Wildly inaccurate Voron Tap

[Cheetah512]

I've just upgraded my Voron 2.4 to have a BTT SB2209 RP2040 hotend board with CAN, and at the same time I took the opportunity to switch to an OptoTap.
In hindsight changing two variables at the same time doesn't make troubleshooting easier.

The SB2209 seems to be running perfectly, hotend, temp and accelerometer sensors all work perfectly.
The CAN bus is running back to the Pi via a U2C at 1mbit.
The Tap also "works" but the results are abysmal with variances of up to a mm!

Here's a Probe Accuracy with the speed reduced right down to 0.6mm/s which is painfully slow, and it's still horrible.

PROBE_ACCURACY at X:150.000 Y:150.000 Z:23.222 (samples=10 retract=2.000 speed=0.6 lift_speed=5.0)
probe at 150.000,150.000 is z=-3.103750
probe at 150.000,150.000 is z=-3.086250
probe at 150.000,150.000 is z=-3.105000
probe at 150.000,150.000 is z=-3.106250
probe at 150.000,150.000 is z=-3.106250
probe at 150.000,150.000 is z=-3.155000
probe at 150.000,150.000 is z=-3.155000
probe at 150.000,150.000 is z=-3.120000
probe at 150.000,150.000 is z=-2.836250
probe at 150.000,150.000 is z=-3.122500
probe accuracy results: maximum -2.836250, minimum -3.155000, range 0.318750, average -3.089625, median -3.106250, standard deviation 0.087035

These results are worse at a more reasonable speed.

I'm a bit stumped with where the issue is. Mechanically the non-moving part of the Tap feels rock solid, and the rest also feels solid with only movement upwards, no side to side movement and nothing seems loose.
Looking at the LED at the back of it, manually moving the tap seems to be very accurate with the LED changing with barely any manual input when I move the hotend up.

Any suggestions on what I should be checking would be greatly appreciated.

 

[Cheetah512]

I've figured out what it was, it seems the plastic body of the optical sensor was "loose" and the soldered connections were not holding it in place.
A little bit of glue to lock it in place and it's suddenly super accurate as expected.

If the pcb rattles slightly when you shake it, it's probably the plastic casing moving changing the trip location.