The W140 S600 with its M120 V12 is a monument to over-engineering, but that engine's biodegradable wiring harness and propensity for expensive internal failures make it a financial minefield past 100k miles. When maintained obsessively it's magnificent; when neglected it's a five-figure repair waiting to happen.
Engine Wiring Harness Disintegration
Common · high severityTypical onset: 80,000-150,000 mi
Symptoms: Random misfires, multiple cylinder codes, Rough idle or stalling, especially when warm, Check engine light carnival with no consistent pattern, Visible cracking or melting of wire insulation in engine bay
Fix: Complete engine harness replacement required. Mercedes used biodegradable insulation that turns to dust. 12-16 hours labor, must remove intake manifold and accessories for access. Aftermarket improved harnesses available.
Estimated cost: $3,500-5,500
M120 V12 Internal Engine Failure (Nikasil Bore Scoring)
Occasional · high severityTypical onset: 100,000-180,000 mi
Symptoms: Excessive oil consumption, 1+ quart per 1,000 miles, Blue smoke on cold start that clears after warmup, Loss of compression on multiple cylinders, Metallic rattling from crankcase at idle
Fix: Early M120s had Nikasil cylinder liners that fail with high-sulfur fuel. Requires complete engine rebuild or replacement. 40-60 hours labor for in-chassis rebuild. Later Alusil blocks are more durable but still see bearing and piston ring failures from neglected oil changes.
Estimated cost: $15,000-25,000
Head Gasket Failure (Both Banks)
Occasional · high severityTypical onset: 120,000-180,000 mi
Symptoms: Coolant loss with no visible external leaks, White smoke from exhaust, especially on startup, Overheating or running hotter than normal, Oil contamination in coolant reservoir or vice versa
Fix: V12 architecture means double the misery. 25-35 hours labor per side, requires removing both exhaust manifolds, significant disassembly. Often find warped heads requiring machine work. Smart to do both banks preventively if one fails.
Estimated cost: $8,000-14,000
Transmission Oil Cooler Line Failure
Common · medium severityTypical onset: 100,000-160,000 mi
Symptoms: Transmission fluid pooling under vehicle, Pink or red fluid dripping near radiator area, Transmission slipping or delayed engagement when low on fluid, Burnt transmission smell if driven low on fluid
Fix: The steel cooler lines rust through where they connect to radiator. 3-5 hours labor including flush and refill. Critical to catch early before transmission runs low and damages itself. Often combined with transmission mount replacement.
Estimated cost: $800-1,400
Hydraulic Suspension (ABC) System Failures
Common · medium severityTypical onset: 90,000-140,000 mi
Symptoms: Car sits low on one corner or entire side, ABC warning light and suspension malfunction message, Fluid leaks visible at shock/strut units, Harsh ride or bouncing, loss of adaptive damping
Fix: Active Body Control uses hydraulic fluid under 3,000 PSI. Accumulators, struts, and pump all fail. Single strut replacement 4-6 hours, pump 6-8 hours. Parts are eye-wateringly expensive from dealer, quality aftermarket options limited.
Estimated cost: $2,500-6,000
Mass Airflow (MAF) Sensor Contamination
Common · low severityTypical onset: 60,000-120,000 mi
Symptoms: Sluggish acceleration, feels like driving with parking brake on, Poor fuel economy, 20-30% worse than normal, Hesitation or stumbling during light throttle, Check engine light with MAF sensor codes
Fix: Hot-wire MAF sensors get contaminated by oil vapor from crankcase ventilation. Clean with MAF-specific cleaner first (30 min job), replace if cleaning doesn't help. Two sensors on V12, one per bank. Common to neglect and misdiagnose as bigger issues.
Estimated cost: $400-800
Fuel System Component Degradation
Occasional · medium severityTypical onset: 100,000+ mi
Symptoms: Extended cranking before engine starts, Rough running or misfires under load, Fuel smell in cabin or engine bay, Loss of power at highway speeds
Fix: Fuel pumps, fuel distributor, accumulator, and injectors all age poorly. V12 has two fuel pumps and complex distribution. Filter replacement 1 hour, pump replacement 6-8 hours (tank drop). Injector service all 12 cylinders adds significantly to any engine-out work.
Estimated cost: $1,200-3,500
Only buy if you have deep pockets, a trusted specialist, and complete service history — this is a $100k car with $100k problems hiding in a $10k package.
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.