OBD-II Diagnostic Trouble Code
P0678 — Cylinder 8 Glow Plug Circuit
P0678 means a fault was detected in the glow plug circuit for cylinder 8. Current or resistance on that circuit was out of range, which typically indicates an open or shorted glow plug or a damaged harness lead. It applies to V8 diesels.
Severity 2/5
Truck-relevant
Service soon
Severity
Service soon
Cylinder 8 Glow Plug Circuit
Code
P0678
Category
Powertrain
Subsystem
Cold Start
Truck-relevant
Yes
What this means for truckers
P0678 — Cylinder 8 glow plug. (Yes, 8-cylinder commercial diesels exist — Cummins Signature 600, some Mack and CAT V8s in older trucks.)
Before you replace parts — try this first
P0678 on the Cold Start system is most often burned-out or open cylinder 8 glow plug — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Harder cold starts with rough idle, misfire and white smoke on cylinder 8 after a cold start. Warm running is normally unaffected.
Common causes
- CommonBurned-out or open cylinder 8 glow plug ($120–$450)
- CommonDamaged or corroded lead or connector at cylinder 8, which on many V8s sits at the rear of the engine near the cab ($60–$300)
- OccasionalGlow plug shorted to ground ($120–$450)
- OccasionalFailed glow plug module output channel ($150–$600)
How to fix
Measure the cylinder 8 glow plug resistance against spec
When to use: First step
Inspect and repair the cylinder 8 lead and connector
When to use: When the plug tests good
Replace the cylinder 8 glow plug using proper removal technique
When to use: When the plug is open or shorted
Test the module's cylinder 8 output
When to use: When the plug and wiring are good
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0670 — Glow Plug Control Module Circuit
- → P0671 — Cylinder 1 Glow Plug Circuit
- → P0672 — Cylinder 2 Glow Plug Circuit
- → P0673 — Cylinder 3 Glow Plug Circuit
- → P0674 — Cylinder 4 Glow Plug Circuit
Frequently Asked Questions
Can I drive with code P0678?
P0678 is severity 2 — service this week. Not an immediate stop, but ignoring it usually compounds the problem. Most common cause for this code is a worn sensor or minor performance fault that won't strand you today but will get more expensive the longer you wait.
How much does it cost to fix P0678 on a commercial truck?
Total repair cost ranges $60–$600 depending on the root cause. The cheapest fix is usually the first one listed below — Burned-out or open cylinder 8 glow plug — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P0678 fail a DOT inspection?
P0678 is generally not a direct DOT inspection fail item by itself. Inspectors focus on safety-critical systems (brakes, steering, tires, lights, hours-of-service). However, the check engine light alone can trigger additional inspection scrutiny. Best practice: fix the code and clear it before a planned inspection.
What's the difference between P0678 and related codes?
P0678 is closely related to: P0670 (Glow Plug Control Module Circuit), P0671 (Cylinder 1 Glow Plug Circuit), P0672 (Cylinder 2 Glow Plug Circuit), P0673 (Cylinder 3 Glow Plug Circuit). These codes share the same subsystem (cold-start) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P0678 is set without the companions, the issue is more isolated.
How do I prevent P0678 from coming back after a repair?
For P0678 (Cylinder 8 Glow Plug Circuit), prevention centers on the cold start system. Test glow plug system annually before winter. Use a battery in good condition — weak batteries make glow plugs work harder. Park indoors below -10°F if possible.