OBD-II Diagnostic Trouble Code
P0312 — Cylinder 12 Misfire Detected
P0312 means the ECM detected misfires on cylinder 12 from crankshaft speed variation during that cylinder's power stroke. Twelve-cylinder engines are uncommon in modern trucks, so on an inline-6 or V8 this code usually points to a software, calibration or scan tool issue rather than a real cylinder fault.
Severity 4/5
Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi
Cylinder 12 Misfire Detected
Code
P0312
Category
Powertrain
Subsystem
Ignition
Truck-relevant
Yes
What this means for truckers
Stop within ~50 miles on a Cummins X15, Detroit DD15, PACCAR MX-13. Driving longer risks damaging adjacent components (turbo housing, head gasket, aftertreatment) and turning a $300 fix into a $3,000 fix.
Before you replace parts — try this first
P0312 on the Ignition system is most often ignition coil, spark plug or injector failure on cylinder 12 (12-cylinder 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
On a 12-cylinder engine: rough idle, vibration, check-engine light, lower power and poor fuel economy. On engines with fewer cylinders, the code typically appears with no matching drivability problem.
Common causes
- CommonIgnition coil, spark plug or injector failure on cylinder 12 (12-cylinder engines) ($150–$1400)
- OccasionalWiring or connector damage in the cylinder 12 circuit ($100–$450)
- OccasionalIncorrect ECM calibration or scan tool decoding error ($0–$400)
- RareMechanical low compression on cylinder 12 ($1500–$6000)
How to fix
Verify the cylinder count and confirm the code with an OEM-level scan tool
When to use: First step on any engine
Swap cylinder 12 coil, plug or injector with another cylinder and recheck
When to use: On a confirmed 12-cylinder engine
Inspect the cylinder 12 harness and connector for damage
When to use: For intermittent codes
Do compression and leak-down tests on cylinder 12
When to use: When fuel and ignition 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
- → 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 P0312?
Drive with caution. P0312 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 P0312 on a commercial truck?
Total repair cost ranges varies depending on the root cause. The cheapest fix is usually the first one listed below — Ignition coil, spark plug or injector failure on cylinder 12 (12-cylinder 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 P0312 fail a DOT inspection?
P0312 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 P0312 and related codes?
P0312 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 P0312 is set without the companions, the issue is more isolated.
How do I prevent P0312 from coming back after a repair?
For P0312 (Cylinder 12 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.