how does one read the codes on a 1994 first gen t10/s10 blazer?

i have the service engine soon light come on at red lights/stops. and then at about 40km/hr it shuts off.

 

You straighten out the paperclip so it makes a "U" shape. Then you take the key out, stick the paperclip into the top right 2 holes "jumping" them together, and stick your key in and dont start it, just turn it forward to the run position.

Now you are going to get the codes in blinks. Here is an example of how it will flash for a code 34

blink (small pause) blink blink - this will be 12, this means the system is OK, if this is all you get, there are no codes
blink blink blink (small pause) blink blink blink blink - this will happen 3 times - 3, then 4 = code 34
then if there is more codes it will blink them in succession.

Here, this guy has a good video on checking the codes