P0306 on Cadillac: How to Diagnose & Fix
Cylinder 6 Misfire Detected
2 Cadillac models · 2 documented issues · $400-$1,000 repair
P0306 on Cadillac by model: symptoms, diagnosis and fix
The 3.6L V6 (LFX naturally-aspirated and LF3 twin-turbo) uses direct fuel injection, so no fuel washes over the backs of the intake valves. Combined with a PCV system that routes oil vapor into the intake tract, carbon coking accumulates on the intake valves over time. By roughly 60,000-100,000 miles this can cause rough idle, misfires, hesitation, and reduced fuel economy/power. This is separate from timing-chain-stretch and high-pressure fuel-pump issues; it is a fueling/airflow deposit problem inherent to GDI engines.
Common Symptoms
- Rough or unstable idle
- Cold-start misfire or stumble
- Hesitation on acceleration
- Reduced fuel economy
- Check engine light with misfire codes
- Loss of power
How to Fix
Clean the intake valves by walnut-shell media blasting (the accepted fix for GDI coking); chemical intake cleaners help mildly but do not remove hardened deposits. To slow recurrence, install an oil catch can on the PCV line and/or a reduced-orifice PCV to keep oil vapor out of the intake. Periodic top-end cleaning is preventive maintenance on these DI engines.
Sources for this issue (1)
The direct-injected 3.0L LF1 fitted to U.S.-market 2010-2011 SRX vehicles can develop valve deposits that contribute to rough running or misfires. The U.S. SRX changed to a 3.6L engine for model year 2012. GM preliminary information PIP5029 lists the 2010-2014 SRX with the 3.0L LF1 among direct-injected engines where major carbon deposits on the intake and/or exhaust valves can cause misfires, often more apparent on a cold start. Misfires have several possible causes; valve deposits should be confirmed rather than inferred from codes alone. GM bulletin 16-NA-383 superseded PIP5029 and addresses carbon-related valve sticking across a broader gasoline-engine population.
Common Symptoms
- Misfire, often more apparent on a cold start
- Check engine light with misfire codes (P0300-P0306)
- Rough idle
- Extended crank or crank-no-start
How to Fix
Follow current GM diagnostic and cleaning instructions after confirming deposits or sticking valves as the cause. Bulletin 16-NA-383 includes an approved chemical cleaning process; do not describe walnut blasting as the only accepted remedy or all chemical cleaners as ineffective. More extensive mechanical work depends on inspection and the response to the specified procedure. Confirm injector, ignition and mechanical condition as part of misfire diagnosis, and check VIN-specific coverage separately.
Sources for this issue (2)
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 Cadillac?
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 Cadillac it is documented across 2 models: XTS, SRX.
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 Cadillac?
Published repair estimates on Cadillac range from $400 to $1,000 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.