This is true for a lot of OBD 1 cars - in the C900 check codes by jumping the OBD terminals under the rear seat and counting check engine light flashes.
In some OBD 1 and 2 cars, you can read codes through the dashboard display and scroll through them using the A/C control panel buttons, my 93 Buick Riviera did not need a scanner tool to read or reset codes.
I don’t know how this works in a 9-5 though other than A/C off+auto getting you ACC codes…
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