How do you know GPs are not operating?... If you actually READ my past appends... the GPs ARE powered for several minutes AFTER starting the engine. This is when the ECU does its current-sensing and may throw a code.
If you have any knowledge of electronics... the ONLY way to detect current is to measure voltage across a known resistance. (There is no such thing as measuring current .... only measuring voltage and then applying Ohms law.)
Further, to measure the relatively large current that the GPs draw, the resistance has to be very low. . Ideally, the resistor for measuring the current should be less than 1/10 of the resistance of the glowplugs.
(Otherwise, the voltage-drop would be too much)
If the GPs are 1.0 ohms... this means the resistor may be 0.1 ohms (or less) Even with these paramators, that would make the operating voltage at the GP almost as low as 11.5V (theretecally)
Also, you must account for the fact that the GPs are resistive heating elements. This means that their resistance GOES UP as the GPs get hotter. (if 1.0 Ohm cold.... may be as high as 100 Ohms hot)
I hope this answers your question. (I really dont want to get much deeper into electronics theory here