Bosch Dishwasher model SMS40T42UK/34 Check Water light flashing

Cannot find an answer elsewhere so hope someone can help with this.
Bosch sms40t42uk/34 2014 mfr, but used very little.

I press ‘on’ button, machine does self check – lights
flash a bit – all turn off, but check water light begins to flash
continuously along with drain pump humming/emptying. If I pour water
into machine, it empties fine. Checked for blockages – none. Have
replaced drain pump and flood switch – no errors found on the original
ones – and have confirmed no evidence of water ever having been in flood
tray – polystyrene float as clean and dry as the day it was put in. No
water in bottom of case at all. Have removed matrix and swilled out the
very small amount of crud in there. Flow meter spinning freely.

Everything I read says a flashing light indicates a drain/flood problem.
Pressed start/reset button for 10 seconds – does nothing – flashing ‘check
water’ light stays on. If I press on/off button, after a few seconds
machine powers down and lights go off. Switch on again and the whole
problem described above repeats. Have had this machine stripped down –
tilted it 90 degrees to sides, front and back. It has no leaks, it
hasn’t flooded, filters clean as new, pipes into and from machine flow
freely.
Are there any other reasons for a flashing ‘check water’ light?