Only a very small flame appears after igniting. Then the rest doesn't light. Or the back burner. S'up?

Check to see gas pressure, also clean all burner

I'm not sure where the flame sensor is on that one, but the problem is the flame sensor is dirty. Also called the flame rod, flame proving rod, flame proving sensor. Should be a small rod that the flame from the main burner touches. Clean it with some sandpaper or replace it

The clicking sound is from the ignition, but you do not have gas at the burner, I think it could be thermocouple.

you will notice small holes around your burner. Look at the burners holes next to your spark igniter. A paper clip will fit perfectly into the small hole to clean them. This is very common, boil overs clog these holes with debris and does not allow gas flow to ignite properly.
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Hello,
Your right side burners probably have small vertical lighting holes that are clogged where the pilot lights the burner. Use a sewing needle to poke each hole clean. While you're at it, poke clean the horizontal burner holes just to make sure they are not clogged either. Are there also little tunnels from the burners to each pilot flame? Make sure those are clean inside and attached properly on each end. The pilot flame should be approx 3/8" tall. Compare the working pilot flame to the non-working pilot flames and adjust as necessary. Each pilot has a small aluminum tubing running to it, the adjustment screw will be on this tube. You'll just need a small flat head screw driver to turn it counter-clockwise to increase flame. Hope this helps. Let us know if we can be of further assistance. Douglas

It sounds like it needs a good clean!

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

Between the burners is a small rail in the pressed steel where the flame runs across to light the other burners and it sounds like there is dirt or debris in the step over rails causing the problem. You will need to use some compressed air on those rails to clear them out. Once the rails are clear the other two burners should ignite right away.

The way it lights is that the gas is spread from one burner to the next by either a flame tray or small holes drilled in the burner tubes. If the flame does not get all the way across the burner tubes within a few seconds, the unit goes out on failure. Clean and inspect each burner tube, not just the orifice. Clean any tray that connects the burners. You can use a small wire brush or tooth brush but be careful near the glow igniter as it is very fragile.

If you have ayellowish powder it is most likely the refridgerant and does not always smell like ammonia. You should pull the unit out and do a through inspection of the cooling coil and boiler