P0303 on Genesis: How to Diagnose & Fix
Cylinder 3 Misfire Detected
3 Genesis models · 3 documented issues · $400-$900 repair
P0303 on Genesis by model: symptoms, diagnosis and fix
GV70 models with the Theta III / Smartstream 2.5L T-GDI engine may illuminate the check engine light during a cold start with misfire DTCs P0300, P0301, P0302, P0303 or P0304. Genesis addresses this with an engine control module software update with improved misfire logic, and the California Air Resources Board determined the vehicles may be releasing air pollutants exceeding Federal and California standards. Genesis issued emissions Service Campaign 914G in August 2025 covering 2022-2023 and 2025 GV70 (JK1, Korean-built, VIN starting KMU) produced 03/29/2021-03/04/2025 and 2024-2025 GV70 (JK1A, US-built, VIN starting 5NM) produced 05/10/2023-02/13/2025, along with 2021-2024 G80 and GV80 2.5T.
Common Symptoms
- Check engine light on after a cold start
- Misfire DTC P0300 (random) or P0301-P0304 stored
- Vehicle may fail an emissions inspection with the MIL on
How to Fix
Ask a Genesis retailer to check the VIN for open campaign 914G. Eligible vehicles receive the ECM software update and campaign documentation at no charge using TSB25-01-051G or its current replacement. The update improves cold-start misfire logic; it does not prove that an actual rough-running engine has no mechanical fault. Have drivability symptoms or persistent codes diagnosed rather than assuming all misfires are false. Any separately required parts repair has its own warranty eligibility; the campaign bulletin permits a separate warranty claim or prior goodwill request, not automatic coverage.
Sources for this issue (4)
- Genesis Service Campaign 914G: ECM Update - P030X Misfire, Retailer Notification (TSB 25-01-051G) - NHTSA MC-11021790-0001tsb
- Genesis TSB 25-01-051G: ECM software update - P030X misfire (Service Campaign 914G), NHTSA MC-11021791-0001tsb
- GV80 ECM Software Update P030X Misfire Letter - Genesis GV Forums (campaign letter for the same 2.5T campaign)forum
- Engine light on 5 month old GV70 - Genesis Forums (2022 GV70 2.5T misfire CEL)forum
Like most gasoline direct-injection engines without supplemental port injection, the G70's 2.0T and 3.3T accumulate carbon on the backs of the intake valves over time because no fuel washes them. Owners report heavy valve coking by ~24,000 miles in some cases, especially when combined with oil consumption. The deposits disrupt airflow and seating, causing rough idle, hesitation, cold-start stumble, and cylinder misfire codes. Removing the intake to walnut-blast the valves is the definitive fix.
Common Symptoms
- Rough or uneven idle, especially cold
- Hesitation / stumble at low speed
- Cylinder misfire codes, sometimes a check engine light
- Reduced fuel economy and power
- Hard / rough cold start
How to Fix
Definitive repair is mechanical intake-valve cleaning (walnut-blasting / media blasting) after removing the intake manifold — typically due around 40,000-80,000 miles depending on driving and oil use. Address any contributing oil consumption (PCV/valve guides) at the same time. A quality top-tier fuel, regular oil changes, and an occasional Italian tune-up slow accumulation; catch-can installation reduces future buildup. Replace fouled spark plugs and clear misfire codes after cleaning.
Sources for this issue (1)
Theta III 2.5L T-GDI engines in GV80 JX1 (and related G80/GV70) illuminate the Check Engine light with random or specific cylinder misfire codes P0300-P0304 during cold start. Root cause is identified as ECM misfire-detection logic; Genesis issued service campaign 914G covering 2021-2024MY GV80 built 07/27/2020-03/04/2024 for a free ECM reflash.
Common Symptoms
- Check Engine Light on after cold start
- Rough idle during warm-up
- Brief stumble or shudder on first start of the day
- Multi-cylinder or cylinder-specific misfire codes
How to Fix
Free dealer remedy under Service Campaign 914G / TSB 25-01-051G: ~30 minute ECM software reflash with improved misfire logic. Persistent misfire after reflash warrants coil/injector swap-test on the affected cylinder.
Sources for this issue (1)
P0303 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 P0303 mean on a Genesis?
P0303 stands for "Cylinder 3 Misfire Detected." This code means your engine's computer detected a misfire specifically in cylinder number 3 — that cylinder isn't burning its fuel-air mixture properly or completely. The computer senses misfires by watching for tiny fluctuations in crankshaft speed each time a cylinder is supposed to fire. A misfire wastes fuel, can cause a noticeable shake, rough idle or loss of power, and can flood raw fuel into the exhaust where it may overheat and damage the catalytic converter. Because the fault is isolated to one cylinder, the cause is usually something specific to that cylinder's ignition, fuel delivery, or compression. On Genesis it is documented across 3 models: GV70, G70, GV80.
What are the most common causes of P0303?
The most common causes of P0303 are: Worn or fouled spark plug in cylinder 3, Failed or weak ignition coil on cylinder 3, Bad spark plug wire or boot (where applicable), Clogged or leaking fuel injector for cylinder 3, Vacuum or intake air leak affecting that cylinder, Low compression (worn rings, burnt/leaking valve, head gasket), Wiring or connector fault at the coil or injector. The specific cause varies by vehicle.
How much does it cost to fix P0303 on a Genesis?
Published repair estimates on Genesis range from $400 to $900 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 P0303 on a different make?
View P0303 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.