What Does P0309 Mean?
The ECM has detected a misfire condition specifically in cylinder 9. This code applies to engines with 9 or more cylinders (V10, V12, W12). The crankshaft position sensor detects a deceleration in crankshaft rotational velocity during the cylinder 9 power stroke. The driver may notice rough running, vibration, and reduced power. These high-cylinder-count engines are typically found in performance and luxury vehicles.
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. On high-performance engines, continued misfires can also damage expensive exhaust components.
Common Causes
30%
Worn or fouled spark plug on cylinder 9 unable to produce adequate spark
25%
Failing ignition coil on cylinder 9 producing weak or intermittent spark
20%
Fuel injector on cylinder 9 restricted or malfunctioning
15%
Compression loss on cylinder 9 from worn rings, leaking valve, or head gasket failure
10%
Wiring harness damage to cylinder 9 coil or injector — common in tightly packaged V10/V12 engine bays
Diagnostic Steps
1
Swap the ignition coil from cylinder 9 with an adjacent cylinder — if the misfire follows the coil, replace it; if it stays on cylinder 9, the coil is not the cause.
2
Remove and inspect the cylinder 9 spark plug — check gap, electrode condition, and fouling pattern; on V10/V12 engines access may require intake manifold removal.
3
Check cylinder 9 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 9 — minimum is typically 100 PSI and should be within 10% of the highest cylinder; low compression requires a leak-down test.
5
Inspect the wiring harness routing to cylinder 9 — in tightly packaged engine bays, chafing against engine components is common and can cause intermittent open circuits.
Estimated Repair Cost
$100 - $1,200
Parts + labor, varies by vehicle and location
The ECM has detected a misfire condition specifically in cylinder 9. This code applies to engines with 9 or more cylinders (V10, V12, W12). The crankshaft position sensor detects a deceleration in crankshaft rotational velocity during the cylinder 9 power stroke. The driver may notice rough running,...
The most common cause of P0309 (Cylinder 9 Misfire Detected) is: Worn or fouled spark plug on cylinder 9 unable to produce adequate spark
Typical repair costs for P0309 range from $100 to $1,200, 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. On high-performance engines, continued misfires can also damage expensive exhaust components.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0309 to identify the root cause.
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Quick Info
Category
Powertrain
System
Ignition System
Difficulty
Type
Generic (SAE)
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