What a maintenance plan actually covers

Set out in full rather than summarised into a slogan, because “annual tune-up” is a phrase that means whatever the person saying it wants it to mean.

The cooling visit

Booked for March or April, before the season loads the equipment.

Condenser coilInspected and cleaned. In this county it is the single most common reason a working system stops keeping up.
Evaporator coil and drainCondition checked, the drain cleared and the trap and float switch tested rather than looked at.
Refrigerant behaviourMeasured with gauges and temperatures. Where a system is low, we look for the leak rather than topping it up and leaving.
AirflowMeasured, not assumed. Filter condition, blower condition and static pressure across the system.
ElectricalCapacitors, contactors, connections and running current, with the readings written down so next year has something to compare against.
ControlsThermostat operation, staging and schedule, and whether any of it matches how the building is actually used.

Booked for October, before the first genuinely cold night rather than during it.

Burners and ignitionCleaned and tested, including the flame sensor that causes a large share of no-heat calls.
Heat exchangerInspected. If there is any doubt we say so plainly and we do not leave the appliance running.
Flue and combustion airCondition, support, termination and anything that has built a nest in it since March.
Gas pressure and safetiesMeasured and tested in operation.
Airflow across the exchangerThe commonest cause of a furnace tripping its limit, and the commonest thing skipped on a cheap tune-up.
Unit heaters, where there are anyStarted and run. A warehouse heater that has sat since spring is not a winter job.

Not an insurance policyMaintenance reduces how often things fail and finds faults while they are still cheap. It does not mean nothing will ever break, and anybody promising that is selling something else.
Not a licence to skip the diagnosisIf something fails between visits it still gets measured and priced properly. Being on a plan does not turn a repair into a guess.
Not priced on this pagePlans are priced after a survey, per unit, in writing. A figure published here would only be wrong for your building.
Not a lock-inThe file is yours. If you leave, you take the record with you, and that is the point of keeping it in the first place.