I have an Armstrong Model UHPA090AE1 hanging unit in garage. The unit has a spark ignition and the unit fire the flame last for 25-30 seconds than goes out and re ignites. I checked the High limit its good and the flame sensor looks brand new and was replaced last year. I did clean the flame sensor with steel wool but unit still does the same. Any ideas?

Make sure flame sensor is lined up with burnner

It could be possible the one of the roll out limit sensor's are malfunctioning. They are the one's surrounding the outside of the heat exchanger with one or two wires going to each. Yours would have to automatically reset otherwise you can eliminate them as the culprit simply because manual reset types are just as described, you have to physically push a button to reset them. Hope this helps and good luck, gary hazher

My apology I should have proofed my reply before sending it. They are the sensors either dime size or fifty cent size around the manifold. What they do is if the manifold has received too much fuel and it has ignited they will shut the furnace off until you reset them by pushing a small button located on the back where the wire(s) connect. Like I said before if there not automatic you can eliminate them.