It is not uncommon for backflow preventers to fail their annual test. 
As part of my testing fee, I will attempt to disassemble and clean or repair the failed components free of charge for up to 30 minutes.  In 95+ percent of cases, this is more than adequate time to determine the problem and either resolve the issue immediately or identify what parts need to be ordered.
I do carry a small collection of common parts.  But if additional parts do need to be ordered, I have a very reputable supplier who can deliver in two business days, and will ship overnight if necessary.
Your preventer will be put back in service until parts arrive.  I will pass the price of the parts directly on without any additional charges.  And I will return to your location to complete the installation of the new parts and retest the preventer without charging any additional visitation fee.
If a diagnostic session exceeds 30 minutes, or a return visit to install parts is required, I will charge a reasonable labor fee.
Disassembly and repair of a preventer requires that the water which flows through it be turned off for the duration of the repair.  For an irrigation or fire suppression system, this is not usually an issue.  However, if the preventer is part of your domestic water supply line, we may have to make special arrangements to accomplish the repair outside of normal business hours.  Any additional charges will be on a case-by-case basis.