unit ignites 3 times shuts off with 7 blinking led code. Fame sensor replaced less then 1 year ago. cleaned flame sensor unit with pencil eraser (doesn't damage like steel wool or sandpaper) runs fine for two three days then back to error. no blockages in intake air/ vent air. had service tech out 6 months ago and found no issue with combustion air, gas pressure.
CO2 % 6.3.
O2% 9.8
CO PPM 21 burners and ignitor checked cleaned and all in great condition. Pressure switch?
nothing shows what the voltage or ohm should be for the pressure switch located in upper left hand corner.

Most pressure switches close once the venter starts. This being the case, you should read 24v across the switch with the hose disconnected & zero with the hose connected & venter running.

Since the furnace fires then drops out, it would seem you have a flame sensing problem. Use very fine steel wool or I prefer a scotch brite pad to clean the flame rod. Also check all connections, especially grounds, to insure they are clean & tight.