Wanted to know if anyone could help me troubleshoot this problem..
This furnace will not fire up, basically what happens is the igniter lights up, but will shut down after a 5 seconds or so. It will do this three times until it goes into lock down mode for three hours, and then flashes code 34. Please tell me if Im on the right path here.

1. Cleaned flame sensor with scotch brite. NO CHANGE
2. Check to see if I have .5 UAMPS between flame sensor harness and flame sensor. DO NOT HAVE ANY AMPERAGE READING ON THE METER WHEN FURNACE TRIES TO LIGHT?


I noticed that one or both(hard to tell) of the solenoids that control gas flow into the burner are clicking when the igniter is on, so I am assuming gas is going into the burner, although I don't hear any hissing of gas flowing? I also checked to confirm that I have power coming out of the transformer, and I have 28VAC. At this point I don't know if the gas valve is defective, or the computer board?

Thanks for any help guys. Fernando

Cheapest first test is replace the sensor. Sometimes cleaning does not get the job done.

This may sound silly, but... is the gas on? Has there been recent work to gas lines in the area? If there has been local utility work outside, you may have air in the line and it is causing the flame fault. If the main burner is not igniting, the burners may be clogged. The flame sensor can only prove flame if it's present, but the board will give the same "flame sensor" code regardless-especially if there's no ignition. Are you getting 24-26VAC at the gas valve when you hear the solenoid "click"? If so, the problem is in the gas valve, gas supply, or the burners. Assuming the burners are clear, this leaves us only the gas valve and gas supply. You can check the valve by unplugging the hot surface ignitor and cycling the furnace. Quickly run outside to the vent and see if the scent of gas is coming out the vent. If so, then it's not the valve and leaves us only the gas supply to consider-in which place you'll need a qualified tech to check the gas pressure. (*tip*... many local gas companies will adjust your pressure regulator at no charge)

Try this... put out your cigarette first... SHUT OFF THE GAS TO THE FURNACE AND EXTINGUISH ALL FLAMES IN THE ROOM (Yep, *especially* the pilot on the water heater). Crack the union leading to the furnace with 2 pipe wrenches and open it up a bit. Turn the gas back on and see if you have scent/gas. If you detect faint scent, let it bleed out for a bit-say 5-10 seconds until you get the strong nauseating scent of natural gas odorant. Once gas has displaced the air in the line, you will hear a faint change in pitch. Close and tighten the union with the gas flowing. LEAVE THE ROOM FOR 10 MINUTES so the gas can dissipate and come back and try to cycle the furnace again. Be sure to check the union for leaks when done using very soapy warm water as a bubbles test.

If air in your lines is the culprit, it may take some bleeding to get it all out. If after a couple of bleeding attempts you gain no ground, you'll need to get a tech in to look at it, as it may be gas pressure related and require adjustment of the house/appliance regulator (a common problem in our area, especially in times of high gas usage).

Hope this helps.

NOTE: Cleaning the flame sensor with "00" or "double ought" steel wool is much more effective than scotch brite, but as HVAC1000 has already pointed out, sometimes cleaning just doesn't do it-especially if the porcelain is cracked. Also, the .5 uAmps is generated only during main burner operation. The flame rectifies the neutral AC current to a DC current which the circuit board interprets as proof of flame as this is the ONLY DC current involved in the operation of the system.

I am having a similar problem. In my case the pilot lights, stays on a while and then the main burner lights and stays on about 30 seconds or so and then everything shuts down and the 34 error flashes. This happens several times in a row and then it will usually start working normally again for a few months and start again. This has happened 4 or 5 times over the past two years. Since it always starts working I have not done anything until yesterday when I thought I'd better because if it doesn't start working it could get pretty cold.

I took out the piliot igniter assembly and cleaned it as instructed. It didn't look dirty at all buy I steel wooled it and put it back in and on the first attempt the problem happened again and then after that it has been working fine for the past day. I was going to go out and get a new pilot assembly but then read your post again regarding air in the lines and thought this could be the problem. Is it possible from time to time that the gas company may get air in the lines that would then reach my house and cause the intermittant shutdowns? Could the gas meter/regulator have a problem? As far as I know the meter hasn't been serviced for over 20 years. Thanks for your help.

[You probably already heard this but if Gas control is clicking then either voltage ot available current not adequate. Increase thermopile capacity, ie: bigger pile, more output.QUOTE=;][/QUOTE]

It sounds like you don't have any gas going to the burner manifold. A possibility could be "A" you have an obstruction in the orfice closest to you ignitor or "B" the ignitor isn't sitting in the proper place when the gas valve opens.These are not likely but could be things to look out for. More then likely you have a gas issue, do you have any other appliances that use gas? Check to see how the flame looks. If you don't have any gas going to any of your other appliances then more then likely you have a regulator issue and you should contact your energy company. If you have gas going to your other appliances, check to see if the gas "shut off" closest to your furnace is in the open position.

I tried everyone's suggestion when I was having the same problem. The reason why it was shutting down or not igniting was from the limited amount of air being taken in. I live in Pa where we had a big stink bug problem and they advised to put a screen on the furnace intake and exhaust so they could not get into the furnace. The screen I had on there froze after a big snow and ice storm and there was next to no air being taken in. I pulled the screen off, you could tell the flow of air had greatly increased and by the time I walked back into my house, the furnace was ignited. Hope this helps!

I had the same type problem, code 34, then into a code 12 on the board, I cleaned the sensor rod, didn't work, it ended up being the igniter module, a very easy fix, and quite common with propane gas. Carrier model 9200