Also, I believe that the TPM measures "relative" pressure and not primarily a pre-set pressure. That means that if your last pressure reset was at 33psi then it will look for a -4psi change (just an example number) and it will warn below 29 psi. But if you reset it last time when the tires had 35psi, then it will warn you below 31psi and not below 29psi.
That's why in moderate climates I need to add air when winter comes, and take out air when spring gets here. In your case in Canada: add air in early September and take air out in late June
