Whirlpool g2p DWI F4 draining fault indication

I have had this dishwasher for ten years now and it has been faultless until recently. The unit stops with an F4 error message now. So far I have done the following:

Ensured that it is actually pumping out in a timely manner.

Removed, stripped, cleaned and tested the pressure switch. I have had test leads connected to it, monitoring its operation on a meter. It appeared to be OK, but the spring action was feeble, so replaced it anyway.

A test run then gave two complete cycles with no error. At this point it was put back into use but failed again next time it was used with the same error. Draining is still complete and timely.

Next job was to strip the door and look at the control unit. The plug was pulled off and the operation of the switch monitored there, whilst the door was flexed. The idea was to look for an intermittent o/c or short in the door wiring. No problem was detected there, so a test run was done. That passed OK. Another run in normal mode 2 was carried out and agin it passed OK. The assumption is that there was some corrosion on the PCB contacts that caused it, but once again it failed the next time it was run.

When the sump is empty, the switch is open and so the control unit should sense that, but the machine continues pumping until it times out, suggesting a short on the switch wiring to the control head. I have looked in vain for a service manual or a circuit diagram.

Does anyone have any more suggestions please that will save me having to replace the unit?