Been sitting for a while, filled with fuel, turn on runs for 5-10 secs then shuts off and light blinks ?? model 3ve51

You need to pull a little maintenance on this unit. Locate and remove the burner tray, on the tray you will find a flame sensor, this detects flame and permits the unit to run. These collect carbon deposits, dirt and the occasional insect seeking shelter. So clean it with windex and a soft terry cloth rag. Next is the fuel nozzle, find it , remove it, and they say you can not clean these but I say q-tips, tooth pick and carb cleaner will do the trick just be careful not to damage the nozzle. Directly behind the nozzle is a screen these like to corrode durring storage, so clean it good too. Put assembly back together and inspect all fuel lines for holes, leaks,or dry rotten conditions. Now inspect fuel lines to the filter, if an in line filter (small plastic filter) clean with compressed air and blow gun, blow air backwards thru the filter instantly cleaned. If a large oil type filter replace it with a new one and do not forget to fill the filter before putting it on.Make sure that your fuel is clean and no water has developed in it from condensation, if water is present drain fuel and clean out the tank put new fuel in and it should now work for you. Happy heating