The brief
The 420i was in for its routine maintenance interval. No reported faults from the owner, no warning lights, just the regular check-up.
The job is preventive, but preventive does not mean just an oil change. It is: refresh the consumables that wear out by design, run a full scan-tool sweep so any pending fault gets caught now rather than on the road, and inspect everything that wears unpredictably, brakes, suspension, belts, hoses, so the things that are getting close get flagged before they become a problem. Sometimes that inspection turns up nothing. Sometimes it turns up a couple of items that are due.
The diagnosis
The pre-service scan returned no stored faults. The visual inspection of the underbody, the suspension boots and the belts came back clean.
But two things were due. The brakes: the pads were worn and the discs were scored and lipped, past serviceable condition, so they needed doing rather than left for the next interval. And the battery: a load test showed it past its service life, the kind of reading that says it will let the owner down sometime soon if it is not changed. So the scope grew from the standard service to include a brake disc-and-pad set and a new battery.
The work
Drained the engine oil, fitted a new BMW-spec oil filter, refilled to grade and volume. Swapped the air and cabin filters. Topped the fluids where needed: coolant, brake fluid, washer.
Replaced the worn brake discs and pads with new BMW-spec parts, cleaned and lubed the slider pins, and bedded the new brakes in on a controlled road test. Fitted a new BMW-spec AGM battery and registered it to the car's charging system so it gets the right charge profile.
Set tyre pressures, reset the service interval indicator, and ran a confirming scan after the work.
The outcome
Engine running smooth on the road test, throttle response sharp, no warning lights. Brakes solid, no squeal, full bite. The new battery cranking cleanly.
The 420i went home with the service record signed off and two items that would have become problems sorted before they did. For the owner, that is what a proper service buys: not just the oil changed, but the brakes and the battery handled at the point the inspection said they were due, so the car keeps running the way it should and nothing arrives as a surprise.
The kind of record that also holds the car's value when it changes hands.