I had this issue on my 2.4 brera, used a multimeter to check each plug individually and it was just one faulty plug. If you look on the forum there is more info including what the glow plugs should read.
Generic instructions below:
Glow plug test:
"Perform this test with key off!
Set the multi-meter to ohms (conductivity) lowest scale (X1 is best)
Connect one meter lead to a good ground point (use the jumper wire if necessary)
Momentarily test the meter connections by grounding to another point the second test lead. The meter should read zero ohms.
Remove the lead from the glow plug.
Connect the non-grounded multi-meter lead to the glow plug. Use the #1 jumper cable as required.
The meter should read low resistance, 0.8 to 1.0 ohms is the nominal resistance
Any glow plug that reads high resistance should be replaced. The current glowplug specification is AC-60G.
Perform steps 5 through 8 on the remaining seven glow plugs."
"Think of it as Angelina Jolie. You've heard she's mad and eats nothing but wallpaper paste. But you would, wouldn't you?"