P0304 on 2021-2024 Genesis G80: Cylinder 4 Misfire Causes and Fixes

What's Unique About the 2021-2024 Genesis G80

For the 2021-2024 G80 equipped with the Theta III 2.5L T-GDI engine, a primary cause of P0304 and other misfire codes is a known software issue, especially during a cold start. Genesis has issued a Technical Service Bulletin (TSB 25-01-051G) to address this by updating the Engine Control Module (ECM) logic. This is also part of Service Campaign 914G, which notes that these vehicles may exceed federal and California emissions standards without the update. This unique, platform-specific issue should be the first consideration before replacing any physical parts on the 2.5T models.

Symptoms You May Notice

• Check Engine Light is on or flashing
• Rough or shaky idle
• Engine hesitation or stumbling during acceleration
• Reduced engine power
• Poor fuel economy
• Smell of unburned fuel from the exhaust
• Engine running unevenly

Most Likely Causes

1. Engine Control Module (ECM) Software Update Needed (2.5L T-GDI Only) 🔴 High Probability → Shop Engine Control Module (ECM) Genesis issued TSB 25-01-051G specifically for 2021-2024 G80 models with the 2.5L Turbo engine, which can throw misfire codes (including P0304) during cold starts due to software logic. This is also related to Service Campaign 914G for exceeding emissions standards.
   How to confirm: A Genesis dealer can check if Service Campaign 914G or TSB 25-01-051G is open for your vehicle's VIN. The campaign involves an ECM software update with improved misfire logic.
   Typical fix: The dealer will update the ECM software. This is typically performed at no cost under the service campaign and takes about 0.4 labor hours.
   Est. part cost: $0
2. Faulty Ignition Coil 🟡 Medium Probability → Shop Ignition Coil Ignition coils are a common failure point on most modern engines, including Hyundai/Genesis GDI engines, leading to misfires. Over time, the insulation can break down from heat and electrical stress.
   How to confirm: Swap the ignition coil from cylinder 4 with another cylinder (e.g., cylinder 1). Clear the codes and drive the vehicle. If the code changes to P0301, the coil is bad. For the 2.5L I4, the cylinders are numbered 1-2-3-4 from front (pulleys) to back (transmission). For the 3.5L V6, cylinder 4 is the front-most cylinder on the driver's side (left bank).
   Typical fix: Replace the faulty ignition coil. It is often recommended to replace all coils at the same time for preventative maintenance, especially if they are original and high-mileage.
   Est. part cost: $40-$80
3. Worn or Fouled Spark Plug 🟡 Medium Probability → Shop Spark Plug Spark plugs are a maintenance item. The recommended replacement interval for the G80 can be as early as 45,000 miles or as long as 97,500-100,000 miles depending on the source, so they can be a common cause on vehicles within this range.
   How to confirm: Remove the spark plug from cylinder 4 and inspect it for wear (worn electrode), carbon buildup, or oil/fuel fouling. Compare it to a plug from another cylinder. If it looks bad, it needs replacement.
   Typical fix: Replace the spark plug in cylinder 4. It is best practice to replace all spark plugs at the same time as a set to ensure consistent performance.
   Est. part cost: $15-$30
4. Clogged or Faulty Fuel Injector ⚪ Low Probability → Shop Fuel Injector Gasoline Direct Injection (GDI) engines like those in the G80 operate at very high fuel pressures, and injectors can become clogged with deposits over time, disrupting the spray pattern and causing misfires.
   How to confirm: This is more complex to diagnose. A professional can perform a fuel injector balance test. A DIY method involves listening to the injector with a mechanic's stethoscope to ensure it's clicking; however, this doesn't confirm flow rate or spray pattern. One could also swap injectors between cylinders, but this is a much more labor-intensive job than swapping coils or plugs.
   Typical fix: Clean or replace the faulty fuel injector.
   Est. part cost: $100-$200

Rare But Worth Checking

• Internal Engine Mechanical Problem: Although rare on a newer vehicle, issues like low compression from worn piston rings, a bad head gasket, or valve train problems (bent valves, valve seat issues) can cause a persistent misfire that doesn't resolve with ignition or fuel system parts. A compression test and leak-down test would be needed to confirm this.
• Failed Low-Pressure Fuel Pump: → Shop Fuel Pump There is a recall for a failing low-pressure fuel pump on 2022-2023 G80 models that can cause a loss of drive power. While this typically causes multiple misfires (P0300) or stalling, it could potentially manifest as a single cylinder misfire initially as the fuel pressure begins to drop.
• Carbon Buildup on Intake Valves: A known issue with GDI engines is that carbon can build up on the back of the intake valves over time, as fuel is injected directly into the cylinder and doesn't wash over the valves. 🎬 Watch: Why GDI engines misfire and how to fix them This can restrict airflow to a specific cylinder and cause a misfire. This usually requires a manual (walnut blasting) or chemical cleaning service.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0304 and check for any other codes. Note if the Check Engine Light is steady or flashing.
2. (For 2.5L Turbo engines) Contact a Genesis dealer with your VIN to see if TSB 25-01-051G / Service Campaign 914G is open for your vehicle. If so, have the ECM update performed first, as this is a known issue and a no-cost fix.
3. If the ECM update is not applicable or does not fix the issue, proceed with mechanical diagnosis.
4. Swap the ignition coil from cylinder 4 with another cylinder (e.g., cylinder 2). Cylinder 4 is the rearmost cylinder on the 2.5L I4, and the front cylinder on the driver's side on the 3.5L V6. Clear the codes and run the engine. If the code changes to P0302, the ignition coil is the faulty part.
5. If the misfire remains on cylinder 4, inspect the spark plug. Remove it and check for excessive wear, fouling, or damage. If it is suspect, replace it. It's best practice to replace all plugs as a set.
🎬 See this walkthrough for replacing spark plugs on a V6
6. If the misfire still remains on cylinder 4, inspect the fuel injector wiring and connector for damage. Listen for a consistent clicking sound from the injector with a mechanic's stethoscope.
7. If ignition and fuel injector seem okay, a compression test and/or a cylinder leak-down test should be performed to check for mechanical engine problems like bad rings or valves. This is best left to a professional.

Parts You'll Likely Need

• Ignition Coil (2.5L T-GDI) (OEM #27300-2T000) — A common cause of a single-cylinder misfire. The coil is responsible for generating the high voltage needed for the spark plug.
  Trusted brands: Hyundai/Genesis OE, Bosch, Duralast
  OEM price range: $60-$90
  Aftermarket price range: $40-$70
• Ignition Coil (3.5L Twin-Turbo V6) (OEM #27301-3L100) — A common cause of a single-cylinder misfire on the V6 engine.
  Trusted brands: Hyundai/Genesis OE, NGK, Delphi
  OEM price range: $50-$80
  Aftermarket price range: $35-$60
• Spark Plug (2.5L T-GDI) (OEM #18875-09085) — Spark plugs are wear items and can become fouled or worn out, preventing a proper spark.
  Trusted brands: Hyundai/Genesis OE (NGK), Denso
  OEM price range: $25-$40
  Aftermarket price range: $15-$25
• Spark Plug (3.5L Twin-Turbo V6) (OEM #18877-09085) — Spark plugs are wear items and must be replaced at scheduled intervals.
  Trusted brands: Hyundai/Genesis OE (NGK), Champion
  OEM price range: $25-$40
  Aftermarket price range: $15-$25

Related Codes That Often Appear With This One

• P0300 — If the misfire is random or affects multiple cylinders, P0300 (Random/Multiple Cylinder Misfire Detected) will be stored alongside or instead of P0304.
• P0301, P0302, P0303 — These codes indicate misfires in cylinders 1, 2, and 3, respectively. The presence of these codes along with P0304 points to a problem affecting the entire engine, such as a fuel delivery issue, major vacuum leak, or the ECM software issue on the 2.5T.

Technical Service Bulletins (TSBs) & Recalls

• 25-01-051G DN / 25-01-051G TSB: Addresses misfire DTCs P0300-P0304 on 2.5L T-GDI engines during cold start by providing an ECM software update.
• Service Campaign 914G: The customer-facing campaign related to TSB 25-01-051G, addressing emissions and cold-start misfires with an ECM update.

Platform-Specific Known Issues

• A significant number of 2021-2024 G80 models with the 2.5L T-GDI engine are subject to Service Campaign 914G and TSB 25-01-051G, which addresses misfire codes (P0304) via an ECM software update.

Mechanic-Grade Diagnostic Values

• High-Pressure Fuel Pump (HPFP) Operating Pressure — expected: >2,000 PSI, can exceed 2,500 PSI. Failure: Significantly lower pressure readings under load, which may not always set a specific fuel pressure code but can cause lean misfires.
• Low-Pressure Fuel Pump (LPFP) In-Tank Pressure — expected: 50-80 PSI. Failure: Pressure below 50 PSI indicates a failing in-tank pump, which will starve the HPFP.
• Fuel Injector Resistance — expected: While no exact spec was found, all injectors should have nearly identical resistance (within 0.5 ohms of each other). A common range for similar GDI injectors is 12-17 ohms.. Failure: An injector with a resistance reading that is significantly higher or lower than the others is faulty.
• Misfire Count (Mode $06) — expected: 0 or very low single digits during normal operation.. Failure: A rapidly increasing count on a specific cylinder (e.g., TID $AC for cylinder 4) confirms a persistent misfire, even before the count is high enough to trigger a CEL.

Hidden / Shadow Codes Worth Checking

• Mode $06, TID $AC: Misfire count for cylinder 4 for the last/current driving cycle. This is not a 'code' but raw data from the ECU's self-test monitor. (see via A professional scan tool or an advanced OBD-II reader that can display Mode $06 test results. The value represents the number of misfire events counted.)

Scan Tool Commands That Help

• Genesis GDS (or equivalent professional scanner): Cylinder Power Balance Test — Use this active test when a misfire is felt but the specific cylinder is uncertain, or to confirm a P0304. The tool deactivates each cylinder one by one and measures the RPM drop. If deactivating cylinder 4 causes little or no RPM drop, it confirms that cylinder was not contributing power.
• Genesis GDS (or equivalent professional scanner): Fuel Injector Kill — This is a specific method for performing the Power Balance Test. Disabling the fuel injector is safer for the catalytic converter than disabling spark, as it prevents unburned fuel from entering the exhaust.

Wiring & Ground Locations

• Engine Compartment Fuse Box — Located in the engine bay, contains relays and fuses critical to engine operation, including those for the ECU and ignition system.. A corroded terminal or blown fuse in this box could cut power to the ignition coils or fuel injectors.
• Passenger Compartment Fuse Box — Located behind the driver's side dash panel bolster.. Contains fuses for various control modules, including the Data Link Connector (OBD-II port) itself, which is necessary for diagnosis.
• Ignition Coil Wiring Harness — Runs along the top of the engine to each individual ignition coil.. This harness is susceptible to heat damage or rodent damage. A chewed or broken wire leading to the cylinder 4 ignition coil will cause a P0304 and will not be fixed by replacing the coil itself.