P0303 on 2021-2024 Genesis G80: Cylinder 3 Misfire Causes and Fixes

What's Unique About the 2021-2024 Genesis G80

For the 2021-2024 G80 equipped with the 2.5L Turbo (Theta III T-GDI) engine, this code is often not due to a hardware failure. Genesis issued Technical Service Bulletin 25-01-051G, as part of Service Campaign 914G, acknowledging that cold start misfires (P0300-P0304) can be caused by the engine's software logic. This campaign also applies to the Genesis GV70 and GV80 with the same engine. The official fix is an ECM software update performed by a dealer, which is a unique and critical first step in diagnosis for this specific powertrain.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Rough or vibrating idle
• Hesitation or stumbling during acceleration
• Noticeable loss of engine power
• Check Engine Light is on and steady
• Check Engine Light is flashing (indicates a severe misfire)
• Reduced fuel economy
• Smell of raw fuel from the exhaust

Most Likely Causes

1. ECM Software Update Needed (2.5L T-GDI Engine Only) 🔴 High Probability Genesis issued Service Campaign 914G (related to TSB 25-01-051G) for 2021-2024 G80, GV70, and GV80 models with the 2.5L engine, which can set misfire codes like P0303 during a cold start due to software logic.
   How to confirm: Contact a Genesis dealership and provide your vehicle's VIN. They can verify if Service Campaign 914G is open for your car. This can also be checked on the Genesis recall website.
   Typical fix: A Genesis dealer will update the Engine Control Module (ECM) software. This procedure is performed at no charge to the owner and takes less than 30 minutes.
   Est. part cost: $0
2. Faulty Ignition Coil 🟡 Medium Probability Coil-on-plug ignition coils are a common failure point on most modern engines due to constant exposure to heat and vibration. They are a frequent cause of cylinder-specific misfires after ruling out the software TSB.
   How to confirm: Swap the ignition coil from cylinder 3 with another cylinder (e.g., cylinder 1). Clear the codes and drive the vehicle. If the code changes to P0301 (Cylinder 1 Misfire), the ignition coil is faulty.
   Typical fix: Replace the failed ignition coil. On the G80 (both 2.5T and 3.5T), all ignition coils are located on top of the engine and are relatively easy to access and replace. 🎬 See this walkthrough on how to replace an ignition coil. It is often recommended to replace all coils if they are original and the vehicle has high mileage.
   Est. part cost: $50-$100
3. Worn or Fouled Spark Plug 🟡 Medium Probability Spark plugs are a regular maintenance item. An old, worn, or oil-fouled spark plug can fail to produce a strong enough spark to ignite the air-fuel mixture, causing a misfire. A technician in a video noted that a misfiring cylinder's spark plug was full of black soot and looked wet, indicating it was not firing. 🎬 Watch a technician diagnose a similar Genesis misfire code.
   How to confirm: After confirming the coil is good, remove the spark plug from cylinder 3. Inspect it for wear, carbon buildup, or oil/fuel fouling. You can also swap it with a plug from another cylinder to see if the misfire follows.
   Typical fix: Replace the spark plug in cylinder 3. It is best practice to replace all spark plugs at the same time to ensure even performance and avoid future misfires.
   Est. part cost: $15-$30
4. Faulty Fuel Injector ⚪ Low Probability Both the 2.5T and 3.5T engines use Gasoline Direct Injection (GDI), which operates under high pressure and can be susceptible to carbon buildup or electronic failure. A clogged or failing injector can prevent the correct amount of fuel from reaching cylinder 3. Forum discussions mention injectors as a potential cause for misfires on the 3.5T engine.
   How to confirm: This is more complex. After ruling out ignition issues, a technician can perform a fuel injector balance test or measure the injector's internal resistance. Swapping injectors is possible but is very labor-intensive on the V6 engine, often requiring removal of the intake manifold.
   Typical fix: Replace the faulty fuel injector for cylinder 3. On the 3.5L V6, this is a significant job with estimated labor times of 8+ hours.
   Est. part cost: $100-$250

Rare But Worth Checking

• Low Engine Compression: This indicates a more serious mechanical problem within the engine, such as a bad valve, worn piston rings, or a head gasket issue. A compression test is needed to diagnose this.
• Vacuum Leak: A vacuum leak near the intake manifold runner for cylinder 3 could lean out the air-fuel mixture and cause a misfire. This is less common for a single-cylinder misfire but still possible.
• Carbon Buildup on Intake Valves: As GDI engines age, carbon can build up on the back of the intake valves, disrupting airflow into the cylinder and causing cold-start misfires. The dual injection system (GDI + MPI) on these engines is designed to mitigate this, but it can still occur.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner to confirm P0303 is the primary code.
🎬 Watch this helpful overview of P0303 causes and common fixes.
2. For 2.5L Turbo engines: Contact a Genesis dealer with your VIN to check if Service Campaign 914G (TSB 25-01-051G) is active for your vehicle. If so, have the ECM update performed. This is the most likely fix and is done at no cost.
3. For all engines (if TSB does not apply/fix): Identify cylinder 3. On the 2.5L I4, it is the third cylinder from the front of the engine. On the 3.5L V6, Bank 1 is the passenger side (right side) and cylinders are numbered 1, 3, 5 from front to back. Cylinder 3 is the middle cylinder on the passenger side.
4. Swap the ignition coil from cylinder 3 with an adjacent, easily accessible cylinder (e.g., cylinder 1). Clear the codes and run the engine. If the code changes to P0301, the ignition coil is bad and needs replacement.
5. If the code remains P0303, swap the spark plug from cylinder 3 with the one from the cylinder you just used for the coil swap. If the misfire code now follows the spark plug, the plug is faulty.
6. If the misfire still remains on cylinder 3, the issue is likely related to the fuel system (fuel injector) or a mechanical problem.
7. Inspect the wiring and connector for the cylinder 3 fuel injector and ignition coil for any visible damage, looseness, or corrosion.
8. If ignition and basic wiring are ruled out, a professional diagnosis is recommended. This may involve a fuel injector balance test, a GDI injector resistance check, or an engine compression test. Be aware that injector replacement on the 3.5L V6 is a labor-intensive job.

Parts You'll Likely Need

• ECM Software Update (OEM #Campaign 914G) — For the 2.5L T-GDI engine, this is the most probable solution for cold-start misfires as per a manufacturer service campaign.
  OEM price range: $0
  Aftermarket price range: N/A
• Ignition Coil (OEM #27300-2T000 (Example for 2.5L, verify by VIN)) — This is a very common failure point for cylinder-specific misfires after the TSB has been addressed or for the 3.5L V6 engine.
  Trusted brands: Hyundai/Genesis OE, NGK, Delphi
  OEM price range: $80-$120
  Aftermarket price range: $50-$90
• Spark Plug (OEM #18849-11070 (Example, verify by VIN)) — Spark plugs are a maintenance item and can cause misfires when they wear out. They are an inexpensive and logical part to replace during diagnosis.
  Trusted brands: NGK, Denso, Autolite
  OEM price range: $25-$40
  Aftermarket price range: $15-$25

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or begins to affect other cylinders, a P0300 (Random/Multiple Cylinder Misfire) code may appear alongside P0303.
• P0301, P0302, P0304 — The TSB for the 2.5L engine specifically mentions that any of the individual cylinder misfire codes (P0301-P0304) can be triggered by the software issue.
• P219E — On the 3.5L V6, a related code for 'Cylinder #3 Air-Fuel Ratio Imbalance' could appear, pointing more directly towards a fuel injector problem. A TSB for the Kia Carnival with this engine addresses this exact issue.

Technical Service Bulletins (TSBs) & Recalls

• Bulletin # 25-01-051G / Service Campaign 914G: Addresses misfire DTCs P0300, P0301, P0302, P0303, or P0304 on 2.5L T-GDI engines in G80, GV70, and GV80 models by updating the ECM software.

Platform-Specific Known Issues

• A significant issue for the 2021-2024 G80 with the 2.5L T-GDI engine is a known software problem that can cause misfire codes (P0300, P0301, P0302, P0303, P0304) during a cold start. Genesis has addressed this with Service Campaign 914G to update the ECM software at no cost.
• Some 2021 G80 and GV80 models with the 2.5L engine were recalled for a high-pressure fuel tube that could have an incomplete seal, potentially causing a fuel leak (NHTSA Campaign 21V208000). While less likely to cause a single-cylinder misfire, it's a known fuel system issue for this platform.
• Owners of vehicles with the 3.5T V6 have reported misfires that were ultimately traced to faulty GDI fuel injectors.

Mechanic-Grade Diagnostic Values

• GDI Fuel Injector Resistance (3.5L V6) — expected: ~2.1 - 2.2 Ohms. Failure: A reading of several Kilo-ohms (KΩ) or 'O.L.' (Open Loop) indicates an open circuit and a failed injector.
• Low-Pressure Fuel System — expected: ~50 - 80 PSI. Failure: Significantly lower pressure indicates a failing in-tank low-pressure fuel pump (LPFP).
• High-Pressure Fuel System (GDI) — expected: Over 2,500 PSI. Failure: Pressure that does not build to specification can indicate a failing high-pressure fuel pump (HPFP). This requires a specialized gauge and scan tool data.

Hidden / Shadow Codes Worth Checking

• Misfire Counter (Live Data): While not a 'code', dealer-level scanners like the Genesis GDS can display live misfire counts for each individual cylinder. This allows a technician to see which cylinder is misfiring in real-time, even before a specific P030x code is stored. (see via Genesis GDS (Global Diagnostic System) or advanced aftermarket bidirectional scan tools.)

Scan Tool Commands That Help

• Genesis GDS / Advanced Aftermarket Scanners: Actuation Test: Fuel Pump Relay — To verify the low-pressure fuel pump can be commanded on, helping to rule out a relay or circuit issue when diagnosing a fuel-related misfire.
• Genesis GDS: Guided Diagnostics / Active Tests — The official dealer tool provides step-by-step troubleshooting procedures for DTCs like P0303, which can include automated tests of components.
• Genesis GDS / Advanced Aftermarket Scanners: Injector Balance Test — This function commands each injector to fire for a set duration and measures the corresponding drop in fuel pressure. An injector that causes a smaller or larger drop than the others is likely faulty. This is a key step before attempting a labor-intensive injector replacement.

Wiring & Ground Locations

• Cylinder Numbering (3.5L V6) — Bank 1 (Right/Passenger Side) contains cylinders 1, 3, 5, from front to back. Bank 2 (Left/Driver Side) contains cylinders 2, 4, 6, from front to back.. Correctly identifying Cylinder 3 as the middle cylinder on the passenger side is critical for performing any physical diagnosis, such as swapping coils or plugs.
• Main Ground Point (2.5L I4) — For jump-starting purposes, an exposed bolt near the driver's side fuse box is designated as the primary negative ground terminal, as the battery is in the rear.. While not a common cause, a poor main engine ground can cause a variety of electrical issues, including misfires. This identified point is a key part of the main grounding circuit.
• OEM Wiring Diagrams — Available in the official, VIN-specific factory service manual in PDF format.. To diagnose a persistent P0303 caused by a wiring issue, a technician needs the factory manual to trace the specific wires for the ignition coil and fuel injector of cylinder 3 from the component to the ECM, including any intermediate connectors.

Real Owner Repair Stories

• Reddit user in r/GenesisMotors (2022 Genesis G80, 32,000 miles) — Check engine light with code P0303.
  ❌ Tried (didn't work) The owner took the car directly to the dealership.
  ✅ What actually fixed it The dealership performed a fuel injection cleaning service, and the code did not return.
• YouTube technician video (Genesis GV80 with 3.5L V6 (same engine)) — Check engine light, history code for cylinder #2 misfire (P0302).
  ❌ Tried (didn't work) The misfire was not present when the vehicle first came in, making it hard to diagnose.
  ✅ What actually fixed it The technician swapped the ignition coil from cylinder #2 to cylinder #4 as a preventative measure. The customer returned later with a P0304 code, confirming the ignition coil was faulty. The coil was then replaced.

Model Year Variations Within This Range

• 2021-2024: There were no significant mechanical changes to the 2.5T or 3.5T engines within this period that would alter the diagnosis of a P0303 code. A recall for a high-pressure fuel tube (21V208000) primarily affected early 2021 models with the 2.5T engine. All-wheel drive became standard on all G80 models for 2024. A major refresh with different interior tech and minor exterior changes occurred for the 2025 model year, which is outside this vehicle range.