Cannot seem to get the computer to give the fault code?

I turn on the key, turn the switch to the diagnostic mode, one, blink, two blink, three blink, four blink, then turn the switch back and wait, and wait, and nothing happens, disconnected the battery for three minutes to reset the computer, it did not reset. Removed the battery for the night to see it that works. The truck has been sitting for 6 year had to replace the fuel pump assembly to get it to run, runs great drove it about 150 miles. Just need to know what the check engine light is doing on. Is there an OBD, OBD1 code reader that I can use? You have to be in computer mode three to read codes then tun off the switch and it works the instructions were wrong or for a different computer.