Most gas furnaces give some warning before a service call. The changes are not dramatic. The blower may run longer after the burners shut off. The start may sound different. The house may take longer to warm after the thermostat calls. These are useful notes to keep, because they help a technician know where to look and they help you decide when to plan.
Log the run pattern
Pick one room that the thermostat sees, and note the time the burners start and the time the blower stops in a simple log. Do this on a mild day and again on a cold one. A furnace that runs in longer cycles than it did when it was new is not necessarily failing, but the change is worth recording and comparing against the install year.
Listen to the start
A healthy start is brief and steady. Note any new rattle, scrape, or pause before the burners light. Note any long hum from the blower motor after the heat turns off. Write down the date you first heard it and whether it repeats. A repeated sound is more useful to a technician than a description given later from memory.
Watch the filter and the airflow
If the filter loads faster than it used to, check for a change in the return path or the duct run before you blame the filter. Restricted airflow can make a furnace run longer and work harder. Note any room that is consistently slower to warm than the others.
Know which signs need a call
If you smell gas, leave the home and call the gas utility or emergency services from outside. If you suspect the furnace is not venting properly or you notice unusual soot, shut the system down and call a qualified technician. Do not try to adjust the burners or the venting yourself. Those checks belong to the technician and the gas supplier.
Use the walk-through and the planning guide
Each fall, run through the fall furnace walk-through and log what you see. Then read the Gas Furnace guide to place the install year on the planning timeline. The early signs do not tell you the day the unit will stop. They tell you whether the plan needs to move up.