It could be a bad board or the capacitor or something else. Me, I'd try to take advantage of the trouble code system and see what it is reporting before throwing more money at it.
Can you get the trouble codes and report back? Otherwise, the next least expensive part would be the capacitor.

 

The furnace worked fine for most of the afternoon, but began blowing cold air again about half an hour ago. The light blinks in fast succession-according to the manual it means normal call for heat. However, when I used a voltmeter between C and the other leads, R and W registered at around 26, but G and Y were only 1.0. I followed the wires they were connected to, and both go into the far reaches of my attic. Would the problem be at the circuit board or at the other end?

 

With the stat calling for heat and the burner door off the furnace describe in detail the sequence of events that happens.

 

OK-once the tstatclicks calling for heat, the draft blower comes on, the ignitor glows red hot, then the hsi dims and the blower motor comes on. The hsi glows again two more times before it goes into lockout mode. The only difference in the sequences when the furnace is producing heat and not is that the burners don't fire-not even a whiff of gas. Everything else is the same, at least visually. I don't hear any strange noises either--the only sound missing is the whoosh of the burners.

 

do you have a volt meter and know how to use it?

 

If your HSI glows and the gas does not come on then I'd be checking the gas valve at that time for 24V. If you don't have 24V at the gas valve, we need to check elsewhere but if you do then your gas valve is suspect.

The fault code when the furnace goes into lockout would help!.

 

When I test the gas valve, where do I place the two leads?

 

You pull the two wires off of valve and test those wires. Just after the HSI glows there should be 24-ish volts sent to the gas valve. If it is receiving 24 volts is the question.

 

I tested the only two wires going into the valve assembly, white and red. The voltmeter registered +/- 37. Does that sound too high? The other wires I tested registered 24ish. I guess I need a new valve?

Now the diagnostic is blinking twice, indicating lockout mode.

THANK YOU ALL!!!

 

Test amperage on the ignitor before you go to jumping too conclusions. Pretty much you want to make sure that it is pulling amps up untill the gas valve is being sent voltage and not cutting off early.

I don't understand how you can get 37 volts when everything else is 24.

 

hey Doc
I understand the amp draw check on the igniter but what stops a valve from opening with a complete 24V circuit for as long as it is powered.. (no idea about the 37V though). Are you thinking that the valve is actually open and spewing gas, not igniting because of a faulty sequence or igniter, and the op is not noticing this?

 

If the ignitor quits before voltage is sent to the gas valve then it could be faulty such as a crack invisible to the naked eye. You can ohm out an HSI and it can show continuity but with a minute hairline fracture as the ignitor gets hotter that crack expands causing the ignitor to prematurely fail (even though voltage is still being applied to the ignitor) and thus when the gas valve is finally sent voltage and of course assuming gas pressure is correct and gas is flowing, the ignitor will not ignite the gas, no flames.

the ignitor has to be pulling amps until after the gas valve opens

 

I checked again...37v, checked multiple times. Furnace is running again now. I also checked the ignitor by unplugging it and putting the leads in the plugs. VM registered 122. When I switched the leads, it didn't register anything. Again, I did it a few times with same results. I followed the ignitor wires to test them at the control board, but I can't get to them. I smell NO gas except right before the burners really do fire, and I have a VERY sensitive nose. If there is gas, I'm smelling it.

 

I checked again...37v, checked multiple times. Furnace is running again now. I also checked the ignitor by unplugging it and putting the leads in the plugs. VM registered 122. When I switched the leads, it didn't register anything. Again, I did it a few times with same results. I followed the ignitor wires to test them at the control board, but I can't get to them. I smell NO gas except right before the burners really do fire, and I have a VERY sensitive nose. If there is gas, I'm smelling it.

You didn't check the ignitor, you checked to see that the control board was sending voltage to the ignitor and yes, it is. Connect your leads to the ignitor leads and OHM it out or test continuity. Then with it plugged back in test amp draw of the ignitor as it's working/glowing. You may need a meter with a clamp that fits around the wire for amp testing. That is how you test the ignitor itself but if it's working then I guess you don't really need to at the moment.

 

I have a Trane TUE080A936K3, and is original to the house, which was built in 2000. I've lived in the house since Dec. 2009, and until this year, it has worked great. Now, it will work perfectly for a while, producing heat, fan blowing, then the burners cease to fire and the fan blows only cold air into the house. When this first started happening in late October I could turn it off for a while at the T-stat, and it would work again for a couple of nights. The amount of time has decreased now, and it produces heat for only a few hours, and will only begin working again once I've turned the circuit breaker off and on. It will also work when I reset it on the actual unit itself. I've replaced the thermostat (Honeywell) which has brand-new, high quality batteries, the ignitor switch, and the flame sensor. The LED light blinks a steady, even pattern, although I've not been up in the attic when it's acting up to look at the light pattern. When the burners do fire, they produce a steady blue flame, so I don't think it's the heat exchanger. I also had a technician come and clean the unit as part of his maintenance agreement. He checked the high limit switch, adjusted the pressure switch, checked for gas leaks, and cleaned the condensate drain. He said that all looked fine (of course, the furnace worked perfectly while he was there). He suggested that I would need a new capacitor soon, but that the present one was fine. I have a carbon monoxide detector in the house that has never been triggered. Could this be a faulty control board?

Bunny, when the technician was there do you know if he cleaned and or read the reading from the flame sensor?

 

harleyrider

The technician just mentioned cleaning the flame sensor, nothing else. I was the one who replaced it. Not exactly brain surgery. Are you able to see the other posts I made?