Dealers have been trying these tricks for years, over 15 years ago I took a customers Renault to the Dealer to read the error codes re an airbag issue, they said it would take an hour and included a "free safety check", an hour later after a "free grotty coffee" I was presented with a bill of nearly £100 and told a long list of things their "free safety check" had found! I asked them about the reason I had come in and they said there were several codes and that it was probably the BCM at a cost of around £280 odd, I asked if that would cure the problem? They said probably not as many other things could be causing it!!!!!This "diagnostics needed" seems to be the latest "cash cow" which is something the ordinary everyday Joe public knows nothing about and therefore can't judge whether it's either needed or whether a relevant price is being charged. I don't disagree that, when relevant, a good starting point when a vehicle is presented with a problem would be to do a quick check for stored codes but honestly, how much time does that check take? minutes only! and the often "standard" charge bears little relationship to the cost charged. If further action is required as a result of reading the codes then that's another matter.
Happily they went bust a year later and I went to their auction to jump on their grave, richly deserved.
The cause of the error code was a loose wire under the passenger seat, a known common problem.