P0304 on Hyundai: How to Diagnose & Fix
Cylinder 4 Misfire Detected
5 Hyundai models · 7 documented issues · 6 critical · $150-$8,000 repair
P0304 on Hyundai by model: symptoms, diagnosis and fix
The Theta II engine can seize due to metal debris blocking oil flow to connecting rod bearings. Hyundai issued recalls and extended warranties. Affected engines may need replacement.
Common Symptoms
- Knocking noise from engine
- Engine stalling
- Check engine light
- Loss of power
- Engine seizure
How to Fix
Check VIN for active recalls - Hyundai provides free engine replacement for affected vehicles. Install the knock sensor detection system (KSDS) software update that monitors for bearing wear. Change oil every 5,000 miles. If engine fails and under warranty, replacement is free. Out of warranty replacement: $4,000-8,000.
Owner tips & cautions
- TipDIY repairs can save significantly - dealer charges $4,000-8,000 but DIY costs are typically 50-70% less
- 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 (1)
- NHTSA Recall 17V-224recall
The Theta II GDI engine in 2011-2019 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. Hyundai issued multiple recalls (19V-601, 20V-150) and extended the engine warranty to 15 years/unlimited miles for original owners.
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 engine has seized, Hyundai will replace it under recall. 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 piston ring issues, 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 covered these vehicles with extended warranty up to 140,000 miles.
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. Contact Hyundai dealer for software update (KSDS knock sensor detection system). Complete engine replacement if bearing failure has occurred. Regular oil changes every 3,000-5,000 miles to minimize sludge buildup.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
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)
- ClassAction.org: New Hyundai, Kia Engine Failure Settlement Covers 2.1M Additional Vehiclesmedia
- MyCarVoice: Hyundai Elantra 1.8L Nu Engine Class Action Settlementmedia
- Sauder Schelkopf: Hyundai Elantra and Kia Forte Nu Engine Failures Class Action Investigationmedia
- Top Class Actions: Kia, Hyundai engine failure class action settlementmedia
Recall 203 / 21V301 covers certain U.S.-market 2019–2020 Elantra 2.0L Nu MPI vehicles produced November 1, 2018 through July 6, 2020. Incorrectly heat-treated oil rings may chip and damage cylinder surfaces, increasing oil consumption and risking engine damage, stalling or fire. VIN confirmation is essential; symptoms need not follow a predictable sequence.
Common Symptoms
- Abnormal engine knocking
- Reduced power or hesitation
- Check-engine or oil-pressure warning
- Unexplained accelerated oil loss
- Burning smell, leaking oil or smoke
- Engine stall
How to Fix
Contact Hyundai for current VIN-specific Recall 203 instructions and inspection history. The documented remedy is inspection and conditional engine replacement at no charge. The October 2021 procedure uses PASS/NO PASS inspection branches, with further diagnosis for abnormally low oil. PNSS was then still under development: verify current availability rather than assuming a completed recall includes a flash or demanding an unverified software version. Ask Hyundai about reimbursement under its applicable plan; do not promise acceptance.
Sources for this issue (2)
The Elantra Sport's 1.6L Gamma T-GDI is a direct-injection-only engine: no fuel ever washes across the back of the intake valves, so oil vapour drawn in through the PCV system bakes onto the valve stems and seats. Owners on the Elantra Sport community report visible deposits by roughly 60,000-90,000 miles, and heavier deposits on cars driven mostly short-trip. The deposits disrupt airflow and swirl, producing a shaky cold idle, hesitation off boost, cylinder-specific misfires and a slow erosion of power and fuel economy that owners often mistake for normal ageing. Turbocharging worsens it because higher blow-by loads the PCV system with more oil mist. This is specific to the direct-injected 1.6T - the port-injected 1.8L and 2.0L Nu MPI engines sold in the same bodies wash their intake valves with fuel and do not accumulate deposits this way.
Common Symptoms
- Rough or shaky idle when cold
- Hesitation or stumble under light throttle
- Cylinder-specific misfire codes
- Gradual loss of power and fuel economy
- Long crank on a cold start
How to Fix
Confirm by borescoping the intake ports through a removed intake manifold rather than guessing from symptoms. The only durable fix is mechanical cleaning - walnut-shell blasting of the intake ports and valve backs, typically repeated every 50,000-70,000 miles; chemical intake cleaners and fuel additives cannot reach the intake valves on a DI-only engine. Preventively, owners fit an oil catch can on the PCV side, keep oil-change intervals near 5,000 miles with a quality low-volatility oil, and avoid long strings of short cold trips. No recall or Hyundai campaign covers carbon cleaning; it is a maintenance item on this engine.
Sources for this issue (3)
On the 2.5L Smartstream GDI+MPI engine (G4KR) 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). The dealer procedure under TSB 23-FL-002H-1 is a leak-down/injector-balance test, then replacement of the affected direct injectors (many techs replace all four, since one failed screen usually means the rest are the same production lot) plus new injector seals and combustion seals. 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 5 models: Sonata, Tucson, Elantra, Santa Fe, Accent.
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 $150 to $8,000 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 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.