Flame symbol showing on theremostat

You said it heats up but never comes to the thermostat temp. Is it blowing hot air through the vents? If not it may be the fan limit shutting it down because its too hot. It could also be a pressure switch in the venting. Either bad switch or vent pipe is blocked or spill is stuck.

Same problem...then discovered my thermostat was set to Celsius instead of Ferinheight... pressed toe temp up and temp down buttons to change..fixed...

Problem is the thermostat.The heat/cool selector switch might not be making try back and fourth from heat/cool to make a good contact.In heat mode make sure the temp is set LOWER than your cooling temp.If it is too cold in the future just RAISE the cooling temp up and not the heat/cool switch.

Could be under sized unit or fan relay on unit.

You can make sure it is not the thermostat by crossing or jumping the R and W terminals (or associated wires) together. This will keep the furnace calling for heat. As it is approaching set temperature, get in front of the furnace and inspect the air filter to make sure it is clean, watch the control module for any error codes that may be turning off the system prematurely.

On a call for heat, the 24 volt thermostat sends a signal to the control module. The control module will indicate a call for heat with a light on the control either blinking or remain solid depending upon model. The inducer (exhaust) blower will purge all gasses from the furnace and pressurize a pressure switch. Once the pressure switch tells the module to continue, the electronic ignition will energize and send 120 volts to the igniter. The igniter will glow and you will be able to see it if viewed thru the small inspection port. Once the igniter gets hot enough, it sends a signal to the module opening up the gas valve (24 volts). Either a pilot will come on or the burner tube will ignite then spread the flame to all burners. Lastly a safety sensor will be looking for a certain temperature within a few seconds and the furnace will continue to operate and the room air blower will turn on in a minute or two.

What could go wrong? The unit will not run if there is no signal from the thermostat (bad thermostat or broken wire), the control module does not sense a signal from the thermostat (bad control), the inducer does not energize (bad motor), the pressure switch does not close (blocked vent piping, bad switch, plugged condensate hose), the igniter does not energize (bad control, bad igniter), the gas valve does not open or there is no gas (bad gas valve, broken wire, no gas), the pilot does not light (dirty pilot), the burner does not light (bad burner, plugged orifice, not enough combustion air), the flame does not spread to each burner (bad flame spreader, dirty flame spreader, more bad burners), the flame safety sensor does not detect flame (dirty or bad flame spreader, bad flame sensor, broken wire, bad control), or the room air blower does not energize (bad fan motor, bad control).

change the filters first thing. Make sure it is operating correctly. You can use another thermometer and leave in room and see if it matches the temp on theremostat. Forced air is a little cooler. Make sure all your vents are open.

Is there a heat source close to the thermostat, plug in transformer for a phone mabey, table lamp? If not then you have to have a bad thermostat. Any possible heat source from inside the wall? Will it shut off if you turn the system off? Turning power off to the furnace will shut everything down. If no heat source is driving up the indicated room temp, replace t'stat.

I set the thermostat for 70 degrees. It comes on always in auxillary heat and does not shut off when it reaches 70 degrees. It is a heat pump, I am assuming, it might be the thermostat? But when the air conditioner is on, it turns off when it reaches it's temperature.

check the presure switch jump it out and see if it fires all the time

the flame detector rod is dirty? try cleaning with sand paper and scraping also. there should be a small amp. reading threw wire coming from flame detector rod. in milli amp range. increases when running. some times i use a jumper to hold gas valve on once started to allow time to test amps.