HELP! Volvo 2013 780 D13 DEF Issues - 5 Dealership Visits Later and Still No Fix!

Perhaps someone can help with any suggestions? I'll give you as much info as possible (note: if relevant - I have a very heavy load). First, the DEF Fuel Guage cut-out and went to zero. Then it started flicking on and off intermittently. Then I got three fault codes (SPN 1231 FMI9) and (SPN 1231 FMI2) and (SPN 5246 FMI16). Then I got an SCR System Fault Engine De-rate. So I went to the nearest dealership. They found loose wiring and a burnt terminating resistor (807) which had fallen on the exhaust and they replaced it - saying that's the issue following data link test. Then, I carried on, and the next day the same happened with two faults, as well as the DEF Fuel Guage, flicking off (check engine ECU and engine fault with the SCR De-rate - on to the next nearest Volvo Dealership. By this time I am noticing that it's happening when it's hot - as when the truck cools down all is good and the codes go away and the DEF Fuel Guage starts working fine. At this dealership, the truck showed no active codes and they checked the EGR Cooler (reading 0.5 psi at KOEO) and Removed the EGR Pressure Sensor (cleared out ports and refitted - now reading 0.01 psi). Advised to disconnect EDL to see if it's interfering and causing Data Link Codes (so did that). I went on my way bobtail and not even 20 miles down the road had issues again. Back to the same dealership (fault codes reading SPN 5246 FMI0 and FMI15 and FMI16 x8). They checked related wiring harnesses for SCR and DPF (no faults found). They disconnected the DEF Pump Connector and found no leaks into the harness. They reinstalled harnesses and carried out voltage check for ACM (passed ok). They checked the ECM and ACM software (all up to date). They tested the Data Link Wiring and found one pin from the battery voltage pulled out on the ACM side. They pushed them all back and checked that they were locking in place and all was ok. They tightened the master switch cables, disconnected the battery, cleaned them and reconnected and tightened them all down. They sent me on my way and 200 miles down the road - same again. Dealership number three! When I got their (bobtail) they had an active code and found no continuity on Data Link to ECM or ECU from the ACM. They removed the wiring harnesses, pulled off and cut all tape trying to find the CAN BUS Splice. They found it at the hood to the ECM before it connected together. They re-spliced - no code reappeared. They were going to send me on my way and the DEF Fuel Guage went off again. They cleaned all earth terminals and re-tightened them on the chassis and the positives and starter motor. After leaving, bobtail, not even 20 miles, and here we go again! They then bi-passed the master switch and connected the battery directly - after hooking up to my trailer the DEF Guage again went off and I also have fault code active (SPN 1231 FMI9). I told them to try the sender unit but they said that doesn't connect on the same harness. I am now waiting until tomorrow morning for them to try again - but they seem like they don't know where to go from here. Perhaps someone has a suggestion? Everyone seems to be scratching their head! Help!

 

Tha

Thanks
Now the last dealership has done that they are now changing the sender unit from the def tank