The calibration resistor is located in the PMD/FSD connector socket. If you've disconnected the harness, it may have fallen out. In my experience, you can't easily get the dang thing out when you need to, but if you turn your back on it, it will fall out.

Because the PCM "remembers" the value, and doesn't check for it often, there's no telling when it may have gone missing. They do fail on rare ocasions. It's just a small PC board with holes and a single resistor between two of the holes.