Goodman gms80903bna light blinks once fan runs all the time,

It could be anything - did you read where it can be igniter -flame sensor - fan motor - clogged nozzle -fuel delivery will need to test and check these components not just change them great you want to diy it but it it's done wrong or you don't follow proper safety can cause injury or worst so if not comfortable call a licensed service

Youcould be loking at the pressure switch try jumping it out and see if it lights some times you can fool it in to working , check the hoses are tight and nor spilt or take a drill bit and just clean out the 2 ports where the hoses fit some times they get blocked , also check you air filter is clean and your fan

adjust the gas pressure on the gas valve or replace the gas valve

take front panel off where light is and look inside there will be a chart telling you whats wrong

3 blinks is fault with the pressure switch not working try jumping it out some times you can fool it in to working

That blinking 3 times is indicating a pressure switch problem. Try checking for water or a blockage in any of the hoses going to the pressure switch or condensate drain. Check the air intake and exhaust pipe for any blockage including any saggs that may hold water. Make sure condensate drain or pump is working. If you get a glow from the igniter, there may be a blockage in the gas valve or piping.

check out the pressure switch........... i take it glows red gas valve clicks then goes off,

your system has a diagnostic light that blinks a code if there is a problem. look at the lower section of the heater, there should be a small viewing port looking into the cabinet. you may have to remove a cover but if you remove the wrong one, it cuts power to the unit and erases the error code. once you get the code, look on the wiring diagram for a diagnostic chart. this will give you a starting point where the problem is.

Turn the heat up and observe the flash code....there's a legend in the furnace,or in the blower compartment that tells you what the flash sequence means...or in the Manual...........The flash code is likely for the Pressure switch...this part can either be defective or your Vent or Air intake could be blocked with Leaves,rodents,kids rocks etc...check the vent outside and look for obstructions and see that the air coming out the exhaust seems to be normal...........find the pressure switch which is round and has usually a grey small hose coming from the pressure switch and going to the furnace body or the exhaust fan....pull the hose off the opposite end (not the pressure switch...with the power off)...and slightly **** and blow repeatedly...(Really) and you will here the switch clicking....do this several times at least...make sure you're activating the switch by hearing the clicking.....Turn the furnace back on and see if it works...if it does you need a new Pressure switch...if it doesn't your vent could still be blocked or the switch could still be defective

Most all of them furnaces trouble shooting sheets located with the wiring diagram . Count the number of time it blinks and look at it on the guide it will tell you whats wrong