P0674 on the 6.6L Duramax LLY is a glow plug cylinder 4 circuit fault. The LLY uses glow plugs to preheat each cylinder for cold starting, and the glow plug control module monitors each plug's current draw. P0674 sets when the module sees the cylinder 4 glow plug circuit outside its expected range, indicating an open plug, a shorted plug, or a fault in the wiring to that cylinder.
A cylinder-specific glow plug code isolates the fault to one plug and its feed, which makes the repair straightforward. On the Duramax the glow plugs thread into the heads and are fed by wiring near the valve covers. P0674 most often means the cylinder 4 plug has failed open or the feed to it is damaged, both confirmed by checking the plug's resistance and inspecting the circuit before replacing parts.
What P0674 means
The glow plug control module energizes each plug and monitors current. P0674 sets when the cylinder 4 circuit falls outside the acceptable range, indicating an open glow plug, a shorted plug, or an open or short in the wiring to cylinder 4. It is a per-cylinder code that isolates the fault to that plug and feed.
Common causes
- Open or burned-out cylinder 4 glow plug
- Shorted glow plug drawing excess current
- Damaged or chafed wiring to cylinder 4
- Corroded or loose glow plug terminal
- Poor connection at the glow plug feed
Symptoms
- Hard cold starting and longer cranking
- White smoke and a rough cold idle
- Check engine light with P0674 stored
- One cylinder slow to contribute on a cold start
- Normal running once warm
How to diagnose it
Locate the cylinder 4 glow plug and measure its resistance; a healthy plug reads low while an open plug reads very high. Inspect the terminal and wiring to cylinder 4 for corrosion, chafing, and a secure connection. Check the feed from the glow plug control module for an open or a short. Compare cylinder 4 to a known-good cylinder to confirm the fault is isolated rather than a module-wide issue that would affect multiple plugs.
How to fix it
Replace the cylinder 4 glow plug if it reads open or shorted, and repair damaged wiring or a corroded terminal. If the plug and wiring test good but the fault returns, test the glow plug control module. After the repair, clear the code, confirm the glow plug circuits pass, and verify a clean cold start without white smoke. For glow plugs, wiring, and control modules for your LLY Duramax, contact sales@coredieselparts.com.
Frequently asked questions
What causes a single glow plug to fail?
Glow plugs wear with heat cycles and age, and one can burn out or crack before the others. A damaged feed wire or corroded terminal on cylinder 4 can also set P0674.
Will P0674 affect a warm restart?
Little or not at all. Glow plugs matter most on cold starts, so the effect shows up as hard cold starting, white smoke, and rough idle until the engine warms.
Should I test the other glow plugs too?
Yes. While you are there, measure the remaining plugs. If several are worn or open on a high-mileage engine, replacing the full set prevents repeat cold-start problems.
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