P0306 on Porsche: How to Diagnose & Fix
Cylinder 6 Misfire Detected
4 Porsche models · 4 documented issues · 3 critical · $600-$25,000 repair
P0306 on Porsche by model: symptoms, diagnosis and fix
The 997.2 and 991.1 911 with the 9A1 (MA1) direct-injection flat-six can suffer cylinder bore scoring, where the Alusil cylinder bores develop deep vertical scores; owner reports show it happens on these engines but far less often than on the earlier M96/M97 engines. This is caused by inadequate oil supply to the cylinder walls during cold starts and short-trip driving. The issue results in excessive oil consumption and eventually requires engine rebuild or replacement.
Common Symptoms
- Excessive oil consumption (more than 1 quart per 1,000 miles)
- Blue or white smoke on cold start
- Rough idle when engine is cold
- Metallic rattling from engine
- Compression test showing low values on cylinders 1 or 6
How to Fix
Engine must be removed and disassembled. Scored cylinders are repaired by installing Nikasil-plated aluminum cylinders (LN Engineering Nickies) or replacing the cylinder block. Some owners opt for a used or rebuilt engine. Prevention: always warm the engine fully before spirited driving and avoid short trips.
Owner tips & cautions
- WarningWhen buying a used 991, always perform a leak-down and compression test before purchase. Bore scoring is extremely expensive to repair and often not covered under Porsche warranty after the initial period.
Sources for this issue (5)
- 991.1 Bore Scoring - Rennlistforum
- LN Engineering: Are Porsche 997.2 and 991.1 9A1 (MA1) engines susceptible to bore scoring?manufacturer
- The Drive: The Complete History and Current State of the Porsche Bore Scoring Issuemedia
- Porsche Club of America: Bore scoring and how to prevent itmanufacturer
- Flat 6 Innovations: Cylinder Bore Scoringmanufacturer
The M96 and M97 engines in the Boxster develop cylinder bore scoring similar to the 911. Scoring typically starts on bank 2 (cylinders 4-6), most often cylinder 6, when the oil film, silicon bore surface or piston coating breaks down, often accelerated by fuel wash from over-fueling or cold-start enrichment. Symptoms begin with increased oil consumption and progress to misfires and rough running.
Common Symptoms
- Excessive oil consumption
- Blue smoke on startup
- Rough idle
- Misfires on bank 2 cylinders (4-6)
- Low compression on affected cylinders
How to Fix
Engine disassembly and replacement of the cylinders with Nikasil-plated aluminum Nickies, or full engine replacement. Prevention includes extended warm-up periods and avoiding short-trip driving patterns.
Owner tips & cautions
- WarningAlways perform a leak-down test when purchasing a used Boxster. Bore scoring can be present without obvious symptoms in early stages.
Sources for this issue (5)
- LN Engineering: What is Porsche Cylinder Bore Scoringmanufacturer
- LN Engineering: Identifying the Porsche Bore Scoring Problemmanufacturer
- The Drive: The Complete History and Current State of the Porsche Bore Scoring Issuemedia
- Porsche Club of America: Bore scoring and how to prevent itmanufacturer
- Flat 6 Innovations: Cylinder Bore Scoringmanufacturer
The Cayman M97 flat-six engines develop bore scoring, most often on the cylinder 4-6 bank, the same pattern seen in the 911 and Boxster. Scoring on the later MA1 (9A1) engines, including the 3.4L and the GT4's 3.8L, is reported far less often. Oil starvation at the cylinder walls during cold starts is the primary cause.
Common Symptoms
- Increasing oil consumption
- Blue smoke on cold start
- Misfires
- Low compression on affected cylinders
- Rough idle when cold
How to Fix
Engine rebuild with iron cylinder sleeves (Nickies) or engine replacement. Regular borescope inspections can detect scoring before it becomes catastrophic. Allow full warm-up before spirited driving.
Owner tips & cautions
- WarningEven the Cayman GT4 with the 911-derived 3.8L MA1 engine is susceptible to bore scoring. Get a borescope inspection before buying any used Cayman.
Sources for this issue (5)
- LN Engineering: What is Porsche Cylinder Bore Scoringmanufacturer
- LN Engineering: Identifying the Porsche Bore Scoring Problemmanufacturer
- The Drive: The Complete History and Current State of the Porsche Bore Scoring Issuemedia
- Porsche Club of America: Bore scoring and how to prevent itmanufacturer
- Flat 6 Innovations: Cylinder Bore Scoringmanufacturer
Every Panamera engine uses direct fuel injection, so no fuel ever washes across the back of the intake valves. Oil vapour drawn in through the PCV system bakes onto the valve stems and ports and gradually chokes airflow. On the naturally aspirated 3.6 V6 fitted to the base 970 the deposits are the classic case, and the boosted V6s show the same pattern. Symptoms are progressive rather than sudden: rough or lopey idle first, then hesitation and dulled throttle response, then stored misfire codes as the deposits become severe enough to disturb cylinder filling. Owners typically notice it somewhere between roughly 50,000 and 80,000 miles. This is an architectural characteristic of DFI, not a manufacturing defect, and no recall or warranty extension covers it - which is exactly why it catches second and third owners out.
Common Symptoms
- Rough or uneven idle
- Hesitation and reduced throttle response
- Check engine light with random or cylinder-specific misfire codes
- Loss of power at part throttle
- Hard starting when cold
- Reduced fuel economy
How to Fix
Walnut-shell media blasting of the intake ports and valve backs is the accepted fix; chemical cleaners and fuel additives do not reach the intake side on a DI engine. On the V6 this means removing the intake manifold and blasting each port with the valves closed, then replacing manifold and injector seals. Many independents recommend repeating it every 40,000-50,000 miles on cars used for short trips. A catch can and shorter oil-change intervals slow, but do not stop, accumulation. Typical cost is $600-$1,500 at an independent, more at a dealer. If a specific cylinder still misfires after cleaning, look at coils and injectors next.
Sources for this issue (3)
P0306 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0306 mean on a Porsche?
P0306 stands for "Cylinder 6 Misfire Detected." This code means the engine computer detected a misfire on cylinder 6 — that cylinder isn't burning its fuel-air mixture properly. The computer senses misfires by watching for tiny variations in crankshaft speed each time a cylinder should fire, and when cylinder 6 repeatedly fails to contribute, this code sets. You may feel a shake or stumble, especially at idle or under load, along with reduced power and possibly a flashing check-engine light, which signals a misfire severe enough to damage the catalytic converter. A misfire comes down to a problem with spark, fuel, or compression in that cylinder. On Porsche it is documented across 4 models: 911, Boxster, Cayman, Panamera.
What are the most common causes of P0306?
The most common causes of P0306 are: Worn or fouled spark plug on cylinder 6, Failed ignition coil for cylinder 6, Clogged, leaking, or failed fuel injector, Vacuum or intake air leak affecting that cylinder, Low compression (worn rings, burnt valve, head gasket), Damaged plug wire or coil connector, Carbon buildup or worn valvetrain components. The specific cause varies by vehicle.
How much does it cost to fix P0306 on a Porsche?
Published repair estimates on Porsche range from $600 to $25,000 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.