Furnace pressure switch stuck

I don't know which part of Canada you live in. It is sure no fun to have no heat in the coldest part of the year if your furnace die on you. You say it is 19 year old furnace it is time to get a new one especially it is giving trouble already. That is what I will do.

I agree 19 is past due and you are better to shop around when you are not in an emergency (no heat).

However, I had a Payne furnace that started doing this after about year 12. It progressively got worse and it would happen when it started like 3 times a winter and eventually twice a week. I did some googling and found it was a common problem and there was an improved replacement part which I ordered off EBAY and installed myself. Get the part number off the pressure switch you have now and you will find a slightly different part number that replaces it. Two wires and two hoses and it worked fine. However, during the time it was happening I kept a pencil handy and gave the pressure switch a tap with the pencil, reset the furnace and away it would go until the next time. Would happen periodically and the family would start complaining it was cold, go and check, sure enough flashing lights. I don't remember the replacement pressure switch being expensive but couldn't find the receipt and got 8 more years out of that furnace before the fan went. Replaced it with another Payne model and it has been good ($2800 installed).

Actually, I found the ebay receipt...

$300? Does that include labour? These are simple to replace yourself.

The contacts have gone bad on the switch. This kind of wear does not self heal. Expect it to fail again any time. The workaround may or may not work a second time.