Engine starting sluggish when cold, cranks OK, no glow plug light, starts up and runs perfectly when warm.
This has obviously just started to happen in current cold spell..
The associated fuses (based on handbook and Haynes) checked and sound (F2 is 5A, F4 is supposed to be 60A but is only 25A)
There is a good constant supply across large pins 4 & 5 of the Nagares unit socket even when ignition is off.
R4 supposedly Glow Plug Relay, when removed does not affect this supply.
All glow plug resistances are approx 0.5 Ohms (this would say 24 A per plug but I know this resistance will increase rapidly as they heat up. I think this means the very difficult to get at plugs are OK?)
No related fault codes.
GLWPREL commanded and feedback, Forscan PIDs change state when starting but the duty cycle does not.

Whilst I've ordered a new unit, I'm asking if my logic is correct or if I've missed anything?
If this doesn't work then the above may be useful for others and I'll obviously be back with a much deeper question!

Hi, I had a problem with a code referencing glow plugs. It turned out that the fuse, I think it was a 60 amp and was on the front of the battery box or thereabouts took me ages to find the damn thing. Just a mention because it stumped me.

Just to report back on this.
My advice on modules (Nagares, 8 pin)
Either buy from Ford at £107
OR genuine from e-Bay at £60
Don't bother with cheap Chinese knock offs at £30 as it just didn't work, giving same code and confusing the issue unless you feel lucky!
The correct 80A fuse is hidden under a cover at the side of the fuse box!
After testing everything including, as above and getting codes:
P0683:92-6C Performance /incorrect operation
P0683:92-2F Performance/incorrect operation.
Tested:
Individual glow plug resistances (0.5 Ohms) The 4 medium size connectors.
Power supply and earth (the two large connectors) including load test with headlamp bulb on all.
Removing the module and getting "Circuit disconnected" code
P0683:13-2C Circuit open.
This proves the two thin communication wires are OK to the PCM (resistances to Earth checked)
Current clamp on main lead showed that significant current (50A ) with large initial surge was being drawn sometimes (before new genuine module).
Forscan monitoring of :

Showed that:
Duty cycle PID never changed, always zero.
Commanded and feedback should be same (with genuine replacement)
With fault code on, the feedback trace was erratic and did not match commanded.
Glow plugs stay on for about 3 minutes i.e. not just whilst starting
Glow plug light on dash does not always come on.
Summary: do all the tests and if the wiring is proved fine and glow plugs are OK then and only then, change the module? It only lasted 15 years, how bad is that!
At least that was my solution - test don't guess?

Hope this helps someone.

A modern Direct Injection Diesel does not need Glow Plugs to start unless the temperature is well below zero, they are used for the first few minutes for Emission Control purposes though.

To illustrate my assertion my Rover 400 that had a Distributer Pump, not a much higher pressure Common Rail, and only had 3 Glow Plugs and started like a Petrol Engine all year round for the 14 years I owned it.

As for the Electrical Diagnoses, where is @unofix when you need him.

That is supposedly true i.e. direct injection will likely start at low temperature but they may struggle and the glow plugs help.
I notice a significant delay after pressing the start button when there is a cold snap, before cranking starts.
I assume this is to put a bit of heat in and save any extended crank time but that may be just my impression.
Before direct injection, the glowplug in the swirl chamber (Indirect injection, Thank You Harry Ricardo!) was essential to get it going.
I can only report my experience but if there is any improvement or other evidence, I will be glad to hear it!

Take anything above with a barrel of salt...
I believe this is the best explanation of S-MAX (or identical) glowplug systems:
https://www.youtube.com/watch?v=AxksgpgVunE&list=WL&index=2
DON'T PUT 12V ACROSS THEM AS THEY ONLY NEED 4V AND WILL BURN OUT (DON'T ASK HOW I KNOW!)

Despite my best efforts, including two new modules, testing plugs, powers earth's, fuses, using Forscan to test module signal state and feedback (which is still a bit variable) it now seems to have settled at a hard still bad communication code P0683:92-6C.

I'll have to check all again as weather gets warmer but I'm left with going to PCM under wing and load testing the two wires from there to Glowplug module.

Unless there's any other ideas?