OK, so the Profibus media is standard, Profibus connectors' termination is properly set at both ends only, so we can pretty much rule out media problems. Still the VFB indicates BF (bus failure) and yet, if I understand correctly previous explanations, nobody has changed the configuration in the master. You mention there are three nodes (one HMI and 2 VFDs). Since the VFDs are usually Profibus DP slaves, then the HMI is not only HMI but is a Profibus master with HMI capability. Since one of the VFDs is actually working, then the master side has to be working. I can only think that the damage caused by the air conditioner fire has reached somehow the VFD which is not working. Does the documentation of the HMI mention any Profibus DP diagnostics? All Profibus DP slaves must support minimum set of diagnostic services, as it is mandatory in the specification. Many Profibus masters (though unfortunately not all) offer some easy way of looking into diagnostics from Profibus field devices. What is the part number and maker of the HMI/Profibus master being used in this installation?