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The fan normally runs for a few minutes or so after the gas shuts off.
This removes the heat from the furnace and transfers it into the living space instead of just letting it possibly cause overheating of the furnace until it dissipates. If it is a fan that blows outside, that fan also needs to run for a few minutes to clear exhaust gasses from the furnace - to keep them from dissipating into your home.
Sounds like the thermostat, the way to verify is to unplug indoor unit, remove thermostat, take pic of wiring with phone or label the wires as to the terminals. Carefully twist the yellow,(Y) Red (R) and the green (G) together. Plug the furnace back in, if the unit operates normally, replace the thermostat. I am basing this on the fact that the blower speed is changing. This could also be an anomaly within the circuit board in the furnace section, not likely though as the outside unit (condenser) is not controlled n by the indoor board unless this is a heat pump. hope this helps. D
When you say 'older furnace' I am assuming old enough to where there is no circuit board. They will usually have a thermally activated 'fan-limit' switch with a bi-metal sensing device. Most older furnaces had either a Honeywell or a White-Rodgers controller and they look almost identical. With the furnace door off, look towards the top of the furnace vestibule (above the flame area) for a rectangular box approximately 3 1/2" wide by 5" long. Tap the side of the box with a screwdriver and if the thermostat setpoint has been satisfied and the gas burners have been off for more than three minutes, the fan should then shut off. The switch contacts are either stuck together OR the bimetal sensing device is weakening from thousands of cycles.
If it continues to run, squeeze the sides of the 'lid' of the box and remove the cover. Inside you'll see a round moveable dial with three pointers, marked 'fan off, fan on, and limit'. The 'limit' should remain at or near the 200 degree setting, the 'fan on' can be anywhere around 140 and the 'fan off' can be anywhere near 100 degrees. It is the 'fan off' position thats giving you the grief.
Turn the thermostat up again and sit there and watch the dial do its thing and yo'll understand better how this control works. Watch it rotate past the 'fan on' setting and observe the fan turn on. Turn the thermostat below room temperature and watch the gas shut off and the dial rotate back towards the 'fan off' setting. If it hangs up again before twisting past the setpoint, you can slightly increase the setting to compensate for the weakening bi-metal element. But just a tiny bit, not a great amount or you will have the fan shutting off with a lot of residual heat still in the heat exchanger. The idea of the delay in shutting off the fan is to remove all that residual heat and transfer it to the house.
There are other brands that utilized 'timed on' fan controls, and there are a few other variances of controllers but what's described above should cover the vast majority of older furnaces.
Remember there is ELECTRICITY inside the fan control and any adjustments should be done with the power OFF !!!!
It sounds to me like you have problems with the gas valve as you have eliminated pretty much everything else. I would check all wiring to valve,check for any breaks on the insulation and clean all connections.
When the burner goes out is the blower still running or does it quit then the burner go off? If that is the case I would think the thermal overload on motor was tripping. It makes me more inclined to suspect the gas valve due to the lack of any diagnostic code.
One other thing I would try is to check power to gas valve when burner is firing and see what it does when it cuts out.If it maintains power when gas shuts off then I would replace gas valve. Good luck and thank you.
Could be a defective thermostat in the plenum.
When a gas furnace shuts down the fan(blower) will run until the plenum cools to the safe range. Usually 1 and 1/2 minutes.
Intermittent operation is a defective thermostat or relay.