P0304 is a relatively common GMC Sierra trouble code. It stands for:
P0304: Misfire Detected → Cylinder 4
P0304 is certainly a cause for concern, and should be considered a threat to the drivability your Sierra. The nice thing about P0304 is that it’s a cylinder specific trouble code, which makes diagnosing the problems simpler than P0300, which means that the cylinders are randomly misfiring.
Fixing P0304 should be considered a high priority. Unlike a lot of the OBDII codes, this code has to do directly with engine combustion. It also can cost money to ignore it, since driving with a misfire can damage the vehicles catalytic converter. The engine is also not firing all of the fuel, so raw fuel is also passing through Cylinder 4 into the exhaust. Mileage may suffer as well when your Sierra has P0304 as well.
GMC Sierra P0304 Definition
P0304 is a cylinder specific misfire code, which means that Cylinder 4 is misfiring and causing the code. An important thing to understand when finding the location of Cylinder 4 is that it’s going to be the 4’th cylinder in the firing order of the vehicle, and not the 4’th cylinder that you might see when looking at the engine.
Sierra P0304 OBDII Code Symptoms
- The P0304 trouble code will trigger the Sierra’s service engine soon light.
- The vehicle itself may begin to run rough. It depends on how serious the misfire is.
- The vehicle may produce excess vibration, especially at lower RPM.
- Fuel mileage may suffer.
- You may smell raw gas coming from the tail pipe.
- The vehicle may backfire.
P0304 Trouble Code Diagnosis- GMC Sierra
There are quite a few things that can cause the P0304 trouble code to trigger the GMC Sierra. Here are the most common problems that will throw the code. They are presented somewhat in order from most to least likely to be causing the code:
- Bad Spark Plugs– Spark plugs are one of the most common causes of P0304. Take a look at the electrodes and see if they are in good shape. Most vehicles now come with iridium plugs that need changed very infrequently. That being said, the plugs are a great place to start. Here’s a great video on how to see if a spark plug is bad.
- Spark Plug Wires– On most modern engines, the plug wires are not nearly as long as they once were, but they can still go bad. Here’s how to tell if your plug wires are bad (video).
- Coil Packs– Coil packs rarely go bad, but when they do, they can certainly cause P0304 in your GMC Sierra. Replacing a set can be very expensive. Here’s how to test them.
- Bad Fuel Injector– If you have a fuel injector that has gone bad, it won’t be able to properly atomize the fuel and you’ll get the P0304. Here’s a good video on how to diagnose an injector, it can be a little tricky. This is definitely not the place to start.
- Vacuum leak– If your Sierra has a vacuum leak, it can be very difficult for it to get the right air/fuel mixture. This will cause the cylinders to misfire and it’ll throw the P0304 if the leak is around that specific cylinder on the intake manifold. It’s easy (and kind of fun) to chase one down. Popular Mechanics: How to find a vacuum leak.
- Cam or Crank Sensors– This one is very unlikely, but it does happen. If the ECU is not getting the right signal from these sensors, the vehicles timing is not going to sync up and it’ll misfire.
- Low Compression– If you have a leaking head gasket, bent valve, cracked head, etc.. that would cause compression to not be as high as it should, you’re going to get P0304. You should also feel the vehicle is down on power as well.
Most Common P0304 Fixes
If you follow the items listed above you should be able to fix the P0304. Here’s how to go about it:
Inspect and test the parts of the ignition system around the cylinder to determine whether or not you need to replace the coil packs, plugs, or wires. Most of the time one of those three things will end up being the problem.
You need to reset the trouble codes and swap the coil pack, wire, and plug from Cylinder 4 to another. If you get a different P030X code, you’ll know that one of those three components is bad, since by moving those components you have changed which cylinder the trouble code has registered in. After that, Go ahead and replace them or have them tested. It’s an easy, and cheap method of determining if your ignition components have gone bad.
After that you would want to test the fuel injection system. There are links above that’ll show you how to do that. If that doesn’t work, it may be time to do a compression test and see if you have a leak in that cylinder.
Is P0304 a Serious Concern?
P0304 is cause for concern and left unfixed can leave you stranded or make the vehicle virtually un-drivable. Fixing the problem causing the code to fire can save you time and money and keep your GMC Sierra from breaking down. Quick action may also save your catalytic converter from going bad. Good luck diagnosing the issue. If there is anything that you would like to add please leave a comment below.