Help.....!!!

I can't seem to get the codes from my '95. I have the Haynes manual. Jumper according to manual, no codes. They show a hook up with a voltmeter instead of watching the "check engine" light. Hook up the voltmeter, still no codes. Is the setup different for some years?

The book I have shows this....
Jumper the STI to the #2 pin on the DLC. The #2 DLC pin is the right pin on the row that only has two pins.

Just FYI, I've checked codes on several cars and never had this problem. The jumper is good, as are all the connections.

I'd like to get this figured out. My truck has developed a strange problem. Soon after starting, but only when the engine is up to operating temp, I get a stumble. Usually a couple seconds after starting, but sometimes up to 1-2 minutes later. It will stumble like there is a fuel or ignition problem, then clear up and it's fine.
I'm hoping it's just an O2 sensor on it's way out, or something of that nature. Any suggestions would be appreciated.