A diagnostic fee covers the technician's time and skill to find out why your AC stopped cooling; a separate trip charge (or travel fee) covers simply getting them to your home. The national diagnostic fee for residential HVAC runs about $75 to $200, most commonly $99 to $159, and rises to roughly $150 to $300 on an after-hours emergency call.
The widespread and homeowner-friendly practice is to waive the diagnostic fee when you proceed with the repair — the company earns on the fix, not on the visit. That is the script you want to hear: a flat, disclosed diagnostic charge that is credited back if you approve the work that day. Be cautious of the opposite patterns — a shop that won't quote the fee over the phone, that stacks a trip charge and a diagnostic and an after-hours add-on without saying so, or that skips real diagnosis and jumps straight to 'you need a whole new system.'
The honest sequence on an emergency call is diagnose first, then price the specific repair, then let you decide — not a replacement quote before anyone has measured anything. Two questions sort most of this out before the truck arrives: Is your diagnostic fee separate from the trip charge? and Do you credit it toward the repair if I approve the work? A company comfortable answering both is the one you want at your door at midnight.