03 sierra OBDII not working help!!
 

i have a 03 sierra and my check engine light is on . I took it to autozone and the scanner cant read the codes. so i checked all my fuses even the cig lighter and its working. there are 4 or 5 wires going to the obdII connector im getting power to one of them and there is a purple wire that im getting alittle power. Im cheking it with a light tester and the bulb bearly lights up like is not getting enough power ... so I dont know what to do ... any ideas ....thanks