I have an eCos and a number of those mFx+ locos. The main problem I have is that they sometimes don't work at all when placed on the layout for the first time. Here comes the problem - the combination of excitement and stress places me into some sort of diagnostic mode and after trying several things the loco registers and then works perfectly on the eCOs for the rest of its days.
Among the things I remember trying:
- Delete all M4 locos button and reboot the eCos.
- Remove the loco from the programming track, delete the loco from the eCos and then let it reregister on the programming track
- Turn off all track protocols except M4, delete all M4 locos button and the new loco (if created), and then reregister the loco
- Place loco on the track, Walk away and come back after an hour.

As for your problem - it should not detect running out of sand as the eCOS should not understand the World of Operation commands.. Once they get over the initial "I don't know you" problem, the locos all run fine.

Just an update,Tried all of Adrian's suggestions, still no one at home. All the functions are there and working but no drive in either direction.The motor turns freely by hand,all cogs turning in unison.There is a discernable sound when operating the speed but no movement.My mate who owns the central station purchased the new Challenger and had the same problem after introducing it to my layout without having run it on his own.It ran beautifully for a few hours then just stopped.No apparent reason. After taking it home and putting through the central station it came up with no sand.He followed the cab instructions and voila all was great with the world.I'm seeing him tomorrow so hopefully I'll get the same result.I'll keep you all posted.

If the functions work, then (as Tom says) it should still run. I guess the next step is to try a CS2 or CS3 and set the World of Operation to off.. If it still does not work, then there is possibly a problem with the motor or the wiring to the same.

I was able to get all of my mfx+ locos to run on the eCOS when they arrived - with the frustration as noted above:

37200 Railion G 2000 (2015)
37779 BR 403 Lufthanza Express (2015)
37604 VT 11.5 (2016)
39952 VT 95.9 (late 2016)

This means that the eCOS was running version 4.0.1 at the time of the first two locos. For this reason, I believe that the latest version might not be necessary, but it would be a good idea to ensure that you have version 4.2.0 - you can get up to 32 functions with the latest release.

Just an update.I sent my 120.1 to the NZ dealership who determined that the motor had failed thus blowing the chip.Even though I had purchased the loco from a dealer in Germany,he said that Marklin would honour the warranty for parts but I would have to pay for the labour for the installation in NZ.Hey I’m happy with that but still waiting for the parts to come from Marklin,too long if you ask me.

Is it one of those new locos with four powered axles and centrally mounted motor?
Theoretically the motor should last 300+ hours and theoretically there is overload protection in the decoder.
Märklin is the Roll Royce of MRR - theoretically.

Theoretically that should not make a difference when driving the loco with an ECoS.

Hadn't heard from the NZ dealer for sometime so e-mailed him to see what progress had been made .It seems Marklin in their infinite wisdom have changed their picking/packing method so nothing has been shipped out worldwide for the past 4 weeks.Things are apparently moving now so who knows?Maybe I'll be reunited with my 120.1 by July,My birthday.Saves me buying another loco to celebrate,I guess,Clarkie