How the Alternating System Works
The wrench icon and "Service A" or "Service B" designation in the Mercedes instrument cluster comes from the ASSYST oil life monitoring system. ASSYST calculates remaining oil life based on a combination of engine operating hours, temperature cycles, load, and calendar time — not raw mileage. A car that drives mostly freeway miles at steady throttle will have longer service intervals than the same car driven in stop-and-go traffic. The two designations simply alternate: the system does a Service A, then advances to Service B, then back to A, and continues from there.
The interval between services is typically 10,000–13,000 miles or one year, whichever comes first, though the ASSYST system can extend this slightly on favorable driving cycles or shorten it significantly on harsh ones. If the calendar-year mark arrives before the mileage threshold, ASSYST triggers on time regardless.
Service A: The Short Version
Service A is the minor service. At its core: engine oil and filter replacement with the correct MB-approved specification oil, fluid level check and top-off (coolant, washer, power steering if applicable), visual brake pad inspection, tire pressure check, and service indicator reset. That's essentially it. On some models, tire rotation is included if the tires aren't a staggered application.
The oil and filter are the meaningful work. Everything else is inspection and topping off. A Service A at a Mercedes dealer in Los Angeles typically runs $280–$450. An independent Mercedes specialist charging $120–$200 is doing the identical work — the difference is dealer overhead and service advisor margin.
Service B: The Meaningful Difference
Service B includes everything in Service A plus brake fluid replacement — and this is the item that most owners treat as an afterthought while it's actually the most consequential maintenance item in the entire Service B package. Mercedes specifies brake fluid replacement at every Service B interval (approximately every two years) because glycol-based DOT 4 brake fluid absorbs atmospheric moisture continuously. As moisture content rises, boiling point decreases and corrosion risk inside the ABS hydraulic unit increases.
An ABS hydraulic unit replacement on a W212 E-Class or W166 ML-Class is $1,800–$3,200. A brake fluid service at Service B is $80–$140. The maintenance cost relationship between those two numbers is exactly why Mercedes publishes the two-year brake fluid interval — and exactly why it shouldn't be skipped or cheapened out on.
Service B also includes: cabin air filter replacement (important in Southern California where dust accumulation is faster than wetter climates), a more comprehensive visual inspection of all major systems including suspension and brake hardware, and in some cases — when the interval aligns — spark plug replacement. The spark plug replacement at Service B only applies when the engine's spark plug interval has been reached; it isn't included at every B service.
The Dealer vs. Independent Cost Reality
A Service B at a Mercedes-Benz dealer in the Los Angeles area: $600–$950 depending on model. The same Service B at an independent Mercedes specialist: $280–$450. Same oil specification. Same brake fluid specification. Same inspection checklist. Same service indicator reset requiring Mercedes-specific diagnostic equipment. The difference isn't quality — it's the cost structure of operating a franchised dealership versus an independent shop. Dealer service advisors operate on commission or bonus structures tied to revenue. Service bay rates at dealerships are $180–$250 per hour versus $110–$155 at well-equipped independent shops. Neither set of technicians is more qualified than the other for routine maintenance.
What to Verify Regardless of Where You Go
Three things to confirm before authorizing any Mercedes service: the oil specification (ask for the MB approval number — it should be 229.5 or 229.51 depending on your engine, not just "full synthetic"), that the service indicator reset is performed with a proper Mercedes-capable tool (not a generic OBD-II code reader — only a Mercedes-specific tool correctly resets the ASSYST system and logs the service), and that brake fluid is actually being replaced at Service B — not just inspected or topped off. Each of these is a place where corners can be cut invisibly.