P0306 on Toyota: How to Diagnose & Fix
Cylinder 6 Misfire Detected
1 Toyota model · 1 documented issue · 1 critical · $1,500-$3,500 repair
P0306 on Toyota by model: symptoms, diagnosis and fix
The first model years of the 4.0L 1GR-FE V6 used an early head gasket design whose sealing material between the cylinder and the coolant jacket degrades over time, allowing a small coolant leak into a cylinder. The classic signature is a cold-start misfire, most often reported on cylinder 6 (other cylinders such as 5 also occur), that 'steam-cleans' the spark plug and sets a cylinder-specific misfire code such as P0306, together with slow coolant loss from the reservoir (owners report topping off every few weeks) and a gurgling sound from the heater core on startup. Because the leak is small early on, it is frequently misdiagnosed as a bad coil or plug before the head gasket is identified. Left unaddressed it progresses to overheating and possible head/block damage.
Common Symptoms
- Cold-start misfire, most often cylinder 6
- Slowly disappearing coolant with no visible external leak
- Need to add coolant every few weeks
- Gurgling from heater core on startup
- Check Engine Light with a cylinder misfire code (e.g. P0306)
- White exhaust vapor on cold start
- Eventual overheating
How to Fix
Diagnose with a cylinder leak-down/compression test, a combustion-gas (block) test of the coolant, and cooling-system pressure testing to confirm internal coolant loss before replacing parts. The repair is to remove the heads and replace both head gaskets with the updated/superseded gasket, resurface and inspect the heads, and refill with the correct coolant. Replace head bolts per spec if torque-to-yield. Verify no oil-in-coolant or coolant-in-oil contamination that would indicate further damage.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Toyota 04112-0P061 genuine 1GR-FE valve-grind gasket kit
supply the coordinated top-end seals for a confirmed head-gasket repair after inspection and machine work Fitment/availability: 2005-2012 Toyota Tacoma 4.0L 1GR-FE top-end gasket kit; supersedes 04112-31080 and 04112-0P060, with VIN conf…
- Toyota 90910-02162 / 90910-A2006 genuine cylinder-head bolt
replace each head fastener that the factory procedure identifies as non-reusable or out of specification Fitment/availability: Individual 1GR-FE cylinder-head fastener application; 90910-02162 is superseded by 90910-A2006 and the repair …
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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 Toyota?
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 Toyota it is documented across 1 model: Tacoma.
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 Toyota?
Published repair estimates on Toyota range from $1,500 to $3,500 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.