P0300 on Hyundai: How to Diagnose & Fix
Random/Multiple Cylinder Misfire Detected
7 Hyundai models · 7 documented issues · 3 critical · $250-$7,500 repair
P0300 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
The direct-injected (GDI) 3.8L and 5.0L engines in the 2012+ Genesis accumulate carbon deposits on the intake valve faces. Since fuel is injected directly into the cylinder rather than over the intake valves, there is no fuel washing action to clean the valve backs. Carbon deposits restrict airflow, cause rough idle, misfires, and reduce power output. The problem is progressive and becomes noticeable around 40,000-60,000 miles. Short-trip driving and extended idle time worsen the buildup.
Common Symptoms
- Rough idle that smooths out after engine warms
- Misfires at idle or low RPM
- Reduced power output and throttle response
- Reduced fuel economy (2-4 MPG drop)
- Check engine light with random misfire codes
How to Fix
Walnut shell blasting of the intake valves is the most effective cleaning method — the intake manifold is removed and crushed walnut shells are blasted through the intake ports to remove carbon. This should be done every 40,000-60,000 miles as preventive maintenance. Chemical cleaning products (CRC GDI Cleaner) provide a partial solution between blastings. Some owners install an oil catch can to reduce the PCV oil vapor contributing to carbon buildup.
Owner tips & cautions
- TipSchedule walnut shell blasting every 50,000 miles — this is preventive maintenance on ALL GDI engines, not just when symptoms appear
- TipUse Top Tier gasoline (Shell, Chevron, Costco) — the higher detergent levels help reduce intake tract deposits even on GDI engines
Sources for this issue (2)
The 3.5L V6 intake manifold gaskets deteriorate causing vacuum leaks, rough idle, and lean running conditions. Upper plenum gasket is most common failure point. Can trigger multiple misfire codes.
Common Symptoms
- Rough idle
- Lean misfire
- Hissing noise from engine
- Check engine light
- Poor acceleration
How to Fix
Replace upper and lower intake manifold gaskets. Clean manifold mating surfaces. Inspect PCV system and vacuum hoses while manifold is removed. Use OEM gaskets for best seal.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Fel-Pro lower and upper intake-manifold gasket setOEMFel-Pro MS 96460
complete upper/lower intake sealing set after smoke testing confirms manifold-gasket leakage Fitment/availability: Fel-Pro MS 96460 is cataloged as a lower-and-upper intake-manifold gasket set for 2002-2005 Hyundai XG350 3.5L V6. Confirm…
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Sources for this issue (3)
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)
Hyundai TSB 25-FL-001H covers specified production ranges of model-year 2021-2023 Sonata vehicles equipped with the 2.5-liter GDI Theta III non-turbo engine. Some affected vehicles may store one or more misfire DTCs due to GDI injectors leaking internally; the bulletin says internal filter breakage in an injector may cause the leak.
Common Symptoms
- Misfire diagnostic trouble code stored
- Check Engine warning-light illumination
How to Fix
Confirm that the vehicle falls within the bulletin's production range and has one of the listed misfire DTCs. For an applicable vehicle, Hyundai directs technicians to replace all four GDI injectors using the specified kit. If the listed DTCs are absent or the vehicle is outside the range, the bulletin says to continue normal diagnosis instead.
Sources for this issue (1)
P0300 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 P0300 mean on a Hyundai?
P0300 stands for "Random/Multiple Cylinder Misfire Detected." P0300 means the engine computer detected misfires occurring randomly or across multiple cylinders rather than in one specific cylinder. A misfire is when a cylinder fails to ignite its fuel-air mixture properly, which the computer senses through small fluctuations in crankshaft speed. Because it's not isolated to one cylinder, the cause is usually something that affects the whole engine — like fuel, air, or ignition system problems — rather than a single coil or plug. Persistent or heavy misfiring wastes fuel, runs rough, and can damage the catalytic converter, which is why a flashing check engine light should be taken seriously. On Hyundai it is documented across 7 models: Tucson, Elantra, Genesis, XG350, Santa Fe, Sonata, Genesis Coupe.
What are the most common causes of P0300?
The most common causes of P0300 are: Worn or fouled spark plugs (across cylinders), Vacuum or intake air leak, Weak fuel pump, clogged fuel filter, or low fuel pressure, Failing ignition coils or worn spark plug wires, Dirty or faulty mass airflow (MAF) sensor, Clogged or dirty fuel injectors, Faulty crankshaft/camshaft position sensor, Low compression or EGR/PCV system faults. The specific cause varies by vehicle.
How much does it cost to fix P0300 on a Hyundai?
Published repair estimates on Hyundai range from $250 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.