What actually happens during a car service?
A defined list of checks and replacements from the manufacturer's schedule — and knowing what is on that list is the only way to judge whether a quote is reasonable, because "a service" means completely different things at different prices.
The tiers, which vary by garage:
Interim or minor — oil and filter, fluid levels, tyres, lights, brakes inspected, a visual check. Typically every six months or a set mileage.
Full or major — the above plus air filter, cabin filter, fuel filter where applicable, spark plugs on some schedules, more thorough brake inspection, suspension and steering check, and a longer inspection list.
Manufacturer schedule items at specific intervals — brake fluid, coolant, transmission fluid, timing belt, which is the expensive one and the one that destroys an engine if it fails.
What is actually done to the car: drained and refilled with the correct specification oil, which matters more than brand; filters replaced; a lift inspection of brakes, tyres, suspension, exhaust and leaks; diagnostic scan on modern cars; and a service record stamped or entered into the digital record.
Why a service is not an MOT. An MOT is a legal roadworthiness test at a point in time and includes no maintenance whatsoever. A car can pass an MOT while needing every service item.
What to know as a customer:
Ask for the schedule. A reputable garage will tell you what the manufacturer specifies at your mileage, which is checkable.
Ask for old parts back, which is a reasonable request and a deterrent.
Authorise additional work only with a call, and ask for photographs — most garages will send them.
Independent garages do not void warranties in most jurisdictions, provided the schedule is followed and correct-specification parts are used.
Keep the record, since a documented history is worth real money at resale.
The genuinely safety-critical items: brakes, tyres, steering and suspension. Everything else can usually wait; these cannot.