P0307 is a fairly common trouble code with the GMC Sierra. It’s and OBD2 code and means that a misfire has been detected in the 7th cylinder in the firing order. You should only be seeing this code if you have an engine with at least 7 cylinders. So, it’s mostly a code for V8 engines. P0307 is typically caused by bad spark plugs, plug wires, or coil packs.
P0307 is typically caused by bad spark plugs, plug wires, or coil packs
This code is certainly a cause for concern, and should be considered a threat to the drivability your Sierra. The nice thing about P0307 is that it’s a cylinder specific trouble code, which makes diagnosing the problems much more simple than P0300, which means that the cylinders are randomly misfiring.
Fixing P0307 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 Sierra’s catalytic converter. The engine is also not firing all of the fuel, so raw fuel is also passing through cylinder seven into the exhaust. Mileage may suffer as well when your Sierra has P0307 as well.
GMC Sierra P0307 Definition
P0307 is a cylinder specific misfire code, which means that Cylinder 7 is misfiring and causing it. An important thing to understand when finding the location of Cylinder 7 is that it’s going to be the seventh cylinder in the firing order.
Sierra P0307 OBDII Code Symptoms
- The P0307 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 tailpipe.
- The Sierra may backfire.
P0307 Trouble Code Diagnosis- GMC Sierra
There are quite a few things that can cause the P0307 trouble code to trigger the GMC Sierra. Here are the most common problems that may cause it. 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 P0307. 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. GMC Sierra Spark Plugs.
- 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 P0307 in your GMC Sierra. Replacing a set can be very expensive. Here’s how to test them.
- 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 P0307 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 P0307. You should also feel the vehicle is down on power as well.
Most Common P0307 Fixes
If you follow the items listed above you should be able to fix the P0307. 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 7 to another cylinder. 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 the seventh cylinder.
Is P0307 a Serious Concern?
P0307 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.