What immediate step should be taken if you suspect a gas leak from the furnace?

Evacuating the area is the most critical and immediate step to take if a gas leak from the furnace is suspected. The presence of gas poses an immediate risk of explosion, fire, or poisoning, making it essential to prioritize the safety of everyone in the vicinity. By evacuating the area, you ensure that individuals can move to a safe distance away from the hazard.

While turning off all appliances might seem prudent, it can sometimes compound the situation if ignition occurs. Checking the furnace for errors can be important after ensuring safety, but doing so during a suspected gas leak could put you at risk. Similarly, contacting the manufacturer might be necessary for troubleshooting or future references, but it doesn't address the urgent immediate danger posed by a suspected gas leak. Thus, prioritizing evacuation is the correct action to take in such a critical situation.