We are a contract monitoring station. 
    We have visited some of the Subscribers homes, and observed that some of their systems, installed between 2 and 20 years ago, in addition to burglary protection, include smoke/heat detectors.  It appears that some of the systems are not compliant with today's fire codes: fire-wire not used (instead 4-cond. burglar wiring), sounders not fire-approved, no end-of-line power-supervisory modules, smoke detectors 10+ years old, too few detectors, etc.
    For a few commercial subscribers, alarm panels which were residential-fire approved, but never approved for commercial-fire use.
    These are all systems that we did not specify, design, sell, install, program, or ever service... only monitored signals for a dealer who has now abandoned its subscribers [yes the same dealer as yesterday] 
    What liability issues should we consider when making simple repairs (such as replacing a non-operative smoke detector) for such systems? How can we minimize our liability?
    Can you discuss liability issues should we agree to continue to monitor those systems?  Would you ask those on this forum for their comments/opinions, on how they would handle such a situation?
    Once you become the subscriber's "dealer", taking over the system, you are responsible.  You need a direct contract with the subscriber, the All in One [residential, commercial or commercial fire, depending on the subscriber].  
    I want to comment on an issue that you mention.  As the CS you've  I visited subscriber premises to check out the alarm systems.  Once you perform an inspection, visit the premises, you are going to be open to a lawsuit if that system fails. If you find deficiencies or departure from code, custom and practice, manufacturer guidelines or laboratory guidelines, you have a duty to speak up and warn the subscriber [and dealer].  
    Central station's don't typically visit dealer accounts for whom the cs is providing monitoring.  By not performing inspections the cs does not invite claims that it should have reported problems or fixed those problems.  As it is the cs faces claims that the cs should have known and reported problems based on signals received or not received.  Inspecting the systems is an entirely other and broader responsibility.
    If you think about it, it may be a good idea for a cs to routinely inspect a dealer's systems.  After all, the cs has an on-going relationship with the dealer, will be accepting monitoring accounts on a regular basis, and the cs may want to know first hand [before the lawsuits start] what kind of job the dealer does.  Poor installation and service can cause the cs problems by involvement in lawsuits, even if they are ultimately successfully defended.   But, I'm not aware of any cs that performs these types of inspections or surveys [and for those lawyers trolling these articles to buttress cases against the alarm industry, I make it clear that cs inspection of accounts is not industry practice, nor should it be]. 
    All this analysis is out the window however once the cs becomes its own dealer.  Then it has all the responsibility of the dealer, and it better have a contract for its new duties.