Number 3 cylinder miss firing - 2004 GMC Sierra

Did you change the spark plug? Do that first or swap plugs with number 3 and see if the miss moves if not it's time for you to perform a Compression Test and Leak Down test to see if it's a bad valve or piston rings. I hope this helps, Take Care.

Firing order is correct, (1, 3, 4, 2). Cylinders are numbered as 1, 2, 3, 4. Number One cylinder is nearest the belts and pulleys and number 4 is nearest the transmission/gearbox end. Stand in front of the car and face the rear of the vehicle. Cylinder 1 is to your left and Cylinder 4 is to your right.

check the ignition coil for that cylinder

Is there a code stored in the PCM isolating #3 cylinder? When was the last time the spark plugs were inspected or replaced? How many miles are on your vehicle?

Firing order is 1, 5, 3, 6, 2, 4. Number 1 cylinder is up front. Why would you need the firing order, this engine does not have plug wires, each plug has its own coil pack, each coil pack is controlled by the computer.

CYLINDER 3 MISFIRING. MEANS NOT FIRING .CYLINDER 3 IS MISSING FIRE.COULD BE FAULTY OR DIRTY FUEL INJECTOR.YOU COULD HAVE BAD SPARK PLUG OR COIL AND WIRE.CLEAN FUEL INJECTORS CHANGE PLUGS AND COIL TO NUMBER 3 CYLINDER .WHEN YOU DO A LITTLE TUNE UP.THAT WAY YOU WILL KNOW IF YOU HAVE A MECHANICAL PROBLEM TO 3 CYLINDER LIKE LOW COMPRESSION.LEAKING INTAKE AT 3 CYLINDER OR BURNED OUT VALVE.

first code means random/multiple misfire cylinder (s) detected
second code means misfire detected number 1 cylinder, third code, miss=fire number 3 cylinder fourth code misfire number 4 cylinder
causes can be spark plugs, HT leads, ignition coils, injector, wiring fault, low compression/ engine mechanical fault

Number 3 is the 3rd plug back on the passenger side. Coils are notorious for going bad, I would recommend replacing the spark plugs, while you are there. Use Auto Lite, or Motorcraft Platinum Spark plugs only.

check your firing sequence.

firing order is 1-3-4-2, cylinder number 1 is closest to the timing belt