P0304 on Buick: How to Diagnose & Fix
Cylinder 4 Misfire Detected
3 Buick models · 3 documented issues · 1 critical · $400-$7,500 repair
P0304 on Buick by model: symptoms, diagnosis and fix
Like other GM direct-injected engines, the Regal's 2.0T and 2.4L DI engines accumulate carbon deposits on the back of the intake valves because fuel no longer washes the valves clean. Over tens of thousands of miles this causes cold-start misfires, rough idle, hesitation, and reduced power and economy. It is a well-documented owner complaint on the turbo cars in particular.
Common Symptoms
- Cold-start misfire / stumble
- Rough idle
- Hesitation on acceleration
- Reduced power and fuel economy
- Misfire codes
How to Fix
Perform a walnut-blasting / manual intake valve cleaning (or chemical induction service in milder cases). Updated engine-management software and periodic intake cleaning help slow recurrence; catch-can installation reduces future buildup.
Sources for this issue (1)
Owners report misfires, rough running and a flashing check-engine light traced to the one-piece coil pack that sits over all four plugs. They describe a broken coil tower, or a boot, spring and plug that had corroded on one cylinder while the others looked clean. Applies to: Encores with the 1.4L turbo LUJ or LUV engine, which uses a one-piece coil pack. The linked reports come from owners of 2013, 2014 and 2016 Encores. One owner notes that the 1.4L LE2 engine uses individual coil packs; the LE2 was fitted to the 2016–2019 Sport Touring, offered as an option on all but the base model in 2017–2018, and used in later-build 2021 Encores, so those cars are not covered here. Mileage when it happened, as stated in the linked owner reports: 62,000 miles on a 2014.
Common Symptoms
- Engine suddenly starts chugging
- Check-engine light comes on and flashes
- Traction control (TCS) light comes on with the check-engine light
- Misfires felt under acceleration, sometimes with no code stored
How to Fix
Have the misfire diagnosed before buying parts. In these reports the fix was a new coil pack and plugs, and one owner traced a misfire after the repair to non-OEM plugs. Owners report adding dielectric grease to the boots on reassembly. GM's PIP5105A warns that removing the coil pack without special tool EN-6009 can break the coil towers or boots, so this is best left to a technician with the tool.
Sources for this issue (5)
- 2014 Encore - Cylinder 4 misfire - corroded coil and plug (BuickForums, Mar 2021)forum
- 2013 Encore - Gone Sluggish (BuickForums)forum
- 2016 Encore Misfires & Underboost (BuickForums)forum
- GM Preliminary Information PIP5105A — damaged ignition coil assemblies due to improper service procedures (Jul 2014)tsb
- Time sensitive, desperate for help with 2013 Buick Encore (BuickForums)forum
The 4.6L Northstar V8 in 2006-2011 Lucerne CXS/Super (and STS, DTS, Cadillac SLS) is known for head-bolt thread pull from the aluminum block. Bolts back out of the block (rather than head gasket failing outright), allowing combustion gases into the coolant. Symptoms: coolant loss, white smoke, overheating, eventually no-start.
Common Symptoms
- white smoke
- coolant loss
- overheating
- sweet exhaust smell
- rough running
How to Fix
Northstar "time-sert" head bolt repair is the durable fix — block must be removed (engine out or in some cases above-engine work), threads drilled out, larger steel inserts installed. $3,500-$6,000 specialist labor. Some owners try "Block-Tite" / sealants as a band-aid; not durable. Watch coolant level monthly.
Sources for this issue (1)
P0304 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 P0304 mean on a Buick?
P0304 stands for "Cylinder 4 Misfire Detected." This code means the engine computer detected a misfire in cylinder number 4 — that cylinder isn't igniting its air-fuel mixture fully or at all. The system detects this by monitoring small changes in crankshaft rotation speed as each cylinder fires. You may feel a rough idle, shaking, hesitation or reduced power, and a persistent misfire can dump unburned fuel into the exhaust and overheat or damage the catalytic converter. Since it's pinpointed to one cylinder, the problem typically lies in that cylinder's spark, fuel, or compression. On Buick it is documented across 3 models: Regal, Encore, Lucerne.
What are the most common causes of P0304?
The most common causes of P0304 are: Worn or fouled spark plug in cylinder 4, Failed or weak ignition coil on cylinder 4, Faulty spark plug wire or boot (where applicable), Clogged, stuck or leaking fuel injector for cylinder 4, Vacuum or intake air leak near that cylinder, Low compression (worn rings, burnt/leaking valve, head gasket), Damaged wiring or connector at the coil or injector. The specific cause varies by vehicle.
How much does it cost to fix P0304 on a Buick?
Published repair estimates on Buick range from $400 to $7,500 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.
Looking for P0304 on a different make?
View P0304 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.