All four glow plugs were replaced just over a month ago. A few days later after the car had been idle for a couple of days I started the car and notice the glow plug light came on for a second or so and the car started normally. But when the car was running the outside temp was only reading 0 degrees.

Went out to the car this morning and it was all covered in frost. It had been idle for only 12 hours. I started the car, no glow plug light, the car started normally but when it was running the outside temp was reading -3 degrees.

Now I am confused, why did the glow plug light come on at 0 degrees but not at -3.

Is there a way of doing an all lights test on the dash to confirm all the warning lights are working. Because under normal conditions the glow plug light doesn’t come on.

I’ve hardly ever seen the glow plug light on in 11 years of owning my FL2. So other than the odd occurrence at 0 degrees, it would seem that your car is behaving normally.

The manual says the glow plugs sense Coolant Temperature, not Air Temperature.

Quote

OVERVIEW

A glow plug is installed in the inlet side of each cylinder, to heat the combustion chambers before and during cranking.
This aids cold starting, reduces emissions and engine noise when idling from a cold engine.

A wiring harness on each bank of glow plugs is connected to a separate relay and fusible link in the BJB (battery junction
box) . Each glow plug is grounded through its fixing in the cylinder head. Operation of the glow plug relays is controlled by
the ECM , which also controls the illumination of the glow plug indicator in the instrument cluster.

Each glow plug is a tubular heating element which contains a spiral filament encased in magnesium oxide powder. At the
tip of the tubular heating element is the heater coil. Behind the heater coil, and connected in series, is a control coil. The
control coil regulates the current to the heater coil to safeguard against overheating.

OPERATION

There are three phases of glow plug heating: Pre heating, crank heating and post heating. The ECM (engine control
module) determines the heating times from the ECT (engine coolant temperature) . The lower the ECT (engine coolant
temperature) , the longer the heating times.

When the ignition switch is switched to mode II, the ECM (engine control module) calculates any required heating times
and, if heating is required, energizes the glow plug relays in the BJB (battery junction box) . When pre heating is required,
the ECM (engine control module) also sends a message to the instrument cluster, on the high speed CAN (controller area
network) bus, to request illumination of the glow plug indicator. The glow plug indicator remains illuminated for the
duration of the pre heating phase, or until the ignition switch is turned to the crank position, whichever occurs first. If
required, the ECM (engine control module) keeps the glow plug relays energized during cranking and for the duration of any
post heating phase.

The ECM (engine control module) monitors the drive circuit of the glow plug relays for plausibility of operation, continuity,
and short and open circuits. If a fault is detected, the ECM (engine control module) stores a related fault code and
permanently illuminates the glow plug indicator while the ignition switch is in mode II.

https://www.freel2.com/forum/topic37673.html FL2 XS SD4 Auto 2010 2012-2017 (21k - 91k miles) (MY2011)
FL2 Metropolis SD4 Auto 2014 2017- (16k - 76k+ miles) (MY2015)

This is probably true. The glow relay bases its operation on coolant temperature. I have the same experience as you. Seems like there are some heat left in the engine mass for a long time after operation, and there is not much glow plug operation seen the day after when starting in a few cold degrees. Seems to me that the car need to stand nearly two days before its "stone cold". I started mine up a few days ago in -6. She hadn't been used for a few days then, so I got glow plug operation for a longer time than usual and a delayed starter operation. I nearly never experience the delayed start when starting the day after in -5-10 blue.
As written several places, its a good practice to perform a "double glow" when you know its cold outside