OBD-II trouble code
P0309: Cylinder 9 Misfire Detected
Cylinder 9 is missing combustion events. There is no generic advice for this cylinder — it only exists on V10 and V12 engines, and those engines share almost nothing with each other, including how they number their cylinders.
Quick facts
- System
- Powertrain
- Category
- Ignition / Misfire
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $150 – $2,500
- DIY difficulty
- Advanced DIY
Browse every code in P0300–P0394, or start from the full code library.
What does P0309 mean?
The misfire monitor watches crankshaft acceleration across every power stroke and attributes a shortfall to the specific cylinder that should have produced it. P0309 means cylinder 9 fell short often enough to set a code. The mechanism is the same one behind every cylinder-specific misfire code, but the population is not, and the population is what makes this page different from the ones for cylinders 1 through 8.
Only a V10 or a V12 has a cylinder 9. That is a small and extraordinarily varied group: heavy-duty and recreational-vehicle trucks with the Ford 6.8-litre modular V10, the Dodge Viper's pushrod V10, the Audi, Lamborghini and BMW high-revving V10s, and Mercedes, BMW and Jaguar V12s. These engines have essentially nothing in common — different valvetrains, different induction, different ignition architecture, different service culture. A repair procedure that is correct for one of them can be badly wrong for another.
The practical trap is cylinder numbering. There is no universal convention. Some manufacturers number one entire bank before starting the second, so cylinder 9 sits well back on the second bank. Others alternate side to side down the block, which puts cylinder 9 somewhere else entirely. On the Ford modular V10, cylinders 1 through 5 are the right-hand bank and 6 through 10 the left, making cylinder 9 the fourth cylinder back on the driver's side of a left-hand-drive truck. Do not carry that over to a European V10 or a V12. Look up the diagram for the specific engine before a single fastener comes out, because the cost of getting it wrong on an engine this crowded is an entire afternoon spent working on the wrong bank.
The other thing worth knowing before the job starts is that ignition parts on these engines are not priced like V8 parts. Some V12s group their coils into per-bank modules or cassettes rather than fitting individually replaceable coil-on-plug units, which means a single failed coil can force the replacement of a much larger and much more expensive assembly. Confirm what your engine actually uses before assuming a coil is a small, separate part.
Common causes
- Failed ignition coil or coil module serving cylinder 9
- Worn, fouled or damaged spark plug
- Clogged, leaking or electrically failed fuel injector
- Spark plug thread damage or a seized plug on Ford modular V10 heads
- Vacuum leak at an intake runner or plenum gasket on the affected bank
- Low compression from a burned valve, worn ring pack or head gasket failure
- Chafed or heat-damaged wiring at the coil connector on a crowded bank
- Carbon buildup on the intake valve on direct-injection V10 and V12 engines
- Extended high-idle duty on RV, ambulance and utility applications accelerating plug and valve fouling
Symptoms
- Warning lamp flashing while this cylinder misses, then steady
- Rough idle and a noticeable change in the engine's normally smooth character
- Power loss under sustained load, such as towing or climbing a grade
- Vibration through the floor and seat
- Fuel economy dropping on an engine that already has little to spare
- Smell of unburned fuel from the exhaust
- Hesitation or stumble when accelerating from low speed
Diagnostic steps
- 1.Look up the cylinder numbering diagram for your exact engine family before touching anything. This is the single highest-value five minutes in the whole job on a ten- or twelve-cylinder engine.
- 2.Read the misfire counters for all cylinders, not just cylinder 9. One cylinder well ahead of the rest is a component fault; several cylinders on the same bank climbing together points at fuel supply, a plenum leak or an injector driver group.
- 3.Establish what ignition hardware the engine actually uses — individual coil-on-plug, a shared coil pack, or a per-bank cassette — before quoting or buying anything. The repair strategy and the cost differ completely between them.
- 4.Swap the cylinder 9 ignition component with the same component from a healthy cylinder on the same bank, clear the codes, and re-drive. If the misfire moves with the part, the diagnosis is finished and it cost nothing.
- 5.If the misfire stays on cylinder 9 after the ignition swap, repeat the exercise with the injector where the engine's design allows it, or check injector resistance and drive signal.
- 6.On a Ford modular V10, treat spark plug removal as its own risk assessment. The two-valve heads have a shallow thread engagement that has been known to strip and eject a plug, and the later three-valve design uses a two-piece plug that commonly breaks off in the head during removal. Soak, cool-engine removal and the correct extraction tooling belong in the plan, not in the recovery.
- 7.Before a third parts replacement is authorised, compare cylinder 9 on a compression or relative-compression test. A reading well below the bank average moves this from an ignition job to a mechanical one.
Repair cost
$150 – $2,500
An accessible coil and plug on a V10 is the smaller job. Pulling a broken plug out of a Ford three-valve modular head, or fitting an insert where the thread has stripped, moves the job up. A V12 that uses a per-bank coil module rather than a separate coil makes the ignition part itself the large piece. Injector replacement sits in between. Valve or head-gasket work on a ten- or twelve-cylinder engine is quoted on its own, past the top of this span. The bracket shown is for planning, not a price a shop has quoted. Cost the components and the labor the service manual lists for the engine in front of you.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with ignition coil replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.