A diagnostic fee is charged for establishing what is wrong, separately from putting it right.
Why it is charged separately
Diagnosis is skilled work with a real cost. On a fault that takes an hour to trace and ten minutes to fix, the value delivered is almost entirely in the tracing.
Bundled into the repair price, it is only paid for when the customer proceeds. The customer who takes the diagnosis and does the repair themselves, or shops it elsewhere, has received the expensive part free.
Against the free estimate
A quotation for defined work — replacing a known item — is usually given free, because it is estimating rather than diagnosis.
The distinction is whether skilled investigation was required to know what the work is. Where it was, that is a service.
Crediting it
Commonly applied against the repair if the customer proceeds, which removes the objection while still covering the visit where they do not.
The effect on enquiries
Fewer, and better. A fee filters out callers seeking a free opinion, which raises close rate on the enquiries that remain and frees the capacity that was being spent on the others.
Businesses introducing one usually find the lost enquiries were the ones that were never going to buy.
Stating it up front
Always, at booking. A diagnostic fee revealed after the visit reads as a charge for nothing, which is how a fair fee produces a complaint.
Where it sits against the minimum
A minimum charge covers attending. A diagnostic fee covers the skill applied once there. A business can charge one, the other, or both, provided the customer knows which.