Stop troubleshooting and act now if you notice any of these.
Diagnostic codes are for furnaces that are safely failing. The six situations below are not troubleshooting situations — they're act-now situations. Bookmark this before furnace season.
Do this: Leave the house now. Don't flip any switches or use anything electrical on the way out. Call your gas utility's emergency line from outside — it's free and staffed 24/7.
A switch spark can ignite accumulated gas. The utility will come shut off and check for free; this is exactly what their emergency line exists for.
Do this: Get everyone (pets too) outside immediately and call 911 from outside. Don't re-enter to open windows or hunt for the source.
CO is invisible, odorless, and disorients you before it kills — the alarm is the only warning you get.
Do this: Treat it as a CO event: get out, get fresh air, call 911. Do not run the furnace again until it's inspected.
Most CO poisonings happen in homes with no working detector. "Flu that gets better outside" while the furnace runs is the classic pattern — this is the sign the four-item lists on other sites skip.
Do this: Shut the furnace off (switch or breaker) and book a technician before running it again.
Flame rollout is what codes like Goodman 6 flashes, Carrier code 33 (rollout switch), and E3 protect against — and it can indicate a cracked heat exchanger, which is a CO source.
Related codes: Goodman 6 flashes · Carrier code 33 · Goodman E3
Do this: Kill power at the breaker and call for service. (A faint dusty smell for the first hour of the season is normal; anything electrical or plastic-smelling is not.)
Overheated wiring and failing blower motors announce themselves by smell before they fail dangerously.
Do this: Stop resetting. Look up the code, fix the cause, or call a pro.
Every failed ignition attempt can leave unburned gas in the heat exchanger. Lockouts exist to stop the retry loop — repeatedly overriding them defeats the one safety the control is enforcing. No code table on the internet tells you this; the service manuals do.
After the immediate danger is handled
The $25 insurance policy
Half the list above assumes you have a working CO alarm. If you don't have one on every sleeping floor — or yours is older than 7 years — fix that before you fix the furnace.
These signs summarize the safety warnings that appear throughout manufacturer service literature — the same manuals our code pages cite. Your local gas utility's emergency line and 911 always outrank anything on this page.