OBD-II Diagnostic Trouble Code
P0308 — Cylinder 8 Misfire Detected
P0308 means the ECM saw crankshaft speed fluctuation consistent with misfires on cylinder 8. It applies to V8 and V10 engines, such as gas and diesel V8s in medium-duty trucks and V10 gas engines in cutaway and motorhome chassis.
Severity 4/5
Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi
Cylinder 8 Misfire Detected
Code
P0308
Category
Powertrain
Subsystem
Ignition
Truck-relevant
Yes
What this means for truckers
Cylinder 8 Misfire — same.
Before you replace parts — try this first
P0308 on the Ignition system is most often failed ignition coil or worn or fouled spark plug on cylinder 8 (gas engines) — 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
Rough running at idle and under load, a check-engine light that may flash on gas engines during heavy misfire, reduced power, and a miss you can feel through the cab. Diesels may smoke white or gray at cold start and show poor fuel economy. Prolonged misfire on a gas engine can overheat the catalytic converter.
Common causes
- CommonFailed ignition coil or worn or fouled spark plug on cylinder 8 (gas engines) ($120–$400)
- CommonFaulty fuel injector on cylinder 8 (leaking, sticking or low delivery) ($350–$1400)
- OccasionalWiring or connector fault in the cylinder 8 coil or injector circuit ($100–$450)
- OccasionalMechanical problem on cylinder 8 such as a burnt valve, worn cam lobe or head gasket leak ($1500–$6000)
How to fix
Swap the cylinder 8 coil and plug with a neighboring cylinder and recheck which cylinder misfires
When to use: Gas engines, first step
Perform a scan-tool cylinder cutout or injector balance test and replace the injector if it does not contribute normally
When to use: Diesel engines or when ignition parts test good
Inspect the cylinder 8 harness, connector and under-valve-cover wiring for damage or oil contamination
When to use: When the code is intermittent
Run compression and leak-down tests on cylinder 8
When to use: When fuel and ignition are confirmed 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
- → P0300 — Random/Multiple Cylinder Misfire Detected
- → P0301 — Cylinder 1 Misfire Detected
- → P0302 — Cylinder 2 Misfire Detected
- → P0303 — Cylinder 3 Misfire Detected
- → P0304 — Cylinder 4 Misfire Detected
Frequently Asked Questions
Can I drive with code P0308?
Drive with caution. P0308 is severity 4 — stop within ~50 miles. Most commercial diesel codes at this severity (turbo, fuel rail, transmission solenoid) can damage adjacent expensive components if left unrepaired. Get to a service location before highway-speed loads.
How much does it cost to fix P0308 on a commercial truck?
Total repair cost ranges $100–$6000 depending on the root cause. The cheapest fix is usually the first one listed below — Failed ignition coil or worn or fouled spark plug on cylinder 8 (gas engines) — 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 P0308 fail a DOT inspection?
P0308 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 P0308 and related codes?
P0308 is closely related to: P0300 (Random/Multiple Cylinder Misfire Detected), P0301 (Cylinder 1 Misfire Detected), P0302 (Cylinder 2 Misfire Detected), P0303 (Cylinder 3 Misfire Detected). These codes share the same subsystem (ignition) 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 P0308 is set without the companions, the issue is more isolated.
How do I prevent P0308 from coming back after a repair?
For P0308 (Cylinder 8 Misfire Detected), prevention centers on the ignition system. Use OEM-spec injectors and spark plugs (gas). Address fuel quality issues immediately — bad fuel kills injectors. Cylinder balance test annually for early warning.