I need to ask a simple question to tack onto the end of this subject.

I have always wanted to build the simple code reader that I have seen on this site over the years, but have never understood why I need one.

I have a 1990 300te California car. It has the push button diagnostic unit near the battery with 16 numbered holes (2 of which are used by the push button and the red LED).

I push the button and count the flashes to find out any stored codes. I then fix the problem and clear the codes by holding down for 8 secs and turning off the key.

What more would the home built code reader do for me?