Have a carrier 58DR075-AA Series 121. Pilot was lighting but main gas to burners was not opening. I replaced pilot/thermocoupler unit, its all one piece. Now the pilot lights after a few seconds I hear the burner valve open, and the burners try to light but go right out. Other times the pilot clicks off before the main valve opens and it keeps retring. Only occationaly does it light and come to temperature but after it retries I face the same problem. Am I missing something? I have checked the voltage on the circuit board and it is sending a voltage to the valve to open.

If it is sending a steady 24 volts to the gas valve when the gas valve turns off no reason it would seen to me to be a gas problem valve. All other requirements for unit operation must be met for the unit get the 24 volts to operate the gas valve to start with.

Replacing the gas valve is never my first choice in a repair because they usually last for a very long time and cost some $$$.

If you feel that all other devices on the furnace are working properly then you will have to make the choice as to weather or not you replace the valve.

Another important thing to check is proper gas pressure to the burners as per the ID plate spec. on that unit, as well as incoming gas pressure to the gas valve...

Have a carrier 58DR075-AA Series 121. Pilot was lighting but main gas to burners was not opening. I replaced pilot/thermocoupler unit, its all one piece. Now the pilot lights after a few seconds I hear the burner valve open, and the burners try to light but go right out. other times the pilot clicks off before the main valve opens and it keeps retring. Only occationaly does it light and come to temperture but after it retries I face the same problem. Am I missing something? I have checked the voltage on the circut board and it is sending a voltage to the valve to open.
hvac1000 Thanks. The valve seems to be working but the 24v from the board to the valve is only on momentarily when it tries to ignigte. When it does lite it is blowing right out immediately and the voltage to the valve ends. It seems that it is tiring to lite less often and the pilot is clicking off before the burner valve opens. Only sometimes does it acually fire up. Then only for one cycle.

How is the pilot height? It needs to be high enough to cover the flame rod and the back metal bracket of the pilot assembly?

Another important thing to check is proper gas pressure to the burners as per the ID plate spec. on that unit, as well as incoming gas pressure to the gas valve...

Thanks tsa7man, The furnace is Nat gas from the street. Comes in on 3/4" feeds two furnaces. one to the house and one to the shop. it is the one to the shop giving the problem. It has a 1/2" line going 50'+- It has always worked to this point. I'm not sure where a press. Drop could be. Is the pressure adjustable at the valve some how?

How is the pilot height ?? It needs to be high enough to cover the flame rod and the back metal bracket of the pilot assembly ??

Thanks again; this seems like it could be a possablity; where as I just replaced the pilot. Thanks. I will check it. I mounted it in the same position as the old but noticed the flame is larger than the old pilot was. About 3/4' to 1"..