To continue with my testing:
I repaired the old pcb (replaced the Mosfet in the H-bridge that drives the pump motor and put wire links across the two corroded tracks) and also replaced the pressure sensor and the leaky expansion tank by using the complete pump assembly from the 2018 Crossland X tank except for the pcb.
Tried this in the car and it still didn't work even during an extended road test.
Also tried the 2018 pcb, but came back (as expected) with the "No communication to the Denox system" fault.
I was unsure if I still had faults on the pcb, if hadn't properly commissioned (refill, bleeding, resetting) the tank, or if I was not using Diagbox correctly, so as Paul-R was not too far away I asked if I could have the pcb from his old faulty tank. He very kindly let me have the complete tank
Note at the start of the process, I was unsure how the tank could be tested. So I'm not sure exactly when it was working or not, but here is what I did:
1. My tank wouldn't run even when driving the car
2. Inspected Paul's tank's pcb & wiring. Couldn't see anything wrong.
3. Tried Paul's pump assembly (pump and pcb) on my tank, Using Diagbox showed -40C tank temp! I thought it was a bad connection or thermal sensor not connected. Pulled it apart again and found nothing wrong.
4. Tried swapping my tank for Paul's. Still -40C.
5. As my pcb had shown the correct temperature, I tried my pcb with Paul's pump and my tank. Showed +19C.
6. Swapped the two pcbs. Now Paul's showed +20C, but the parameters on Diagbox still showed tank heating, so the car wouldn't allow the pump to run. Note I don't believe that tank heating is required much above -11C (freezing point of adblue).
7. Disconnected and reconnected Paul's pcb. Diagbox box showed that it was no longer in heating mode.
Note I had tried resetting the urea tank faults on Diagbox on the above faults but it hadn't changed temperatures or the heating requirement.
8. Tried the actuator tests again and the "check of the Denox flow" and it started the pump!! Note that the outlet was just connected to a bottle, so would be unable to pressurise.
9. Tried my repaired pcb with the 2018 pump again, it also worked on the check of the flow!
I have found that this test is best for testing the system. I also tried the other actuator tests. The control relay test does nothing, but doesn't fail. The heating tests sometimes pass, sometimes fail, sometimes it doesn't start. I tried combinations of Paul's pcb, tank and wiring and mine all with inconsistent results. I'm totally confused by that as the resistances of my tank, Paul's and the Grandland x are all the same.
10. My last combination was basically Paul's tank & pcb, so I tried a test drive. It worked.
11. The next day it didn't work on a test drive or start the pump on the flow test.
12. The day after that, I pulled it apart, but couldn't find anything wrong or with the parameters on Diagbox and it wouldn't run the pump with the flow test. I was talking to my next door neighbour and was telling him about it, saying that it felt like the problem seems to be one or more poor connections. So I said if I just tapped the pump assembly it would work and guess what, with the screwdriver handle I gently tapped it and it worked!
I put it back together, gave it a test run which worked fine. I was going to do some more testing when it stopped again, but it has now been working for just over a fortnight and over 600 miles.
Note that the original faults on my leaking pump were:
P20F6 Denox leak detection, P249E Depollution of denox - excess amount, P3052 Pressure of urea too low.
These were followed the next day by a host of other faults including P20E8 "low Denox pressure" and U029E "Fault in communication with the denox system - no communication" and the 700 mile countdown. Unfortunately I didn't record them all. I guess this is when the adblue completely submerged the pcb.
My conclusion at the moment is that Paul's tank wouldn't run because of a bad connection. My guess would be the relay contact, but the main wiring connector to the tank wasn't great either, or maybe a soldered joint? I'm waiting for it to fail again to track this down. Note that that cable on the tank had been replaced as it was fitted with one dated 2016. There was a recall for the cabling harness.
I think that the problem of the -40C was the corruption of some parameters when connecting the pcb. I'm still not sure if the heaters will work ok as it's not cold enough to test these properly.
I can't explain why my repaired pcb with the 2018 pump wouldn't originally run on the car (unless it was in the commissioning) as it did work on the actuator test (step 9)
I have stopped testing as the car is still working and I've got to get on with some other jobs, but more to follow.
Thanks again to Paul-R