Get your notepad and smartphone out and check out a 1993 Cadillac Allante with Jack McGinnis of HiTech Investigations.

"Yep, they're still out there and the service technician is expected to diagnose and repair the old ones as well as the new. It doesn't matter--the diagnostic process is still the same," Jack McGinnis told me.
The owner of the Allante (actually, he owns one and his better half does, too) bought the Allante and a great price--unfortunately, someone hacked the electrical system, turning it into a tangle of useless wiring. The shop owner that worked on it, "J", did a fabulous job of retracing every wire and after a fair amount of time invested, repaired it--except for one annoying problem.

J: "It keeps flagging a trouble code--a P080--meaning the Throttle Position Sensor Learn hasn't happened. I use the on-board diagnostics to perform the relearn, the vehicle computer responds by saying the Learn Process is complete, but when I cycle the ignition key the code resets, saying it hasn't been learned."

Me: "The TP sensor is okay?"
J: "Yep--I checked the wiring and the sensor with an oscilloscope."
Me: "No add-on systems that shouldn't be on the car?"
J: "Nope."
Me: "did you try disconnecting the battery?"
J: "Yeah--I even shorted the battery cables together after disconnecting the battery."
Me: "Did you do your research?"
J: "Yeah--no technical service bulletins, no answers on the forums, no search engine results."
Me: "Sounds like you are thorough," McGinnis said, walking away as he looks through his note pad. "Oh--just one more thing, sir--did you check the engine ground strap?"
J: "I will. Hey--the engine ground is bad. Let me try using a jumper wire and then perform the TP Learn."
Me: "Did you try it?"
J: "Yeah--it's fixed! Why did a bad engine ground cause that to happen?"
Me: "I suppose that then the module does a TP Learn, it sweeps the TP sensor to reference low based upon ground. So, if the engine ground is missing, then the PCM uses a perceived ground and learns the TP value based on the perceived ground. Unfortunately, it discards the values at the next key-up because it has nothing to compare ground to. Perceived ground is not a memory ground."
J: I'm just glad to get the car out of my hair."
Me: "Yeah--me, too. Miller time?"
J: "You bet!"

Always remember to check those engine grounds.