The economics of the no-cost visit
Most companies will look at your system at no charge, and it's worth being clear-eyed about what that visit is: a sales appointment. The technician's hour gets paid for either way. On a no-cost visit it gets paid for by whatever you buy at the end of it. That's not an accusation of bad faith; it's just the incentive structure. Your doctor charges for the exam and your mechanic charges for the diagnostic hour for the same reason: it keeps the diagnosis independent of the sale.
Two ways to buy a diagnosis
We sell measurement at two depths. The $99 service call is for a broken system: travel within 25 road miles of Soddy Daisy, an instrument-based diagnosis of the problem you called about, and a written price to fix before any repair starts. The $249 SystemIQ Evaluation is the full workup: static pressure, total system airflow measured with a TrueFlow grid, temperature split, refrigerant charge against measured superheat and subcooling in cooling season, and combustion on gas equipment in heating season, ending in the Home Comfort Report, every reading in plain language with a prioritized plan. There's a sample report on this site so you can judge that deliverable before you spend a dollar. And the two stack: the $99 credits toward the $249 if you upgrade.
When you should skip it
A system under a year old should go back to the installer under warranty. A system that won't turn on at all may just need a breaker, a float switch, or a filter. Check those first, it takes five minutes. And if you've already decided to replace no matter what, put the diagnostic money toward a Manual J load calculation instead, because sizing is where replacement decisions go wrong.