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Trigger valve most likely problem. You could also just replace the two orings on the trigger stem within the triger valve. They are very small (2mm). There are two or three other orings inside the gun that need to be replaced as well. The yellow vinyl orings used by dewalt do not last as long as the black rubber.
Nailers that use strips of nails (one nail directly behind the other) will sometimes drive two nails at once and is caused by damage to the driver. The nails are so close together that the driver will hit the first and part of the second nail head. Coil nailers, as your an451, use nails seperated by welded wire making it very difficult the the driver to hit the second nail. Usually when a coil nailer shoots two nails, it is acutally the trigger valve that acutates twice (two very rapid fires). Two very rapid fires is caused by faulty trigger valve. I have seen a roofing coil nailer fire one nail and a second nail drops out of the gun. Caused by weak / malfunction of nail feeder or wire used to weld nails together is too thick.
Hi... mine had the same problem when I bought it cheap second-hand. I made a spacer out of a washer that slips over the gas stem adapter and so far 100% fire rate. It holds the adapter closer to the cartridge nozzle and opens the valve that releases the gas when the trigger is pulled. Too easy! Good luck.
Orings /other parts tend to stick when nailer is new. Dewalt uses some vinyl orings that have been causing premature failure. Be sure to add a few drops of oil when used. Too much oil is not good. Wrong oil will cause orings to swell and cause problems such as yours. Your nailer's firing valve can be reset by rapping the nailer against rubber surface such as a thick rugger mat or coiled rubber air hose. Remove airline and nails, being carefull not to damage the head, rap had against rubber mat and retest gun. If it did not reset rap against nose of gun and test. If no joy, you will most likely need to remove head and manually move cylinder up/down and /or check vinyl orings for failure. Before you try to reset the gun, make sure that the safety is pushing the firing pin all the way in. Air removed, look under the trigger as you push down on safey, if it does go all the way in try bump-firing the gun (for test have nails removed). Hold in trigger and quicky /firmly strike gun against rubber pad. Good luck
First look at the end of the rod which pushes the nail. Is it bent or worn?
If not, look at the guide for the rod. Is it worn?
If both pieces above are good then the top or bottom of your jaws are worn out. You should replace both of them. The cost depends on your make and model. Some are cheap to replace, others get pricey.
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