The W140 S420 is Mercedes' last over-engineered flagship—built like a vault but plagued by aging hydraulics, wiring-harness rot, and expensive M119 V8 engine issues after 100k miles. When maintained, it's bulletproof; when neglected, repair costs exceed the car's value fast.
Engine Wiring Harness Biodegradation
Common · high severityTypical onset: 80,000-150,000 mi
Symptoms: Random misfires, rough idle, stalling—codes for multiple cylinders, Check engine light on constantly or intermittently, Hard start or no-start when hot, sometimes OK when cold, Fuel smell in engine bay from cracked injector wiring
Fix: Complete engine harness replacement—Mercedes used biodegradable insulation that crumbles. Remove intake manifold, replace entire harness with updated version. 12-16 hours labor for competent tech, longer if you fight broken connectors.
Estimated cost: $2,500-4,000
M119 Head Gasket Failure
Occasional · high severityTypical onset: 120,000-180,000 mi
Symptoms: White smoke from exhaust on cold start, Coolant loss with no visible leaks, Overheating or persistent high temp readings, Milky oil or oil in coolant reservoir
Fix: Both head gaskets, head resurfacing if warped, new bolts, timing chain service while you're in there. Check for cracked heads—common if it overheated. 18-24 hours labor, more if heads need machine work or replacement.
Estimated cost: $4,500-7,000
Transmission Valve Body and Conductor Plate Failure
Common · medium severityTypical onset: 100,000-160,000 mi
Symptoms: Harsh or delayed 2-3 shift, slipping between gears, Trans goes into limp mode—stuck in second gear, Erratic shifting when cold, smooths out when hot (or vice versa), Check engine light with transmission fault codes
Fix: Drop pan, replace 13-pin conductor plate and valve body or solenoids. Clean electrical connectors thoroughly—corrosion is common. Filter, fluid, and gasket while you're there. 6-9 hours labor.
Estimated cost: $1,800-3,200
Hydraulic Self-Leveling Suspension (SLS) Leaks
Common · medium severityTypical onset: 90,000-140,000 mi
Symptoms: Rear end sags overnight or after sitting—pumps up when started, 'SLS Inoperative' warning on dash, Groaning or whining from rear suspension pump, Oily residue on rear shock bodies
Fix: Replace rear accumulators (spheres) and/or shock absorbers with hydraulic lines. Pump failure less common but happens. Alternative: convert to coil springs (aftermarket kits available). 4-6 hours for accumulators, 8-10 for full shocks.
Estimated cost: $1,500-3,500
Window Regulator Failure
Common · low severitySymptoms: Window drops into door or moves crooked, Clicking or grinding noise when operating window, Window won't go up—motor runs but no movement, Slow or jerky window operation
Fix: Plastic regulator clips break—common on all four doors but driver's side fails first. Replace entire regulator assembly; trying to repair clips is temporary at best. 2-3 hours per door.
Estimated cost: $400-700
Crankshaft Position Sensor Failure
Occasional · high severityTypical onset: 100,000-180,000 mi
Symptoms: No-start condition—engine cranks but won't fire, Sudden stalling while driving, then won't restart, Intermittent cutting out at highway speed, No fault codes stored or generic crank/cam correlation codes
Fix: Sensor located at bell housing—access is terrible, sometimes requires dropping exhaust or transmission crossmember. Sensor itself is cheap but labor kills you. 3-5 hours depending on approach and rust.
Estimated cost: $500-1,000
M119 Engine Oil Consumption—Piston Ring and Cylinder Bore Wear
Occasional · medium severityTypical onset: 150,000-200,000 mi
Symptoms: Burning a quart every 500-1,000 miles, Blue smoke on deceleration or at startup, Failed emissions test—high hydrocarbons, Low compression readings on multiple cylinders
Fix: Worn Nikasil cylinder linings or ring flutter—M119 weak spot. Needs bore inspection; if scored, you're looking at short block replacement or full rebuild with resleeving. 30-40 hours for complete engine out rebuild.
Estimated cost: $6,000-12,000
Buy one only if you're handy, patient, and have a $5k reserve fund—when sorted, it's the best highway cruiser ever made, but getting there isn't cheap.
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.