HHT means "hand held tester". It is an M-B tool not widely available outside the dealer network. Independent shops, at least the better ones, have one or something equivalent that can access codes other that OBDII codes (i.e. airbag systems, HVAC, ESP, etc.). A regular OBDII code reader cannot read or reset SRS codes. Disconnecting the battery does not work either, in my experience (at least on M-B). The problem is that many folks decide to rock the steering wheel a bit to get the cluster out. If the key is turned on the free the steering lock, and usually by this point the wiring is disconnected, an error code is set for the SRS system (at least on the car referred to in this post and maybe others). That means a trip to a dealer or good independent shop to reset the code. My memory is a bit weak, but I recall Code 019 as being the code that is set.