P0304 on Hyundai: How to Diagnose & Fix
Cylinder 4 Misfire Detected
3 Hyundai models · 3 documented issues · 3 critical · $600-$7,500 repair
P0304 on Hyundai by model: symptoms, diagnosis and fix
The Theta II GDI engine in 2014-2015 and 2018-2019 Tucson models is susceptible to connecting rod bearing failure caused by manufacturing debris restricting oil flow. This can lead to catastrophic engine seizure, often without warning. Under the Theta II engine class-action settlement, Hyundai extended the powertrain warranty to a Lifetime Warranty (Warranty Extension TXXI) for connecting rod bearing damage on vehicles that complete the Knock Sensor Detection System software update (Service Campaign 953).
Common Symptoms
- Metallic knocking from engine
- Engine stalling without warning
- Check engine light with P1326 code
- Loss of power while driving
- Excessive oil consumption
How to Fix
Check VIN for active recalls at NHTSA.gov. Ensure the Knock Sensor Detection System (KSDS) software update has been installed - this monitors for early signs of bearing failure and can put the engine in limp mode. If the engine has failed from connecting rod bearing damage after the KSDS update, Hyundai covers the short-block repair under the lifetime warranty extension. Out-of-warranty replacement runs $4,500-7,500. Use full synthetic 0W-20 oil and change every 5,000 miles.
Owner tips & cautions
- TipGet the KSDS software update immediately if not already done - it can detect bearing failure early and prevent catastrophic seizure
- WarningIf you hear any knocking noise, stop driving immediately. Continued driving can destroy the engine beyond repair
Sources for this issue (1)
The 1.8L Nu engine in 2011-2016 Elantras is prone to premature failure due to manufacturing defects including a piston defect that produces piston slap/scuffing (most often in cold weather), connecting rod bearing failure, and oil passage problems. Metal debris from failed components can circulate through the engine causing catastrophic damage. A class action settlement (Brown v. Hyundai) extended the powertrain warranty on the engine long block for piston-slap damage to 10 years/120,000 miles for original owners and 8 years/80,000 miles for other owners in cold-weather states (campaign TXXK).
Common Symptoms
- Engine knocking or ticking noise that increases with RPM
- Excessive oil consumption
- Check engine light illumination
- Oil pressure warning light
- Engine stalling or loss of power
- Metal shavings in oil
How to Fix
Check for coverage under the Nu Engine Settlement warranty extension (Hyundai campaign TXXK, TSB 22-EM-009H) by giving a Hyundai dealer the VIN. Complete engine replacement if bearing failure has occurred.
Owner tips & cautions
- TipRegular oil changes every 3,000-5,000 miles to minimize sludge buildup.
- 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
Sources for this issue (4)
- Hyundai TSB 22-EM-009H - 1.8L Nu Engine Warranty Extension (TXXK)tsb
- Hyundai dealer notice - Brown Nu Class Action/Warranty Extension TXXKtsb
- MyCarVoice: Hyundai Elantra 1.8L Nu Engine Class Action Settlementmedia
- Sauder Schelkopf: Hyundai Elantra and Kia Forte Nu Engine Failures Class Action Investigationmedia
On the 2.5L Smartstream GDI+MPI engine (G4KN) used in the TM Santa Fe from 2021, the internal filter/screen inside the direct injector can break apart and the injector leaks fuel into the cylinder after shutdown and at idle. Owners report a hard hot start, raw-fuel smell, rough idle, single- or multi-cylinder misfires, loss of power and stalling while driving; the leaking fuel also dilutes the engine oil. Hyundai addressed it with TSB 23-FL-002H-1, which lists the diagnosis and replacement procedure for internally leaking GDI injectors and was later expanded to a wider production date range covering Santa Fe, Tucson and Santa Cruz. A 2025 class action alleges Hyundai has still not recalled the vehicles. Note the earlier 2.4L Theta II GDI injectors are a different part and this pattern is specific to the 2.5L.
Common Symptoms
- Long crank / hard hot start
- Rough idle and shaking at stop
- Misfire, flashing check engine light
- Sudden loss of power or stalling while driving
- Fuel smell; oil level rising and smelling of gasoline
How to Fix
Pull misfire DTCs and identify the offending cylinder(s). TSB 23-FL-002H-1 has been superseded by TSB 25-FL-001H (January 2025), which calls for replacing all four GDI injectors with Hyundai's injector kit (injectors, clips, high-pressure fuel pipe and gaskets) on affected vehicles. Change the engine oil afterward — fuel dilution from the leak contaminates it. Powertrain warranty covers this on most affected vehicles; push for the TSB by number if the dealer resists.
Sources for this issue (3)
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 Hyundai?
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 Hyundai it is documented across 3 models: Tucson, Elantra, Santa Fe.
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 Hyundai?
Published repair estimates on Hyundai range from $600 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.
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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.