Since your car is a 1991 model, it may use the original OBD-1, which began being used on most vehicles in the US in the late 70s. You will need to check your vehicle’s owners manual to see if it is one of those.
You can also look for the 12-Pin OBD-1 port that was usually under the hood. OBD-2, which began in 1996, uses a 16-Pin port located under the dash.
If your vehicle uses OBD-1, then you’ll have to purchase a diagnostic tool with a 12-Pin connector that is designed to read those older codes. You also need to buy an extension cord so you can sit in the drivers seat to carry out the required tests while reading the diagnostic tool.
A dual OBD-1/OBD-2 tool that includes everything you need, including the extension cord for the older models that have the port under the hood, can be found here: https://amzn.to/2M3muA5