My pilot is lit.the gas is on and my thermostat is fine,but the furnace will not ignite. Thanks

check thermostat is on and working correctly and check wiring at unit

Is it natural gas or propane? If propane, make sure you are not out of gas. Does it have electronic ignition or a manual pilot? if pilot, make sure the pilot flame is still lit and the control knob is turned to the "On" position. If it has electronic ignition, shut off the power to the furnace and then turn it back on and see if resets and lights.

Bad igniter? When you 1st try it the gas valve will give enough gas that you would get the burner lit if the igniter was glowing red. If you're desperate you can light the igniter area w/ an aim n flame, but you definitely want to do that w/ caution and crank up your t-stat so the furnace stays on awhile Cuz you'll have to do it again for heat or buy an igniter($40 or so)

I say jumper the thermostat red and white wires. Sounds like the thermostat failed.

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?

Need furnace model number. It sounds like you're looking for a standing pilot. Your furnace might not have one. Very likely to have a spark ignition type system or hot surface igniter.

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

The steps involved can vary slightly depending on your furnace model and pilot valve type, so if possible try and find the instructions inside the furnace door cover or in the instruction manual. This is for a gas fired furnace, but applies to any gas-fueled appliance including water heater. Two of the most common types of pilot valve body assemblies either have a red reset button and a gas valve or no reset button and a valve knob that can be depressed. But whichever type you've got, if you have an older model furnace with a standing pilot (flame is lit all the time) then this is the basic procedure.

• Turn you thermostat to 80 degrees or to a setting that will demand heat. Make sure the thermostat is in "Heat" mode.
• Go to the furnace and find the pilot valve. The pilot valve body is a box shaped device into which the main gas line will run and is located near the gas burners. It will usually have a gas **** or valve knob that reads "On, "Pilot" and "Off." Locate this gas **** or knob.
• Turn the knob or gas **** to the "Off" position and wait about 3 minutes for any residual gas to clear away.
• Find the pilot. It's located near the gas burner tube assembly in the furnace.
• Get your match or lighter ready. Sometimes the pilot is hard to reach. If possible, try and use a long fireplace match when you light it. If you don't have a fireplace match then a butane BBQ grill lighter works well too. If you don't have that, then you can fasten a match to the end of a stick when you light the pilot

• If you have a pilot with a red Reset button, turn the gas valve from "Off," to "Pilot."
• If you have a pilot with no red button, turn the knob from "Off" to "Pilot."
• Place the lit match or ignited lighter tip at the pilot and depress and hold the button or depress the knob as appropriate. Depress the button or knob for about 30 seconds. This maintains gas flow to the pilot until the thermocouple gets hot enough to open the main gas valve.
• Once the pilot stays lit, slowly release the button or knob and turn the gas **** or knob from the "Pilot" position to the "On" position. This will ignite the burners and keep the flow of gas supplied as required for the burners as called for by the thermostat.
• If the burners fail to ignite then it may be because the thermocouple did not get hot enough to open the gas valve. Wait a few minutes and then repeat the above procedure. This time hold the red reset button or depress the knob for about 45-60 seconds.
• Once the main furnace burners ignite, adjust the thermostat to the desired setting.
• If you've followed the previous steps on lighting the pilot and it still does not light after, or will not stay lit, then you probably have a problem with the thermocouple or an adjustment needs to be made to the pilot. You'll need a furnace service technician to replace or repair these parts. You may find that the pilot lights, but it is an anemic looking flame. That's a different problem we'll look at in the next section.
  
• If the pilot lights but the flame is a weak yellow flame, it will not get hot enough to heat the thermocouple to its set point allowing the gas valve to open.
  A natural gas flame should be a bright blue with the tip of the flame having just a tinge of yellow. A propane flame should have a bluish green flame with a tinge of yellow at the tip. The flame should be strong enough to hit the thermocouple tip about 1/2 inch from the tip end. If the flame is weak or shaky looking, check to see that a breeze or draft is not blowing on it. Adjusting the Flame
  There is usually a small screw on the pilot valve body that will adjust the flame. You may have to refer to the manufacturer's instructions to find the screw. Turn the screw as needed to adjust the flame throw. Yellow Flame
  A yellow flame is caused by lack of air and incomplete combustion. It can be caused by a dirty pilot tube tip. Split Flame
  This is caused by dirt in the pilot tube. Take a needle or small nail and gently clean the tube. Flickering or Wavering Flame
  A flame that flickers is usually caused by a draft.
  

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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.

sounds like the sensing bar on the spark ignition is dirty.....a wire brush works nicely.