Thanks David.

What would you suggest as a figure?

So, I've built a little PCB that drives 2 more extruders on the e2 and e3 pins on the expansion connector.

I'm using 2 pololu DRV8825 drivers with all MS pins held high doing x32 microstepping

But, one of the extruders will randomly go backwards or forwards.

What the heck?

I haven't setup M906 for it, it's an external stepper. Does it apply? I thought I just used the little pot on the stepper driver.

I've setup an additional stepper driver (drv8825) and configured it in firmware.
all ms pins are high
M584 X0 Y4 Z2 E3:5 ; Apply custom drive mapping
M350 E16:32 I0
I've set M569 P5 S1 T4 as well

But it buzzes and doesn't turn.

I've got a1 a2 b2 b1 attached to red blue green black respectively.

where am I going wrong?

Well, I deleted my config.g override, and changed to p8. It stopped travel.
Changed back to p5, seems to be working now.
Odd, nothing in the config override should have been doing anything, but it seems to be working now.
Thanks for all your help @dc42

Yes, I can SEE the pulse on the web interface, I can't imagine its not long enough, but I'll try that now.
I USED to have a mechanical z-probe, is there some other setting I might have changed?

Ok, Been farting around.
When I tap the hot-end, the Zprobe on the web interface DOES go to 1000.
But it doesn't stop diving on the G30.

What the heck?

I just re-updated the firmware to the latest 2.02
Same thing.

M558
Z Probe type 5, invert no, dive height 5.0mm, probe speed 1200mm/min, travel speed 6000mm/min, recovery time 0.40 sec, heaters normal, max taps 1, max diff 0.03

G31 T5
Threshold 100, trigger height -0.10, offsets X0.0 Y0.0

Maybe I'll start the config.g from scratch.. But you see what I have there, nothing misconfigured?

@dc42 I get a v+(3,3?) blip on z_probe_in on the duet wifi board when I tap the hot end.

So, either Duet wifi board, or config is wrong.
How can I see what the firmware is seeing?

Can I run the Z_Probe_in to somewhere else?