The low pressure light coming on when it gets cold out is common, and every car or truck forum I visit has multiple posts come up asking about this when the temperature starts dropping. The pressure in the tires is directly related to the outside temperature......as it gets colder the pressure will drop, and as it gets warmer the pressure will increase. In general you can expect the tire pressure to drop around 1 PSI for every 10 degree F drop in temperature....this basically means if you set the tire pressure when it was 90 degrees F out and now it is 40 degrees F out you should expect at least a 5 PSI drop. Add in to the fact that all tires naturally lose a small amount of air over time (expect 1-3 PSI per month). This now means that when you last put air in the tires 4 months ago you should expect it to be 4+ PSI lower on top of the temperature drop.
In regards to when the light triggers and how much you have to pump it up for it go out, those are different threshold values. These numbers are different for different vehicles, but for example if the target PSI is 65 PSI the light may not come on until the tires drops down to 55 PSI, however for the light to go off you usually have to inflate them all the way back to 65 PSI.
In regards to the post above stating that the pressure gauge showed 39 psi and the TPMS readings said 35 psi.......doesn't matter what the gauge stated and only matters what the TPMS thinks is in the tire. I also wouldn't put complete faith in what a manual tire pressure gauge is stating, especially if it's a $5 version you bought from Wal-mart or Autozone, and I'm sure it has never been calibrated. Having two different gauges, or in this case a manual gauge versus the TPMS showing a few PSI different is quite common and should be expected. In any case you will need to inflate the tires so the TPMS reads whatever the target PSI is as stated by the vehicle manufacturer, and that number should be on a sticker inside the driver's side doorjam.
