A fundamentally solid German diesel sedan popular across Europe, but with significant age-related issues. The tried-and-tested CDI engines are durable and can reach 400,000-500,000 km with proper maintenance - post-2003 models with revised injection are most reliable. Critical concerns: pre-2003 cars rust extensively (not galvanized until Q2 2003), notorious electrical problems (SAM modules, ignition switch), and typical diesel issues. The 722.6 automatic transmission has known valve body and conductor plate issues. Parts are widely available and cheaper than modern Mercedes. Best for buyers who can verify rust-free body and complete service history.
Pre-Q2 2003 cars not galvanized, rust forms under arches, doors, sills · more· less
Mercedes did not start galvanizing W203s until Q2 2003. Pre-2003 cars rust badly at wheel arches, door bottoms (inside and out), where front doors meet front wings, rear arches, and underneath rear bumper (estates worse). The issue stems from water-based paint not being impervious to water, and soft clearcoat that chips easily allowing water to seep through. Copper oxide was not used in the metal due to cost. Forum evidence shows this affects nearly all 2000-2003 models. Repair costs: €300-500 per arch for surface rust, €1,000-2,000 for extensive work, €3,000+ if structural damage. Prevention: inspect every hole and spray with anti-rust coating.
SAM module failure (water damage)€300 - €1,200
Front/rear SAM units fail from water ingress, causes electrical chaos · more· less
Signal Acquisition Module (SAM) failure is one of the most common W203 problems. Symptoms: "Visit workshop" warning, no fuel gauge, no turn signals, no wipers, no reverse lights, boot won't open, doors show open when closed, battery drain when parked, taillights stay on constantly. Water or liquid damage causes corrosion and eventual failure. The 64-pin Motorola processor inside fails. Rear SAM most common. Used SAM units cost €100-200, new €400+. Programming required at dealer (€60-100). Some owners do DIY processor replacement (€95). Critical: fix water entry point or problem will repeat. Affects W203 throughout production.
Injector seal failure ("Black Death")€200 - €2,000
Leaking injector seals create black sludge, can destroy engine if ignored · more· less
Common on most CDI Mercedes engines, especially smaller capacity ones. Symptoms: smell of fumes inside car, thick gooey black sludge around injectors when hot, chuffing noise. First signs at 80,000-120,000 km. Sealing washers are cheap (€20 kit), but labor expensive due to special tool required and risk of seized injectors breaking during removal (common design flaw). If left too long, destroys engine, wiring, and surrounding components. DIY tools cost €200. Single Delphi injector €415. Professional replacement: €800-1,200 for seal replacement, €1,500-2,000 if injectors break and need replacing. Check by removing plastic engine cover - should be part of bi-yearly maintenance on high-mileage cars.
CPU chip crashes, prevents starting or key recognition after 120,000-150,000 km · more· less
The electronic ignition switch is a sensitive component that easily becomes damaged. Symptoms: key won't turn in ignition (especially when cold), no dash lights when turning key, car won't start, EIS no longer recognizes key. Terminal 15 & 15R fault codes stored in front SAM. The internal CPU chip fails or crashes like a computer. Jump-starting with key in ignition can cause EIS-EZS to lose memory. Dealer replacement: several hundred to €1,500 with programming. Third-party repair services: €500-700 with 12-month warranty (95% repairable). Cannot use used EIS alone - must swap multiple parts (ECU, shifter) to match VIN. Programming requires Mercedes Xentry (dealer only).
Valve body conductor plate fails, causes harsh shifts, limp mode, no-start · more· less
The 722.6 5-speed automatic has multiple known issues. Most common: electrical conductor plate inside transmission cracks, causing serious shifting problems, intermittent no-crank no-start, irregular shifts due to faulty speed sensors. Symptoms: harsh shifts, torque converter lock harshness, limp mode, jerking/clunking (initially when cold, progresses to constant). Electrical connector/pilot bushing leaks allowing fluid to wick up harness into TCM. Solenoid pressure regulator valve spring breaks on pre-1999 units (cheap €5 fix). Shifter module PCB fails (eco-friendly solder issue). Typical failure range: 80,000-120,000 km. Conductor plate replacement: €300-600. Full transmission rebuild: €2,500-3,500. Dealer remanufactured transmission: €5,500. Manual gearbox W203s significantly more reliable.
Turbocharger failure€1,500 - €3,000
Lack of oil changes or wastegate malfunction causes premature failure · more· less
Turbo relies on proper lubrication. Infrequent oil changes cause oil breakdown and debris accumulation leading to excessive wear, overheating, and failure. Malfunctioning wastegate (stuck or fails to open/close) causes turbo to overwork. Symptoms: significant power decrease, whistling noises, limp mode, smoke during acceleration, boost pressure failure. Before assuming turbo failure, check: EGR valve (common culprit), boost leak in hoses/intercooler, fuel filter (very cheap fix). Turbo actuator repair: €100-600. Full turbo replacement: €1,300 parts + 6 hours labor (€2,500-3,000 total). Prevention: regular high-quality synthetic oil changes, allow engine cool-down after hard driving, occasional spirited driving prevents carbon buildup.
Steering rack leak€800 - €2,000
Power steering fluid leaks typically appear at 100,000-140,000 km · more· less
Steering rack leaks are common on W203 from 100,000-140,000 km. Often the leak is from pipework (combination flex/fixed pipe at rear of rack joint) not the rack itself - check this first. New steering rack: €400-800. Labor: €400-600. Total at dealer: €1,600-2,000. Independent garage: €800-1,200. Used/junkyard parts: €400. Alternative: rebuild original rack (allows pump pressure testing in case pump causing overpressure damage). Stop leak products (€10-30) are temporary only.
EGR valve clogging€150 - €600
Carbon buildup from 60,000 km, causes power loss and limp mode · more· less
EGR valve coking occurs from 60,000 km on CDI engines. Symptoms: power loss, rough idle, limp mode. Short trips and town driving worsen buildup. Professional cleaning recommended over replacement if valve not cracked/leaking. OEM Wahler EGR valve: €185-260. Professional replacement: €450-570. Cleaning service: €150-250. On OM646 engine, proper cleaning requires dismantling intake manifold and gas recirculation lines. Cannot blank EGR as car enters limp mode. After cleaning, performance improves drastically and eliminates fume smell. Reset codes with Star Diagnostics after work.
DPF problems (2004-2005 models only)€300 - €1,500
Only affects late W203s, clogs from short trips, causes limp mode · more· less
Not all W203 C220 CDI models have DPF - only 2004-2005 Euro 4 models. Check: pressure sensor with two rubber hoses on air filter box, black sticker showing MB229.31 (low ash oil required). Symptoms: intermittent limp mode, fault code 2626, power loss, no-start. DPF regeneration requires 60+ km/h at 2,000+ RPM. Short trips prevent proper regeneration. DIY forced regen: 20-minute drive in 4th gear at 2,500-3,000 RPM. Dealer forced regeneration: €300. Professional DPF cleaning: €400-700. New DPF filter: €1,000-1,500. Some owners remove DPF and remap ECU (check legality). Requires MB229.31 spec oil for DPF-equipped cars.
Window regulator failure€150 - €600
Very common on W203, brackets break causing window to drop or stick · more· less
Window regulator failure is extremely common on W203 chassis. Symptoms: window won't move (hear grinding noise), uneven movement, clicking noises, window drops, won't stay up. If only bracket/clip broken, dealer repair kit costs €15 (27-minute job). Window regulator: €50-125 (UK). Body shops quote €400-600 with labor. DIY: 30 minutes per window. If window doesn't move and no noise, likely window motor not regulator. Check window mounting brackets (part #203-720-01-14, rubber gasket and bolt/nut set, €20 each) before replacing entire regulator.
Air conditioning compressor failure€600 - €2,000
AC typically loses capacity at 60,000-80,000 km, compressor seizes · more· less
W203 compressors notoriously fail - Mercedes on 7th iteration of compressor design. AC loses cooling capacity after 60,000-80,000 km from refrigerant loss or compressor failure. SportCoupe and estate variants particularly affected. Compressor seizes after 60,000-70,000 miles typically. When compressor seizes, must flush entire AC system as metal spreads throughout refrigerant circuit. Compressor parts: €200-500. Professional replacement: €1,000-1,600. Dealer: €2,000. Important: complete diagnostic before replacing compressor - may be other AC system issues.
Swirl flap motor/linkage failure€200 - €1,500
Loud knocking at 1,500-2,000 RPM, juddering at idle, limp mode · more· less
Swirl flaps control airflow into cylinders. Symptoms: loud knocking between 1,500-2,000 RPM when driving, light juddering at idle (especially warm), fuse No.104 blows immediately, limp mode, boost leak if flaps completely loose. Dealer repair: €1,500. Specialist fixes: fraction of dealer cost. DIY solutions: swirl flap removal/delete kit (blanking plugs on eBay), 4.7k Ohm resistor bypass for faulty motor (€3 for 6-pack, easy and reversible), manual removal and reconnection of motor (ECU thinks it's working, no EML light). Many owners report smoother running and better MPG after removal/delete.
Glow plug failure€100 - €400
Glow plug light stays on after start, hard cold starts in freezing weather · more· less
Symptoms: glow plug light stays on for 3 minutes after engine start, car fires for 2-3 seconds then cuts out in cold weather (requires extended cranking), light stays on after start-up. When one fails, often causes fusible link inside glow plug relay to blow. Glow plugs: €7 each (€30-40 for set of 4). Glow plug control relay: €60-70. Known issue: outer case can snap off leaving rest stuck due to carbon buildup (design fault). Only remove with very hot engine to prevent snapping. Check resistance (should be 1-2 ohms each). Replace as complete set. Professional replacement: €200-400 total due to difficult labor if seized. CDI engines start even with failed glow plugs unless below freezing with weak battery.
Suspension control arm bushings€300 - €1,000
Worn bushings cause squeaking, clunking, and uneven tire wear near 100,000 km · more· less
Front lower control arms and bushings wear from braking, cornering, and hard acceleration forces. Symptoms: squeaking over bumps, knocking sound (metal-to-metal contact), uneven tire wear, vehicle pulls to one side or wanders. Front sway bar end links fail first (€50 parts). Control arms with bushings: €140. Bushing replacement only: €300-450. Full control arm replacement: €750-1,000 (varies by model). Most common around 100,000 km. Replace ball joints and tie rod ends simultaneously. Wheel alignment required after replacement. DIY: 5 hours for both sides, both arms. Control arm itself may last vehicle lifetime - bushings wear out.
Alternator failure€400 - €1,200
Battery warning light, "Visit Workshop!" message, electrical components losing power · more· less
Symptoms: battery-shaped warning light with "Visit Workshop!" message, all electrical components losing power while driving, eventual stall (jump start won't work), grinding noise from bearing changing with engine speed. Alternator parts: €100-260 (UK/EU). New alternator online: €100-200. Drive belt kit and remanufactured alternator: €500. Voltage regulator only: €56. Dealer replacement: €1,000+. Independent shop: €600-900. DIY replacement: 2-4 hours first time, 1 hour experienced. Test charging system before replacing - may be voltage regulator or connections only.
Reliable diesel engine, but chassis and electrical systems need attention
The OM611/OM646 CDI engines are fundamentally robust and can reach extremely high mileage with proper maintenance. Post-2003 models with revised injection systems are most reliable. However, the W203 chassis has multiple well-documented issues: extensive rust on pre-2003 cars (not galvanized), notorious electrical problems (SAM modules, ignition switch), and the 722.6 automatic transmission has known failure points. Budget extra for potential electrical and rust repairs on older examples. Manual transmission models are significantly more reliable than automatics.
Pre-Purchase Inspection Checklist
General checks
Service history
Complete dealer or specialist records essential, especially oil change intervals. Verify use of correct MB-approved oil (229.5 or 229.51 for DPF models).
Tires
Check tread depth, age codes, uneven wear. W203 uses 205/55R16 to 225/45R17 depending on trim.
Cold start
Must start engine completely cold. Listen for unusual noises, check glow plug light behavior.
Test drive
Minimum 20-30 minutes including highway speeds. Test transmission when hot.
Diagnostic scan
Use Star Diagnostics or equivalent to check for stored fault codes in all modules.
Specific for this vehicle
Rust inspection (critical for pre-2003 cars)
Thoroughly inspect wheel arches (front and rear), door bottoms (inside and out), where front doors meet front wings, rear quarter panels above rear bumper, and sills. Check underneath with torch. Pre-Q2 2003 cars are not galvanized and will rust. Walk away from cars with extensive rust.
Remove engine cover and check for "Black Death"
Look for thick black sludge around injectors. Smell for diesel fumes inside cabin. Essential check - if ignored, destroys engine.
Test all electrical systems thoroughly
Check: all lights, turn signals, wipers, central locking, window operation, fuel gauge, climate control, boot release. SAM module failures cause multiple electrical issues. Check for "Visit Workshop" warnings.
Key and ignition switch test
Test key recognition, check if key turns smoothly in ignition (especially when cold). Try starting several times. EIS/EZS failures common.
Automatic transmission operation (if equipped)
Drive for 20+ minutes. Feel for harsh shifts, jerking, clunking, limp mode. Check transmission fluid for correct level and condition. 722.6 transmission has known issues.
Check for power steering leaks
Inspect steering rack for fluid leaks, especially combination pipe at rear of rack. Check power steering reservoir level. Common from 100,000 km.
Listen for turbo and exhaust issues
Listen for whistling from turbo, check for smoke during acceleration, verify boost pressure under load. Check EGR valve operation. Test for swirl flap knocking (1,500-2,000 RPM).
Verify production date and DPF status
Check if car is pre-Q2 2003 (major rust risk) or 2004-2005 (may have DPF). Look for DPF pressure sensor on air filter box.
Window regulator test
Test all four windows for smooth operation. Listen for grinding or clicking. Very common failure on W203.
Air conditioning function
Test AC thoroughly - compressors fail commonly after 60,000-80,000 km.
Recalls and Technical Service Bulletins
Right rear seat belt latch replacement (2002)Verify completed
Seat belt buckle replacement (2004)Verify completed
Intake air hose replacement (2002)Verify completed
Engine air intake duct not connected (2004)Check if applicable
Multiple TSBs exist for common issues
Contact Mercedes-Benz dealer with VIN to verify all recalls completed. Technical Service Bulletins (TSBs) exist for: harsh transmission engagement, musty AC odors, license plate bulbs burning out, headlamp condensation, cruise control issues, and shock replacement guidelines. TSBs are only covered if car is within warranty period - outside warranty, you pay for repair.
Warranty Status
Factory warranty (2 years)Expired on all W203s
Extended warranty availabilityLimited due to age (18+ years)
Rear subframe rust warranty (USA only)Not applicable to W203
No factory coverage remains
All W203 C-Class models (2000-2007) are outside factory warranty. Extended warranty from third parties may be difficult to obtain due to vehicle age and known issues. Budget for repairs from your own funds. Independent specialists familiar with W203 chassis recommended over dealer for cost-effective maintenance.
This report is for informational purposes only and does not constitute professional advice. Estimates may be inaccurate. Always have a qualified specialist inspect the vehicle before purchase. We accept no liability for decisions made based on this information.