I was doing a PM on a Trane RTU , today...a Ycd060c1lobe..test sequence in heat..Inducer,ignitor, valve opens..combustion for about 2 seconds, and shutter..sometimes a pop..and flameout..propane supply good to valve..need to check ..g v pressure..changed the ignitor..my buddy was wrong....could the CBM be culpret if rpm not right?..My heat background not that strong

Jimbo, it sounds like you are aware of the operational sequence to suggest using a voltmeter to check the actual wires going TO the combustion air fan...if there is no voltage from the board, the board is bad. Just make sure one of the high temp limits are not open... one may be in the blower compartment on some and one will be near the top of the vestibule and a roll out switch should be near the burners. They should all be wired in series.

Always check the manual reset safty switches as the first step in troubleshooting.

More info is needed.
When you say it "Cycles", where does that fall into the following sequence....
1. Call for heat
2. Inducer starts(exhaust)
3. Pressure switch closes( connected to inducer via hose)
4. Ignitor glows or sparks
5. Gas valve opens and burners light
Does the ignitor glow or spark at all?
Do you see the exhaust working?
Sometimes the small hose that runs from the exhaust housing to the pressure switch will get clogged. Blow it out and don't forget the inlet and outlet.

When a furnace has a bad ignitor what I see most of the time is the following sequence of operation:

1. On a call for heat
2.The draft inducer motor starts.

3.The pressure switch which has a small plastic or rubber tube attached yo it senses the negative pressure produced by the draft inducer and closes.

4.The draft inducer runs for 30 seconds to a minute before you hear the gas making a hissing sound. The ignitor did not glow,therefore the flame sensor (a small metal probe about 1/8" in diameter, with a white porcelain base) does not sense the flame, so after 8 to 10 seconds the gas hissing sounds stops with no ignition of gas to heat your home. Your furnace shuts down and goes into a lock out condition until you turn your power switch back off and on again. Then the sequence starts all over again with no ignition of the gas. Solution:You probably need to purchase and install a new ignitor. I would suggest that you inspect your ignitor closely for cracks. Please Make sure you do not touch the ignitor with your bare hands. If you do not visually see a crack, then you could possibly have a furnace control board problem or a bad limit switch or rollout switch problem.

You're pressure switch is open. The sequence of operation starts with the inducer motor and it pressurizes the combustion chamber, and also closes the pressure switch before the burners will come on or before the ignitor will work. Check for a hole in the vent poly hose, you can remove it from the motor end and blow through it and this will most of the time free up the single contact and it will fire up. You can also by pass the switch with a jumper wire if it is bad or stuck and it will fire, Then you will know what to replace, or what the problem is. Very simple for you to do. Good Luck, and please rate my help to you, Thank you.
Shastalaker7
A/C, & Heating Contractor

check for power at gas valve.

Problem: Your furnace will not ignite the gas to produce heat for your home. When a furnace has a bad ignitor what I see most of the time is the following sequence of operation:

1. Thermostat calls for heat. 2. Draft inducer motor starts. 3. Pressure switch attached by a small plastic or rubber tube senses the negative pressure produced by the draft inducer and closes. 4. Draft inducer runs for 30 seconds to a minute before you hear a gas hissing sound. The ignitor did not glow, the flame sensor (a small metal probe about 1/8" in diameter, with a white porcelain base) does not sense the flame, so after 8 to 10 seconds the hissing sounds stops with no ignition of gas to heat your home. Your furnace shuts down and goes into a lock out condition until you turn your power switch back off and on again. Then the sequence starts all over again with no ignition of the gas. Solution:You probably need to purchase and install a new ignitor. I would suggest that you inspect your ignitor closely for cracks. Make sure you do not touch the ignitor with your bare hands. If you do not visually see a crack, then you could have a furnace control board problem or a limit, rollout switch problem. The furnace's control board might not be supplying the voltage to the ignitor. If your furnace lights and the gas stays on for 8 to 10 seconds, then shuts right back off, then you need to clean your flame sensor with light sand paper or steel wool. You might need a new flame sensor, but most of the time they can be cleaned an will work well after cleaning.

Problem: Your furnace will not ignite the gas to produce heat for your home. When a furnace has a bad ignitor what I see most of the time is the following sequence of operation:

1. Thermostat calls for heat. 2. Draft inducer motor starts. 3. Pressure switch attached by a small plastic or rubber tube senses the negative pressure produced by the draft inducer and closes. 4. Draft inducer runs for 30 seconds to a minute before you hear a gas hissing sound. The ignitor did not glow, the flame sensor (a small metal probe about 1/8" in diameter, with a white porcelain base) does not sense the flame, so after 8 to 10 seconds the hissing sounds stops with no ignition of gas to heat your home. Your furnace shuts down and goes into a lock out condition until you turn your power switch back off and on again. Then the sequence starts all over again with no ignition of the gas. Solution: You probably need to purchase and install a new ignitor. I would suggest that you inspect your ignitor closely for cracks. Make sure you do not touch the ignitor with your bare hands. If you do not visually see a crack, then you could have a furnace control board problem or a limit, rollout switch problem. The furnace's control board might not be supplying the voltage to the ignitor. If your furnace lights and the gas stays on for 8 to 10 seconds, then shuts right back off, then you need to clean your flame sensor with light sand paper or steel wool. You might need a new flame sensor, but most of the time they can be cleaned an will work well after cleaning.

check for 24vac between wires on gas valve about 5-10 seconds after ignitor starts to glow. If ignitor does not glow, check for 120 vac to ignitor after inducer goes on.
First...sequence of operation..power to W, inducer motor gets 120v and starts, 10-15 seconds later, ignitor gets 120v and glows, 5-10 seconds, gas valve gets 24v and opens, after 3-4 seconds the flame sensor approves flame and unit stays on. If this is not happening, it has to be in this sequence. Which ever is not happening and you do have voltage to that component, that component is bad. If you are getting a error code on board after it tries to light, pay attention to the code. if a 1 lock out code, that just means it tried 3 times and did not recognize a flame.

Sequence of operation, Gas Furnace.

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 ignitor. The ignitor will glow and you will be able to see it if viewed thru the small inspection port. Once the ignitor 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 ignitor does not energize (bad control, bad ignitor), the gas valve does not open or there is no gas (bad gas valve, broken wire, no gas), the pilot does not lite (dirty pilot), the burner does not lite (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).

Most newer furnaces will have a diagnostic center or control built into the control module. You may be able to view it as it is running thru a small viewing port. Some models will require the removal of an access panel prior to finding it. On the reverse of the access panel, there will be a diagnostic chart that will aide you in understanding any error code recorded (usually a blinking light).

This is why we technicians earn the big bucks!

Check at the gas valve to see if you are getting power to the terminals after you hear the click shortly before the ignitor stops. If no voltage ...replace the board.
Sequence is

induction fan on, if pressure switch is reading ok condition---->

Ignitor is powered for up to 20 seconds (depends on model) ----->

Gas valve is energized ----> if flame is present everything just works away, if no flame in 3 seconds sequence starts over for set number of tries or continuously (again depending on the model)

Hope this helps

if 23/27 volts is present at gas valve then replace valve but bad valve is not very common usually, its the board.