You may find a situation where you arrive to immediately find a primary problem. As an example, a motor has failed. You should quote the repair of the motor but explain to the customer that the diagnostic is not yet complete and cannot be completed until the obvious problem is repaired.
Once the repair has been approved and completed by the technician, it is critical that the diagnostic procedure be completed. Taking the time to evaluate the rest of the system may show you opportunities to offer upgrades.