I have a Rheem RGDA-100A-AR gas furnace, and I believe the pilot light is out and needs to be re-lit, but I'm not sure where it is. Any ideas where it might be or what it might look like? And do I have to light it with a match? The instructions on the inside of the cover just say to "light it" while depressing the manual gas switch in the Pilot position.

That's the next step. Make sure the flame sensor on the pilot assembly is in the blue part of the pilot flame. If it isn't the gas valve will not open the main burner.

By the gas burner near the pilot .

Fix Furnace Flame Sensor (Flame That Goes Off) - YouTube

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https://www.youtube.com/watch?v=fBJ3r9MUa-U
Nov 29, 2011 - Uploaded by RochNoure
A Furnace Flame Sensor is a safety device that reads the temperature and ... easy fix, but the furnace is ...

Troubleshoot the ignitor of the Rheem RGDA model gas ...

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https://www.youtube.com/watch?v=Bx1pXONg2Oc
Dec 12, 2012 - Uploaded by grayfurnaceman
Troubleshoot the ignitor of the Rheem RGDA model gas furnace.... Jun Sim 5 months ago ... My lennox ...Missing: lx2000

Rheem Criterion II No Heat - YouTube

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https://www.youtube.com/watch?v=yOSk6qvpWbA
Jan 7, 2010 - Uploaded by Work12Progress
Rheem Criterion II gas furnace model # RGPH-07EAMGR. When a call for heat is made t wo green led's ...Missing: lx2000

Heater will not kick on unless... [Archive] - Self Help Forums

www.selfhelpforums.com/archive/index.php/t-2153.html

Oct 19, 2004 - 25 posts - ‎9 authors
After a certain amount of time of not running, the heater will refuse to kick back on. ... 2 flashes, then a 2 second pause = pressure switch is open. What do .... On LP gas furnaces, the spark starts and the gas valve opens. ... Buffing them up with a little steel wool (5 minute job) is a good annual PM thing to do.

Not sure but its possible your thermo coupler is shot and needs replacing. If you have a pilot light it could be out but the furnace should not start if the pilot is out as there should be no oower getting to the controls unless the pilot is trying to let it start. A bad thermocoupler will not let the flame start.

it could be a bad flame sensor. if the control board doesn't sense a flame it will shut down the unit

Make sure the pilot light is lit. if the pilot light is not lit I believe there is a safety switch that stops the flow of gas to prevent filling your home with it.

If I recall correctly, model number Rheem furnace you listed has electronic ignition to light the pilot, when the home's thermostat calls for heat. Is this what you're having a problem with?

Most likely the thermocouple which goes from the gas valve to just above the pilot light flame is bad. You can get these from any furnace supply or Lowes/ Home Depot normally. If you can depress the gas valve button and get the pilot light to light, and after holding the button in for about a minute you see it go out, then it is definitely the thermocouple.

if you are by passing stat at the gas valve and it is still not working,i would assume a bad control valve.

If the burners do not fully ignite, the sensor will shut the furnace down. The lack of sufficient burner flame is a gas valve or gas pressure problem within the valve.
Replacing the gas valve with the original one may get you going. I surely suggest a professional service company for safety reasons and a thorough inspection. A 20 year old furnace is past it's normal life span.