2012 MERCEDES-BENZ C350 W204

Typical onset: 80,000-150,000 mi

Symptoms: Check engine light with camshaft position correlation codes (P0016, P0017, P0018), Rough idle or engine stumble on cold start, Metallic rattling from front of engine during startup, Metal shavings in oil or on magnetic drain plug, Sudden catastrophic failure with no warning if balance shaft gears fail completely

Fix: The M272 balance shaft sprocket teeth strip, sending metal throughout the engine. If caught early with codes only, you're looking at 18-24 hours to drop the front cover, replace balance shaft gears, chains, guides, and all timing components. If metal contamination has occurred, you need a full engine rebuild or replacement. Many shops recommend pre-emptive balance shaft delete or upgraded gears at 80k-100k miles as prevention. Figure 20-25 labor hours for rebuild, 12-15 for used engine swap.

Estimated cost: $4,500-8,500 for preventive repair; $8,000-15,000 for full rebuild or engine replacement

Typical onset: 100,000-150,000 mi

Symptoms: Harsh or delayed shifts, especially 2-3 or 3-4, Limp mode with transmission fault codes, Slipping between gears or gear hunting, No reverse or intermittent gear engagement, Transmission warning light illuminated

Fix: The 722.9 seven-speed automatic suffers from conductor plate circuit board failures and valve body wear. Requires dropping the pan, replacing conductor plate, valve body if scored, and fluid/filter. Often both fail together. 6-8 labor hours including fluid refill and adaptation procedure with Star Diagnostics.

Estimated cost: $2,200-4,000

Typical onset: 70,000-110,000 mi

Symptoms: Clunk or thud when shifting from Park to Drive/Reverse, Excessive vibration at idle in gear, Visible sagging of engine or transmission when inspected, Vibration through cabin at highway speeds

Fix: Hydraulic mounts fail, especially the transmission mount and right engine mount. Transmission mount is the worst offender. Each mount is 1.5-2 hours, but smart shops do all three engine mounts plus transmission mount at once (5-6 hours total) to avoid comebacks.

Estimated cost: $800-1,200 for transmission mount alone; $1,800-2,800 for all four mounts

Typical onset: 90,000-140,000 mi

Symptoms: Slow coolant loss with no visible external leaks, Sweet smell from exhaust or coolant odor in cabin, White residue around head gasket area on valve covers, Slight rough idle or misfire codes after engine sits overnight

Fix: M272 head gaskets weep externally more often than internal failure. If caught early, it's head gasket replacement on affected bank (12-14 hours per bank). If both banks need it, you're at 22-26 hours. Must resurface heads and replace timing components while in there since front cover comes off. If coolant has contaminated oil, expect additional bearing/internal damage assessment.

Estimated cost: $3,500-5,500 one bank; $6,500-10,000 both banks

Typical onset: 60,000-120,000 mi

Symptoms: Rough idle or engine stumble, Loss of power under acceleration, Misfires on multiple cylinders (P0300 plus cylinder-specific codes), Failed emissions test, Increased fuel consumption

Fix: Direct injection engines carbon up intake valves since fuel never washes them. Walnut blasting intake ports is 4-5 hours. Injectors often fail simultaneously, especially on higher-mileage examples. Injector replacement is 3-4 hours for the set. Many shops do both services together preventively at 80k-100k miles.

Estimated cost: $800-1,200 for walnut blasting; $1,200-1,800 for injectors; $1,800-2,800 for both

Typical onset: 80,000-130,000 mi

Symptoms: Transmission fluid puddle under vehicle, typically driver side, Low transmission fluid warning, Burnt transmission fluid smell, Transmission slipping or overheating if fluid level drops significantly

Fix: Steel transmission cooler lines rust through where they pass through subframe or rub on chassis. Replacement requires dropping subframe or extensive disassembly depending on which line fails. 4-6 hours labor plus fluid refill and system flush recommended due to contamination risk.

Estimated cost: $900-1,600