P0308 is the OBD-II "cylinder 8 misfire" code, but on the LB7 Duramax the diagnostic path almost always lands on glow plug feedback. Here's the voltage table we use to separate one bad glow plug from a full set, plus when the code is actually pointing at an injector.
What this issue is
P0308 is the OBD-II diagnostic code for cylinder 8 misfire detected. That’s the universal definition across manufacturers — the ECM has counted misfire events on cylinder 8 above its threshold during a drive cycle and stored the code.
On the LB7 Duramax specifically, the diagnostic path almost always leads somewhere different than a gas engine would. There are no spark plugs to swap, no ignition coils to test. When we see P0308 on an LB7, the order of likelihood is:
- Glow plug feedback circuit fault — most common, especially on trucks with original or aged glow plugs
- Injector issue on cylinder 8 — leaky, contaminated, or failed
- Mechanical compression loss on cylinder 8 — rare, more common on high-mileage motors
- Fuel contamination or fuel pressure delivery specific to one cylinder
We have the factory GM scan tool in-house, which is what makes this diagnostic path workable — the glow plug feedback voltage values are only meaningful with a tool that can read them properly.
Symptoms you’ll notice
The symptom pattern depends on which root cause is at play, but the most common combination — glow plug feedback failure — looks like this:
- White smoke or hazing exhaust on cold start, sometimes lingering for the first few minutes of driving
- Hard cold-start that smooths out as the engine warms
- Rough idle when cold, normal idle when warm
- Check Engine Light with P0308 — sometimes paired with codes for other cylinders if multiple glow plugs are failing
- Slight power loss on cold mornings until the engine is fully up to temp
- Lower fuel economy because the unburned fuel from misfiring cylinders isn’t doing useful work
When the underlying cause is an injector or mechanical issue rather than a glow plug, the symptoms typically don’t clear up with operating temperature — the misfire is constant, and you’ll often see white smoke at idle even when warm.
Why it happens
Glow plugs in the LB7 fail with age. These trucks are 20+ years old now. Most original glow plug sets are long past their service life, and even replacement sets installed five or ten years ago are due. Glow plugs degrade slowly — resistance changes, the heating element weakens, and eventually the feedback voltage drops out of spec.
The ECM monitors glow plug condition through the glow plug feedback circuit, reading voltage from the injector circuit on each cylinder. When that voltage falls out of expected range, the ECM doesn’t necessarily set a glow-plug-specific code (P067X family). On the LB7, it commonly sets the misfire code for the affected cylinder — because from the ECM’s perspective, a cold cylinder that’s not lighting fuel cleanly produces a real misfire.
Injector issues on cylinder 8 can produce P0308 the same way they’d produce P0193 on the LB7 (we cover that path in detail in our LB7 P0193 article). Stuck-open nozzles, internal leakage, or contaminated injector tips all cause one cylinder to combust incorrectly. If the glow plug feedback voltage checks out and the misfire persists, injectors are the next stop.
Mechanical compression loss is the rarest cause but worth ruling out on high-mileage trucks. A burned valve, a failing piston ring set, or a head gasket compromise on cylinder 8 will produce a persistent misfire that no amount of glow plug or injector work will fix.
How we diagnose it
Top-to-bottom sequence, simple-to-involved:
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Pull stored codes and look for partner codes. P0308 alone with no other cylinder codes points strongly toward an issue isolated to cylinder 8 (glow plug, injector, or mechanical). P0308 paired with codes for other cylinders means a wider problem — usually multiple aged glow plugs.
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Read glow plug feedback voltage on the factory scan tool. This is the single most useful test on this platform. The threshold table we use:
- 5.00–6.00V — no bad glow plugs
- 2.00–3.00V — likely 1 bad glow plug
- 1.00–2.00V — likely 2 bad
- 0.80–1.70V — likely 3 bad
- less than 1.3V — likely 4 or more bad
If feedback voltage is in the bad range, we move directly to glow plug replacement and skip the more involved tests.
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Bench-test the glow plugs when removed. The reliable method: ground a battery pack to the body of the glow plug (the threaded section that screws into the head), then touch the positive lead to the end of the glow plug that connects to the wiring harness. A good glow plug starts to smoke or glow red/orange within seconds. A bad one stays cold or barely warms. Eye protection on, do not touch — the heat is intense.
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Injector balance and return-flow test on cylinder 8 if glow plugs check out. Same procedure we use for P0193 — measure fuel return at the injector and compare against the other seven. An injector returning significantly more fuel is leaking internally.
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Compression test on cylinder 8 if both glow plug and injector tests pass. This is the rare path. A cylinder reading meaningfully lower than its siblings means a mechanical issue — burned valve, ring set, or head gasket — and we’re into a different conversation about repair scope.
What the repair looks like
The fix depends on which step of the diagnostic identified the fault.
Glow plug replacement is the most common outcome. We typically recommend replacing the full set of eight when we find any bad glow plugs on a truck with the original set, because the others are aging at the same rate and the labor difference between doing one or two versus all eight isn’t large. The job is reasonably accessible on the LB7 — glow plugs are above the valve covers, not beneath them like the injectors.
Injector replacement on cylinder 8 means doing the full top-end teardown that the LB7 platform requires (valve covers off, glow plugs out, injector cup work). At that point we’d typically discuss whether to replace all eight injectors versus just the affected one — see the LB7 P0193 article for the full reasoning. Short version: if injectors have age on them, doing all eight at once is usually the better economic call.
Mechanical repair (head work, ring set) is a major job and we’d quote that with full transparency about what the truck is worth to keep versus what the engine repair will involve.
Pricing
Final pricing depends on labor, parts, and diagnostic findings — we walk through every line item with you before work begins. Call the shop or request an estimate with your year, mileage, and symptoms for a quote on your specific situation.