Nissan Repair: Nissan Frontier OBD system, obd system, nissan frontier

Question
Hello,

I have a 1999 Nissan Frontier, 2.4L 4-cylinder. The onboard diagnostic system seems to be the problem.
About four months ago, the "Service Engine Soon" light came on. Using the Haynes manual for this model, I turned the mode screw on the Powertrain Control Module as directed (clockwise, wait several seconds, turn it counterclockwise)and got a 0505 code from the flashing engine light. The manual showed that this code meant "No Trouble in the System." (The codes are different when read this way, compared to the codes a generic code reader or scan tool would show.
One interesting thing about the mode screw: it didn't stop turning at any point. According to the manual, you're supposed to turn it "all the way" clockwise first, then counter-clockwise. I've used this method several times to try to solve this problem, and I've turned the screw for long stretches, but it has never stopped turning.)
Anyway, the light went out, and I thought everything was cool. The truck has no drivability problems. It starts easily, idles fine, accelerates cleanly, has good fuel economy, and consumes very little oil.
About 10 days later I went to my state emissions test and was told they couldn't test the truck because the PCM wasn't ready to be read. Just drive a while longer and come back, I was told.
I've since connected a OBDII code reader and it shows no trouble code. The engine light is sometimes on steadily, and sometimes flashes, but not with the key on, only with the engine running. And now the Air Bag light flashes as well. By the way, the speedometer and odometer stopped working, but all the other gauges and idiot lights still work.
Maybe I should check the PCM ground wires?
Any advice you can offer will be greatly appreciated. Many thanks for your time and attention!

Ron

Answer
Ron,

It sounds like the computer is not connected properly.  I would disconnect the cable and reconnect it and then drive the vehicle as described in the service manual to get it in test mode.  You have to go through several complete cycles of driving to get the computer to go into ready to test mode.