I have a Coleman Natual gas forced air Model 7970C856 The poilet light stays on but furance burner does not kick on. What could it be?

Did you clean the pilot orfice?? It has a very small opening about the size of a hair. You may have to use a needle to open the opening....Are you getting gas to the pilot orfice assembly from the Gas Valve when you set it to Pilot?? If not then check the pilot adjustment screw on top of the gas valve at the pilot tube. CW decreases flow.

Hi check out the fan control afc see if it is clean , check out pressure switch and the flame sencer

Hi check the flame sensor is clean also check out the pressure switch , and check air filter is clean

if you are getting power to the gas valve, then all of the safety switches should be closed. if you have any drain lines off of the inducer fan and at the bottom of the heat exchanger, make sure they have a trap and have water in them. the pressure switch may be the culprit.

OK, the burners will need to have flame on the entire surface or it will trigger a sensor and shut-off. Get a brush and maybe even one of thoes can of air and blow off the burner. Stay away from the glow ignitor because that is very delicate. Once you get flame on all surface of the burners, the unit should stay lit.

There is a thermocouple "wire" that is supposed to be touching the pilot flame.
The thermocouple senses a flame (hot) and allows the heater to operate. If the thermocouple does not sense that the pilot is on, then it will not allow the furnace to operate.
Bend the thermocouple wire (or the pilot tube) so that the pilot flame is touchingg the thermocouple.

That shouls work now.

The air from the fan should not be blowing through the same cxavity that the pilot is in.
If it is, then you may have a leaking furnace whereby carbon monoxide may be seeping into your living quarters.
If the fix above works and the pilot stays on after the heater starts, then there is probably no leak.

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Once you lit the furnace did you turn the valve to the on position. Russ

almost all thermocouples are universal just need to know the length, ie. 18" , 20", 24" etc...

You either have a bad thermocouple or you need to reset the burner control.