DML FAQ : Electronics : How can I retrieve codes from the computer?
Pulling codes will vary from year to year.
From Jon Smith:
For most trucks, simply turn the key on/off three times within five seconds. Example... On/Off,On/Off,On/Off,ON leave in this last On position***. Gen II trucks will display the codes via a flashing Check Engine light, Gen III trucks will display the codes via the odometer.
On Gen III trucks, this procedure will display the first code, to retrieve any others, simply cycle the ignition switch back ONE click back, then forward. Clear as mud?
From William Hiatt:
I believe it's the 98+'s you can pull the codes from the PCM by doing the following:
The codes will appear in the odometer, flashing the codes, one by one, and then it will flash "P DONE" when you've reached the end of the codes.
From Ron Taylor:
AutoZone can pull the codes for free with a scanner and look up the code, no charge!
To run a diagnostic on your truck simply press in your trip button and hold and turn your key to the on or start position I forget which one it is but don't start it after you turn the key watch the lights where your digital mileage reads and it will start doing a scan and your guages will move. After it is done if there is a problem it will give you a code.