Furnace burners stay on for 20 seconds then shut off
My furnace will cycle on and the burners will stay lit for around 20-25 seconds then shut off. It will cycle back on and repeat about a minute later. Ive removed the flame sensor and cleaned it, cleaned the ground attached to the burner chassis, and the exhaust flue goes into my chimney which has a grate covering it so I dont think a bird got into it or anything. Any ideas before I throw in the towel and call a tech? I dont have a multimeter to test anything.
Also, something interesting that happens is if the furnace has been off for a bit the burners will stay lit for around 20-25 seconds. But when the furnace starts cycling back over the burners will only stay lit for 10-13 seconds.
Edit: Its fixed. Per recommendations below I removed the pressure switch tube and cleared it out as well as cleaned out the nipple that it goes into. Both were full of dirt.