Heating Troubleshooting · 6 min read · Updated June 2026
A furnace that won't start on a cold night is stressful — but the fix is sometimes something you can handle in a few minutes. Work through these checks first (safely), and if it still won't run, we're here 24/7.
Safety first: if you ever smell gas (a rotten-egg odor), don't touch switches or troubleshoot — leave the home and call your gas utility, then us, from outside.
Set it to HEAT and raise the temperature a few degrees above the current room temp. Replace the batteries if it's battery-powered — dead batteries are a surprisingly common cause.
Look for a tripped breaker labeled for the furnace and reset it. Many furnaces also have a standard light-switch-style power switch nearby (often at the top of the basement stairs or on the unit) — make sure it's ON.
A severely clogged filter can trigger a safety shutoff. If it's filthy, replace it and try again.
Most furnaces won't run if the front access panel isn't fully seated — there's a safety switch behind it. Make sure it's closed firmly.
High-efficiency furnaces shut down if the condensate line clogs and the safety float trips. If you see standing water near the unit, that may be the cause.
If you've checked all of the above and your furnace still won't fire — or you're dealing with a pilot/ignitor or gas-valve issue — leave it to a licensed technician. Lumen offers 24/7 emergency heating repair across Philadelphia and the suburbs. Call (267) 252-2969 any time.
Lumen Heating & Cooling — licensed, insured, and serving Philadelphia & the suburbs 24/7.