The brief
The E200 was misfiring under load, worst when it was climbing onto faster roads, fuel economy had dropped on the owner's regular routes, and a check-engine light was on with codes pointing at more than one cylinder. He brought it in to have the actual cause found rather than have parts thrown at it.
A misfire means a cylinder isn't burning its fuel properly, and there are only a few things that cause it: ignition (coil or plug), fuel (injector), or compression. When more than one cylinder is involved and it's worst under load, the smart approach is to work through them in order, rule out the cheap stuff first, and only replace what's actually proven faulty.
The diagnosis
The coils were swap-tested between cylinders first, the quickest way to rule them in or out. The misfires didn't move with the coils, so the coils were healthy. The plugs were clearly old, due for replacement on age, though not the cause of the misfire.
On the scanner, the live data showed two cylinders running rich, which is the fingerprint of an injector spraying badly rather than an ignition fault. Those injectors were bench-tested, and the spray pattern was off-spec on both. A couple of other minor codes were noted to clear at the end. So the fix was injectors, with the worn plugs done at the same time while everything was apart.
The work
Fuel pressure was released and the rail removed. The two failed injectors were replaced, and to keep the bank balanced a matched set went in across all four, with a fresh O-ring set on each to spec. While the rail was off, a new set of Mercedes-spec spark plugs went in to replace the old ones.
Then the injector adaptation routine was run on STAR so the engine controller re-learned its per-cylinder fuel trim on the new injectors, and the stored codes were cleared.
A long road test confirmed it pulled cleanly through the rev range with no misfire.
The outcome
No misfires across a long road test, the flat spots under load gone, fuel economy back across the next tank, and no fault codes after a full drive cycle.
The E200 went home running cleanly through the rev range. Working through the misfire in order, ruling out the coils and plugs as the cause before replacing the injectors that actually tested bad, meant the owner paid for the fix and not for guesswork, with the tired plugs refreshed as a sensible extra while the engine was open.