What Does P0312 Mean?
The ECM has detected a misfire condition specifically in cylinder 12. This code applies exclusively to 12-cylinder engines (V12 and W12) found in luxury and high-performance vehicles. The crankshaft position sensor detects a deceleration during the cylinder 12 power stroke. The driver may notice rough running, vibration, and reduced power. Cylinder 12 is typically the most rearward cylinder and can be the most difficult to access for service.
Safety Warning
A misfiring cylinder sends unburned fuel to the catalytic converter, causing overheating and potential failure. If the check engine light is flashing, minimize driving and avoid high load. V12 repairs are typically expensive — address promptly to prevent cascade damage to catalytic converters.
Common Causes
30%
Worn or fouled spark plug on cylinder 12 unable to produce adequate spark
25%
Failing ignition coil on cylinder 12, often heat-degraded due to rear position
20%
Fuel injector on cylinder 12 restricted or malfunctioning
15%
Compression loss on cylinder 12 from worn rings, leaking valve, or head gasket failure
10%
Wiring harness damage to cylinder 12 coil or injector in the tightly packaged engine bay
Diagnostic Steps
1
Swap the ignition coil from cylinder 12 with an adjacent cylinder — if the misfire follows the coil, replace it; if it stays on cylinder 12, the coil is not the cause.
2
Remove and inspect the cylinder 12 spark plug — check gap, electrode condition, and fouling pattern; this is often the hardest cylinder to reach on V12 engines.
3
Check cylinder 12 fuel injector with a stethoscope or Noid light — verify it clicks rhythmically at idle; measure resistance (typically 11-18 ohms) and compare to other cylinders.
4
Perform a compression test on cylinder 12 — minimum is typically 100 PSI and should be within 10% of the highest cylinder; low compression requires a leak-down test.
5
Review cylinder 12 individual fuel trim data — if the ECM shows high positive correction, suspect a restricted injector; high negative correction suggests a leaking injector.
Estimated Repair Cost
$150 - $1,500
Parts + labor, varies by vehicle and location
The ECM has detected a misfire condition specifically in cylinder 12. This code applies exclusively to 12-cylinder engines (V12 and W12) found in luxury and high-performance vehicles. The crankshaft position sensor detects a deceleration during the cylinder 12 power stroke. The driver may notice rou...
The most common cause of P0312 (Cylinder 12 Misfire Detected) is: Worn or fouled spark plug on cylinder 12 unable to produce adequate spark
Typical repair costs for P0312 range from $150 to $1,500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
A misfiring cylinder sends unburned fuel to the catalytic converter, causing overheating and potential failure. If the check engine light is flashing, minimize driving and avoid high load. V12 repairs are typically expensive — address promptly to prevent cascade damage to catalytic converters.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0312 to identify the root cause.
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Quick Info
Category
Powertrain
System
Ignition System
Difficulty
Type
Generic (SAE)
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