Rossmanm mentioned that in the focus groups he ran people tended to believe that a manufacturer was more competent at repairs by default, even without evidence, but it's worth saying:

Show me any actual data that people being able to see human readable error codes would result in more injuries.

Even if that were somehow the case, demonstrate that the status quo would be less dangerous than putting the company in a position where it had more of an incentive to make ALL repairs safer. If a repair process is hazardous enough to make the machines barely safe enough to use if maintained by authorized repairmen, than that status quo discourages the company from making the repair process safer in general.