2021 Honda Civic Type R Misfire DTC P0304

Symptoms

Owners of affected 2021 Honda Civic Type R vehicles may notice the Malfunction Indicator Lamp (MIL) illuminating. This will be accompanied by a stored diagnostic trouble code (DTC) P0304, indicating a "No. 4 Cylinder Misfire detected."

Possible Cause

Honda has identified that water can enter the hood scoop and accumulate within the No. 4 spark plug hole. This water can corrode critical components such as the ignition coil, plug tube, and spark plug, leading to a misfire in the No. 4 cylinder. If this misfire occurs, the vehicle's system will store the DTC P0304.

To address this, Honda has redesigned the cylinder head cover to divert water away from the No. 4 spark plug hole and ignition coil, mitigating future water intrusion through the hood scoop.

Important Warnings

Customer Information: Honda emphasizes that the information in this bulletin is intended for use exclusively by skilled technicians equipped with the proper tools, equipment, and training. These procedures should not be attempted by "do-it-yourselfers." Owners should contact an authorized Honda automobile dealer to determine if this information applies to their vehicle and to have the condition properly addressed.

Sodium-Filled Valves: For Repair Procedure B, technicians must carefully read and follow the "Precaution for Handling Sodium-Filled Valves" instructions, as the exhaust valves contain metal sodium. This highlights the specialized nature of the repair.

Diagnosis and Inspection

To diagnose the Honda Civic Type R P0304 issue, a technician will connect a Honda Diagnostic System (HDS) to the vehicle's Data Link Connector (DLC) and confirm that DTC P0304 is stored.

Next, the engine cover and the No. 4 ignition coil are removed. The technician will then inspect the No. 4 spark plug tube for any residual water or corrosion. The spark plug is also removed and inspected for corrosion. Following this, the remaining ignition coils and the cylinder head cover are removed for a thorough inspection of the No. 4 ignition coil, plug tube, and spark plug for water or corrosion.

• If no water or corrosion is found: This service bulletin (TSB 25-062) does not apply, and normal troubleshooting procedures should be followed.
• If water or corrosion is found: The technician will clean excessive rust and corrosion from the spark plug tube using a burgundy-colored abrasive pad. After cleaning, the spark plug tube is re-inspected for small holes or cracks. A seal guide tool is used to assess the extent of corrosion.
  • If corrosion on the No. 4 spark plug tube exceeds 12 mm or extends beyond the bottom of the seal tube guide, or if holes/cracks are present, photos of the corrosion/damage are required, and Repair Procedure B is initiated.
  • If corrosion is less than 12 mm and no holes/cracks are found, Repair Procedure A is followed.

Repair Procedure Summary

There are two main repair procedures outlined in TSB 25-062, depending on the severity of the corrosion:

Repair Procedure A: Cylinder Head Cover, Ignition Coil, and Spark Plug Replacement

This procedure is performed if corrosion is minor. It involves:

1. Installing a seal tube guide tool onto the clean No. 4 cylinder plug tube.
2. Applying engine oil to the new head cover plug seal and carefully sliding the new cylinder head cover over the seal tube guide and onto the cylinder head.
3. Using a seal tube setter tool to properly seat the plug tube in the cylinder head cover.
4. Removing the setter tool and seal guide.
5. Installing a new spark plug into the No. 4 cylinder and reinstalling all four ignition coils.
6. Reinstalling the cylinder head cover bolts and other removed parts.
7. Clearing all stored DTCs.

Repair Procedure B: Cylinder Head Replacement

This more extensive procedure is required if severe corrosion (exceeding 12 mm) or damage (holes/cracks) is found on the spark plug tube. Before starting, technicians must submit photos of the excessive rust corrosion or damage. This procedure involves removing and replacing the entire cylinder head, along with associated components. It is a significantly more time-intensive repair.

Required Tools and Parts

Technicians will utilize specific Honda parts and materials for these repairs:

• Key Parts (Procedure A):
  • Cover Assy., Cylinder Head (Part No. 12310-5BA-A01)
  • Coil Assy., Plug Top (Ignition Coil) (Part No. 30520-59B-023)
  • Spark plug (ILZKAR8H8S) (NGK) (Part No. 12290-59B-003)
• Key Parts (Procedure B):
  • General Assy., Cylinder Head (Part No. 10003-RPY-G01)
  • Gasket Kit, Cyl. Head (Part No. 06110-RPY-G02)
  • In addition to many other gaskets, O-rings, and bolts.
• Required Materials:
  • Hondabond HT (Part No. 08718-0004)
  • Burgandy colored abrasive pad (commercially available)
  • Honda Long-Life Antifreeze/Coolant Type 2 (Part No. 0L999-9011)
• Special Tool:
  • Guide, Valve Cover (Set) (Part No. 07AAG-RPYA100)

Owner Action

If your 2021 Honda Civic Type R is displaying the MIL with DTC P0304, or if you suspect a misfire in cylinder No. 4, it is recommended to contact an authorized Honda dealer. They can perform the necessary diagnostic steps outlined in Service Bulletin 25-062 to determine the exact cause and apply the appropriate repair procedure. Repairs performed under warranty will typically be free of charge.

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Source: NHTSA ID 11022041