Furnace runs through normal start up process and I get no fire & 3 blinking LEDs Pres. Switch code; cleaned the flue, reset unit, went through start up process lights runs for approx 15 sec.; shuts off sends Flame sensor code warning replaced sensor unit ran for 15 sec and burner shut off again circulator fan runs no heat. Continued pres switch code.. If control board is bad would unit run start process? It's sending conflicting codea that were remedied... should the control board be replaced?

It's unusual to have so many codes/symptoms at once, unless the circuit board is malfunctioning. It's likely, but somewhat of a gamble without a proper checkout.

First thing I would do is clean the flame senser, then see what codes you get. Let us know what happens after cleaning flame senser, and we will take the next step

Replaced flame sensor; unit now consistently sending 3 flashes pressure switch error & the circulator runs with no ignition... should I replace the pressure switch? Would I be getting ANY trouble codes or heater functionality at all if the controlboard is bad?

replaced flame sensor; unit now consistently sending 3 flashes pressure switch error & the circulator runs with no ignition... should I replace the pressure switch? Would I be getting ANY trouble codes or heater functionality at all if the controlboard is bad?You may, depending on what is wrong with the board. Most do not assume error on their own part for diagnostics, and display codes based on input from sensors, action-switches, etc. The logic circuitry of the board may be fine, but a burned relay or resistor will impact the ability of the board to manage the other components of the furnace.

Three blinks is pressure switch stuck open. I wouldn't replace any pressure switches just yet. Check to make sure combustion and exhaust vents are not plugged or possibly froze up outside. Clean pressure switch hoses and ports. Clean drain tubes. You can bypass the pressure switch by jumping the two wires that hook to it, but you need to wait until the draft inducer starts running, then jump them. If you jump them before the draft inducer starts running, it will give a code saying pressure switch stuck closed.

Jumped out the pressure switch and the unit runs... so what is this telling me? Am I OK bypassing this safety feature?

Only for trouble shooting. Do not leave switch bypassed under any circumstance. It may be time to have in a pro to look at system. They have all the tools and equiptment to fix this quickly and correctly.

Will do Thanks, for your help!

Let us know what they find, and if you can, be around to have them show you what they do to solve the problem. If it's a simple fix that doesn't require replacing any parts, you will be able to do it yourself if it happens again