It’s very easy to check if the person is running a HVAC scam

If there is a leak, he has to shut it down right away, not come back and shut it down. Lol..

You can report to your gas company that there is a potential leak. They will send a worker to check it out. Report leaks line will be on the gas company's web site.

If they confirmed that there is a leak, you call another HVAC guy just for the leak. If at the end, it is the furnace's problem, you shop around for the best deal.

Winter is gone, you have time to sort it out.