P0306 on Subaru: How to Diagnose & Fix
Cylinder 6 Misfire Detected
3 Subaru models · 3 documented issues · 3 critical · $2,500-$12,000 repair
P0306 on Subaru by model: symptoms, diagnosis and fix
The EJ257 engine in the STI is known for rod bearing failures, especially when modified or driven hard. The stock rod bearings have thin oil clearances and can spin under high load conditions. Oil starvation during hard cornering can accelerate bearing wear.
Common Symptoms
- Knocking noise from bottom of engine
- Low oil pressure warning
- Metal shavings in oil
- Sudden loss of power
- Engine seizure in severe cases
How to Fix
Change oil every 3,000 miles with high-quality synthetic (Motul, Rotella T6). Install oil baffle or oil pan with baffle for track use. Consider upgraded rod bearings (ACL Race) if rebuilding. Monitor oil pressure and use oil analysis to catch wear early.
Owner tips & cautions
- WarningThis is a high-severity issue - ignoring it can lead to costly repairs or safety concerns. Address it promptly.
- TipGet a proper diagnosis before replacing parts - similar symptoms can have different causes
The FA20 engine in 2013-2016 BRZs was subject to a safety recall (WQG-66 / NHTSA 18V-845) for defective valve springs that can fracture due to insufficient hardness. A broken valve spring causes a cylinder misfire and can lead to engine stalling. In worst cases, the broken spring allows valve-to-piston contact, causing catastrophic engine damage. Subaru recalls affected VINs for free valve spring replacement. The recall also affects the Toyota 86/Scion FR-S which shares the same engine.
Common Symptoms
- Engine misfire (single cylinder)
- Check engine light with misfire code (P0301-P0304)
- Rough idle or engine shaking
- Loss of power
- Engine stalling at idle or low speed
- Ticking or unusual noise from engine top end
How to Fix
Check recall status by VIN at subaru.com/owners/recalls. Recall WQG-66 (NHTSA 18V-845) covers free replacement of all intake and exhaust valve springs (16 total) at Subaru dealer. If valve-to-piston contact occurred, engine replacement is covered under the recall. Do not delay - a broken valve spring can cause catastrophic engine damage.
Sources for this issue (4)
The 2006-2007 Subaru Tribeca with the 3.0L EZ30 flat-6 engine can develop head gasket leaks, typically at higher mileages (130,000-200,000 miles). While the EZ30 is generally more resistant to head gasket issues than the smaller EJ25 boxer engines, the problem does occur. The flat-6 boxer design means the head gaskets are oriented horizontally, which allows coolant to seep past the gasket under gravity when the engine is off. Symptoms include external oil leaks between the head and block, coolant consumption, and in severe cases, coolant entering the combustion chamber causing white exhaust smoke and overheating. The repair is complex due to the flat-6 layout and cramped engine bay of the Tribeca.
Common Symptoms
- Oil seeping between cylinder head and engine block
- Coolant level dropping without visible external leaks
- White exhaust smoke (coolant burning in combustion chamber)
- Heater blowing cold air intermittently (air trapped in cooling system)
- Engine overheating
- Sweet coolant smell from engine bay
- Temperature gauge fluctuations
How to Fix
Sources for this issue (1)
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 Subaru?
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 Subaru it is documented across 3 models: WRX STI, BRZ, Tribeca.
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 Subaru?
Published repair estimates on Subaru range from $2,500 to $12,000 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.
Looking for P0306 on a different make?
View P0306 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.