Theres a test port in my 1990 240 volvo (center of dash- says SRS) where can I get a tool to use this diag.?

I need the tool to diagnose why the check engine light is on- the test port is under a panel in the center of the dash (it is labeled SRS) supposedly when you put a special "key" into this port, the lights in the instrument panel flash a certain number of times to tell you what is wrong with the car. I need to get the key to use this and also the codes to allow me to diagnose the problem

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