We just had the yearly maintenance done on our furnace. Lately it has been going into lockout mode and I have to reset it. It is an oil furnace and is only 3 years old.
I would call the company that did the yearly service back to check why.
There are a few reasons why it would lock out and they have the means available to properly troubleshoot the problem.
if its still only 3 years it might still have the manufacturer warranty on it, most are 3 years. Your service company should be able to tell you very quickly what it needs, they might not even charge you if its something dumb.