My problem with my Lennox furnace (model G23Q-3/4-100-2) is blowing cool air, and only occasionally heat was diagnosed as needing a Universal Pilot Module, part #53L9001 also known as 53L90.

Question, if we purchase the part how difficult is the installation?

Just change wire for wire. If the control has changed there will be a diagram to follow. Truthfully you are better to have a company do this. If you can buy the part Lennox will not warranty it because you are not authorized to do this kind of service.

I have a Lennox furnace with a Johnston Control ignition control G600ax-3 which I am unable to locate to replace. I ordered a universal White Roberts 50D50-843. The G600ax-3 has a spark igniter connection similar to a spark plug connection and the WR is a spade connector, do I have to replace the igniter or can I replace the existing connector with a spade connector or is it recommended to change the igniter assembly? Also is there a wiring diagram available for the Johnston G600ax-3?

Your instructions are below. I suggest that if you do not know what you are doing STOP and find someone local that can wire it for you. There is a severe liability that follows the changing of these type of SAFETY controls. I suggest you contact White Rodgers in regards to there selection of ignitor and pay close attention about the cable length and stand off requirements. Whatever you do follow the instructions especially the setup and key selection etc. Your old Johnson control wiring has nothing to do with the new White Rodger unit and the old control wiring should not be used as a reference.

http://www.white-rodgers.com/wrdhom/.../0037-6576.pdf