Mercedes-Benz S 420 CDI
Successor to the OM628, from 2005 with revised common rail injection at 1,600 bar and piezo injectors. Power output rose to up to 320 hp at 730 Nm — the gearbox hit its limits early, prompting Mercedes to recalibrate the NAG2 mapping during series production. Mechanically the OM629 shares most challenges with its predecessor but adds its own: the oil cooler in the V-area tends to leak at high mileages and additionally promotes EGR fouling of the intake tract. Cylinder heads come under extreme pressure during high-rpm full-load pulls — warped heads are not uncommon beyond 200,000 km. Piezo injectors demand clean fuel and short oil change intervals (10,000 km, 229.51 or 229.52). Anyone taking on this engine should budget for preventive oil cooler and seal replacement early on.
Silent Power on Four Wheels
730 Nm with subdued diesel refinement in a W221 body — the opposite of attention-seeking. The S 420 CDI drives like the class of silence. More economical than the V8 petrol, more composed than any six-cylinder. No compromise on long distances.
Engine Weaknesses 6
Warped cylinder heads allow combustion gases into the cooling system. Overpressure constantly pushes coolant out through the expansion tank. Engine runs at temperature, coolant level drops steadily -- replacement costs 5,000-16,000 EUR.
Symptoms: Coolant warning light despite stable engine temperature, coolant overflow from expansion tank, no visible external leak.
The NAG2 automatic transmission is rated for approximately 700 Nm; the OM629 delivers up to 730 Nm. Early build years without shift module adaptation showed driveshaft failures and gearbox problems. Later revised.
Symptoms: Shift jerks, gearbox fault messages, missed shifts, vibration on pull-away.
The oil cooler sits at the lowest point in the V-section of the engine and seals with rubber gaskets. At high mileages oil seeps out, pools in the V and drips down. Repair takes approximately 2 days.
Symptoms: Oil spots under the vehicle, oil mist in the V-section of the engine bay, elevated oil consumption.
The rear crankshaft oil seal leaks from approximately 200,000 km. The gearbox must be fully removed — substantial labour. Inspect the oil cooler at the same time and replace gaskets preventively.
Symptoms: Oil puddle under vehicle centre, oil film between engine and gearbox, oil loss without visible external source.
Intake manifold and inlet ports become heavily sooted through EGR recirculation. Engine oil from the V-area promotes coking. Simultaneous intake cleaning is mandatory during oil cooler repair.
Symptoms: Gradual power loss, increased fuel consumption, occasional lurching under load.
The OM629 uses piezo injectors at 1,600 bar injection pressure -- maintenance-intensive high-precision technology. With irregular servicing or fuel contamination, injectors wear prematurely.
Symptoms: Rough running, hesitation during cold operation, misfiring noises, black smoke.
Vehicle Weaknesses 6
The air struts of the W221 wear over time. Between 200,000 and 300,000 km, all four struts typically need replacement.
The Airmatic pressure reservoir tanks at the rear axle can corrode. Removal requires disassembly of the rear axle.
The instrument cluster develops pixel faults due to detached ribbon cables. In worst case, complete display failure.
The W221 shows sporadic CAN bus communication faults leading to failures of various systems: ESP, headlight control, fog lights.
Air spring bags (usually rear) become leaky and lose pressure gradually. The vehicle then sits 3–4 cm lower on one side. Fault code 5285 (fill process too long) appears. Repair is only possible by replacement.
The W221's extensive comfort electronics are fault-prone: electric windows, door mirrors and the optional panoramic sunroof show failures due to aged connectors and control units.
Reports & Tests
233 owner complaints filed with NHTSA (2005–2013). Most reported: Electrical (63), Engine (41), Powertrain (35).