Fisher Paykel DW920 dishwasher has U1 error?

Any help with this problem appreciated:

My DW920 (2006 vintage) shows U1 error. I have the user manual and exploded parts diagrams. U1 means "failed to fill with water to correct level". Machine previously worked ok, not moved or any other change. I checked obvious things eg tap on. A slight trickle of water would enter machine. After a minute or so, machine would pump water out and try to fill again. After second try failed, U1 error appeared. I replaced the water inlet valve - a common problem, I gather - with a compatible 4L/m valve instead of original 5L/m. Also replaced the white plastic push button tray (part 696040177), as it was cracked.

Now good flow of water runs into machine in first minute, then water stops. No other machine action evident. Then machine pumps out, tries to fill again, then U1 error. I tried adding 3L of water manually to the tub during first minute - had no effect. It seems the controller board is at least partly working - attempts initial water fill and pump out.

I gather this machine does not have a water level sensor. Apparently the machine senses water present from the current in the wash pump, which is erratic until enough water is pumping. Apparently current should settle down in first minute.

I'm puzzled that I seem to have two problems, each of which causes U1 error - seems unlikely. I'm an amateur at dishwashers. But my thinking is that problem now might be: faulty controller - no signal to wash pump; or faulty connection from controller to wash pump; or faulty wash pump. Or maybe faulty starter capacitor - stops wash motor from starting?

If it is pump or controller, I don't know whether new parts cost would be justified for a 6 yr old machine with a poor reputation (I gather). But I'd like to investigate problem further, in case there is a low cost fix. I have a multimeter.

Any suggestions?