As title states: furnace lights properly but as soon as the blower kicks on, the furnace turns off. Getting 3 flashes. (Pressure switch open). checked pressure switch no problem. Checks the rollout not tripped. However, I think I've diagnosed the problem but could use some insight.

I took the panel off and it seemed to run better. My thought is better airflow. What I believe corroborates my story is I checked the exhaust outside and it goes back into the intake (very bad I know.). I am not the installer. I'll be moving the exhaust tomorrow so it doesn't flow back into the intake. Just wanted some opinions here: do you think this could be the problem?

Thank you!.

Question from user zacharyo083194 at hvacadvice at reddit.com.

Answer:

80% furnace? 90% furnace?

Answer from user KAG_69 at hvacadvice at reddit.com.

goodman furnace goes out when blower kicks on