P0678 is a common diagnostic trouble code found in diesel engines. It’s a generic code, which means it has the same meaning for the GMC Sierra as it would any other vehicle made for the 1996 model year+.
P0678 indicates that the eighth cylinder glow plug is not heating its assigned combustion chamber. Your Sierra likely has a bad glow plug or wiring issue. We cover troubleshooting steps in detail below.
P0678 Definition: Eighth Cylinder Glow Plug – Circuit
Here’s the definition of P0678 for the GMC Sierra. It has two main parts:
Eighth Cylinder Glow Plug (And What it Does)
Diesel engines don’t use spark plugs. Instead, they use glow plugs to preheat the combustion chamber.
The glow plugs’ job is to heat up the combustion chamber before your Sierra is started. The “glow plug” light lets you know that they are heating the combustion chamber.
Once the engine is started, the heat and compression generated through diesel combustion are enough to keep the cylinders firing without the aid of a glow plug (diesel engines have higher compression ratios than gas-powered motors).
The GMC Sierra’s PCM (powertrain control module) uses data from the engine and transmission temperature sensors to determine if the engine is cold enough to warrant the use of the glow plugs.
They’re usually only needed when a diesel engine has been sitting for a while. It wouldn’t cool off enough while fueling up, for instance. That’s why the symptoms of P0678 are usually only felt when the vehicle is cold started.
Circuit
In the case of P0678, the code means that the circuit that provides power to the eighth cylinder’s glow plug is faulty. That’s it. It doesn’t specify if it’s open, short, low, or high voltage.
P0678 Symptoms: GMC Sierra
If your GMC Sierra has P0678, there are likely to be some noticeable symptoms. Although one bad glow plug is usually not enough to bring a modern diesel engine down.
- Check engine light with P0678
- Hard start
- Misfiring (Until the 8th cylinder warms up)
- No start
- Low engine power until the engine warms up
- Diesel fuel smell from exhaust
If the engine starts, let it idle and build heat before driving it. You want enough heat in there that cylinder 8 starts to fire on its own. While your Sierra may run and drive ok once it’s heated up, it’s still dumping unburned fuel into the exhaust until cylinder eight starts firing.
With most diesel engines, having one glow plug out will not be enough for it not to start.
GMC Sierra P0678 Causes + Diagnosis
While P0678 refers to a circuit issue, it also can be caused by a bad glow plug as well. Here’s how to diagnose this code in your GMC Sierra:
1. Determine if the glow plug is bad
There are a few ways that you can tell if the glow plug is functioning properly. If they are easy to get to, you can do the “swap test.”
Remove the glow plug from the eighth cylinder. Then, swap it with another glow plug (whichever is easiest to access). Once you do that, use your scanner to clear the OBDII memory.
Make sure your Sierra is cold; go ahead and fire it up. Wait for the check engine light to come back on.
If the code stays P0678, you know there is a wiring issue, you’ll need to troubleshoot the harness.
If the code changes to P067X (X being the cylinder with the bad glow plug), replace the glow plug with a new one, and you’re done.
You can also use a test light to determine if a glow plug is bad. Here’s how to do it.
2. Check the Wiring Harness
After following the steps above, you should be reasonably sure your Sierra doesn’t have a bad glow plug. Now, it’s time to check the wiring harness.
Check the harness damage (think cracked wiring, bad ground, or loose connection at the glow plug). The glow plug wiring harness has fusible links, so if the harness is hot on one side of the link, but not on the other, you’ll need to replace it.
If it all looks fine, you’ll need to get an OHM meter and check the resistance at the glow plug. You’ll need to look up the wiring diagram for the exact engine and model year of your particular Sierra.
Conclusion
P0678 in the GMC Sierra is usually caused by the two issues listed above, and replacing the glow plug will more often than not resolve the problem. Good luck!