The hand on my Johnny Mnemonic was reporting No Y Progress on startup this morning. I fixed it, and in the process learned a lot about the causes of this, so I thought I'd share. This is all about the Y axis (forward/back), but the X axis is a larger version of the same assembly.
The error means that the game is applying power to the motor, but is not seeing interruptions on the opto attached to the threaded shaft. That means the carriage is jammed at one end or the other, preventing the shaft from turning.
The most common cause of this seems to be issues with the Home switch. There's a micro switch on the side that is depressed by the mounting plate of the magnet. This tells the computer when the hand is at "0". From here, the computer counts the number of turns on the shaft (via the opto) to know how far the hand can move. If the switch never registers, the game will run the carriage all the way out the end without stopping, and jam it up.
The constant vibration of the hand and the impact of the ball being shot into it can cause the Home switch to malfunction. The switch should be fully activated by the leading edge of the plate contacting the roller on the switch lever. The plate has a small upward bend at the end for the purpose. The plate is slotted and can be adjusted up and down. The switch itself is also mounted on small slots, and can be adjusted.
If the switch is being activated properly by the plate, then the usual switch diagnostics apply- check the wiring, diode, switch continuity, etc.
Furthermore, if the switch is activating, but a little late, the carriage will also jam up against the end. This is because the computer has a fixed number of turns that it thinks it can do from the moment that switch is triggered. If Home is triggered too late, the carriage will crash into the end and No Y Progress will be reported. You can see this happening in gameplay. The hand will move to the end and the motor will start struggling. The carrier will twist slightly under the strain. You should NOT be able to cause the carriage to hit the end using the flipper buttons in gameplay.
If the switch is activating too late, you can adjust its position, or the position of the plate that activates it using the slotted mounting holes. You can also tweak it in software by adjusting the Home Position in the menus. However, if the game was catching the ball properly prior to this error, I would not mess with that software setting. Changing the software's Home to compensate for a misaligned switch seems like it would cause missed catches later on.
The moral of the story is, the root cause of No Y Progress errors seems to be because the Home switch is registering too late or not at all. The game relies solely on counting shaft turns relative to that "home" point to keep the carriage from crashing into the ends.
If it does crash into the end, shut the game off immediately, put the playfield into the lower service position, and twist the metal opto interrupter with your hand to move it back. This is tedious, but once the carriage is jammed at the end, the game refuses to try and free it by moving the other way. It can jam it quite hard, too. It takes strong fingers to back it off by twisting the opto, sometimes.
Hope this helps someone.
Here's the Home switch, properly adjusted, at the right moment of activation.