, cranks but won't fire, Intermittent stalling when engine is hot, Tachometer drops to zero while driving, engine dies, Check engine light with P0320 or P0335 codes
Fix: Sensor fails from heat cycling near the timing belt area. Sensor replacement requires partial timing cover removal and belt inspection, 2.5-3.5 hours. Always replace timing belt and water pump if near service interval while in there.
Estimated cost: $350-550 (sensor only); $850-1,400 (with timing belt service)
Power Steering Rack Leaks
Occasional · medium severityTypical onset: 100,000-160,000 mi
Symptoms: Power steering fluid dripping from inner tie rod boots, Steering whine or groan at full lock, Low fluid level despite no visible external leaks, Stiff steering when cold, improves when warm
Fix: Rack seals deteriorate, causing internal leakage into the boots. Rack replacement requires alignment afterward, 4-5 hours labor. Rebuilt racks are available but quality varies. OE units are NLA, aftermarket is hit-or-miss.
Estimated cost: $800-1,400
Alternator Failure (High-Output 110A)
Occasional · medium severityTypical onset: 90,000-150,000 mi
Symptoms: Battery light illuminated, voltage below 13.5V, Dimming headlights, flickering dash lights, Grinding or squealing from alternator pulley, Electrical accessories shutting off randomly
Fix: Mitsubishi alternators fail from bearing wear and voltage regulator issues. Replacement is straightforward on top of engine, 1.5-2 hours. Use quality remanufactured unit; cheap alternators fail within 6 months on this chassis.
Estimated cost: $400-650
Only buy if transmission and engine have documented recent major work or extremely low miles; otherwise the repair costs will quickly exceed the vehicle's $2,000-3,500 market value.
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.