This is a fan falure code. It can mean the fan has problems (not likley if the fan goes and isn't labored) You have a sensor in your exaust (probally not this if you don't get a good start) The culperet is probally in the negetive pressure sensor. This is a 2-3 inch disc with wires going to the controls and a rubber tube going to the fan. Pull the tube from the fan and make sure the fan fitting is clear (poke with a tooth pic) Be sure their are no kinks in the hose. If the heater is 4 years old the diaprahm could be bad in the pressure sensor. Check the other things first but this is probally your problem. Good luck, hope this helped.

You can check the air flow switch by jumping the wires also look on the back of the control board , if the air flow sensor does not close when fan come on it will set a red LED on the back of the board,
a more likley problem is ,I find with 3 yr old maser temps,
due to the fact that there is no air filter, the diffuser cone down in the combustion chamber becomes clogged and restricts air flow which cuases the air flow switch not to ope,
Also rodent love to get down in side the blower housing and nest in there,

Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/kevin_7b506228149c3829