What does DTC P0304 mean for my Honda Civic Type R?

DTC P0304 indicates a 'Misfire No. 4 Cylinder' has been detected in your 2021 Honda Civic Type R. This often triggers the Malfunction Indicator Lamp (MIL) on your dashboard.

What causes the P0304 misfire in the 2021 Honda Civic Type R?

The misfire is typically caused by water entering through the hood scoop and accumulating in the no. 4 spark plug hole. This leads to corrosion of the ignition coil, plug tube, and spark plug, resulting in the Honda Civic Type R P0304 misfire.

Is there a fix for the Honda Civic Type R P0304 misfire?

Yes, Honda Service Bulletin 25-062 outlines two repair procedures. Depending on the severity of corrosion, it involves either replacing the cylinder head cover, spark plug, and ignition coil, or a more extensive cylinder head replacement. A redesigned cylinder head cover is used to prevent future water intrusion.

Should I attempt to fix the P0304 misfire myself?

No, Honda explicitly states that the procedures in Service Bulletin 25-062 are intended for skilled technicians with proper tools and training. It is recommended to contact an authorized Honda dealer to diagnose and repair the issue.

Boletín de servicio

2021 Honda Civic Type R P0304 Misfire TSB 25-062

Honda has issued Service Bulletin 25-062 for 2021 Civic Type R vehicles experiencing a 'Misfire No. 4 Cylinder' and diagnostic trouble code P0304. This issue is caused by water entering the hood scoop, leading to corrosion of the ignition coil, plug tube, and spark plug.

David Chen2021 Honda Civic Type RApril 25, 2026

2021 Honda Civic Type R — manufacturer_communication document image 1 — Source: NHTSA manufacturer_communication 11024893

DTC P0304Honda TSB 25-062Misfire No. 4 CylinderMIL comes on with stored DTC P0304Honda Civic Type R P0304 misfireignition coil corrosionspark plug tube corrosioncylinder head cover replacement10003-RPY-G0107AAG-RPYA100

Owners of the 2021 Honda Civic Type R may encounter an illuminated Malfunction Indicator Lamp (MIL) accompanied by the diagnostic trouble code (DTC) P0304, indicating a "Misfire No. 4 Cylinder" detection. This issue is addressed by Honda Service Bulletin 25-062, released on October 13, 2025.

Symptoms

The primary symptom owners will notice is the MIL illuminating on the dashboard. When scanned, the vehicle's onboard diagnostic system will have the DTC P0304 stored, specifically identifying a misfire in the number 4 cylinder.

Fault Codes / DTCs

P0304: No. 4 Cylinder Misfire detected.

Diagnosis / Trigger Conditions

The root cause of this Honda Civic Type R P0304 misfire is water intrusion. Water can enter through the hood scoop and accumulate in the no. 4 spark plug hole. This accumulation leads to corrosion of the ignition coil, plug tube, and spark plug, which then causes the misfire condition.

Honda has since redesigned the cylinder head cover to help divert water away from the no. 4 spark plug hole and ignition coil, mitigating future water entry through the hood scoop.

Technicians will inspect the no. 4 cylinder spark plug base for any signs of water accumulation or corrosion. If no water or corrosion is found, this bulletin does not apply, and normal troubleshooting procedures should be followed. If corrosion is present, the repair proceeds based on its severity.

Repair Procedure Summary

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

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

This procedure is performed if corrosion is found but is not excessive (less than 12 mm or no holes/cracks in the plug tube). It involves:

Inspecting the no. 4 spark plug tube and cleaning any rust or corrosion using an abrasive pad.
Installing a seal tube guide tool onto the no. 4 cylinder plug tube.
Applying fresh engine oil to the new head cover plug seal and carefully sliding the new cylinder head cover over the seal tube guide onto the cylinder head.
Using a seal tube setter tool to properly seat the plug tube in the cylinder head cover.
Installing a new spark plug in the no. 4 cylinder and reinstalling all four ignition coils.
Reinstalling the cylinder head cover bolts and other removed components.
Clearing all stored DTCs.

Repair Procedure B: Cylinder Head Replacement

This more extensive procedure is required if the corrosion on the no. 4 spark plug tube exceeds 12 mm or if there are small holes or cracks in the tube. This involves a full cylinder head replacement, including all steps from Procedure A, plus additional work related to removing and installing the cylinder head itself. Photo submission of the excessive rust corrosion or damage to the plug tube, along with the VIN plate, is required for warranty claims.

Required Tools or Parts

For Repair Procedure A:

Cover Assy., Cylinder Head (Part Number: 12310-5BA-A01)
Rubber set, Breather (Part Number: 17136-5BF-A00)
O-ring (10.2 x 1.8) (Part Number: 91307-RCA-A01)
O-ring (14.8 x 1.8) (NOK) (Part Number: 91308-RCA-A01)
Coil Assy., Plug Top (Ignition Coil) (Part Number: 30520-59B-023)
Spark plug (ILZKAR8H8S) (NGK) (Part Number: 12290-59B-003)
Hondabond HT (Part Number: 08718-0004)
Burgandy colored abrasive pad (commercially available)
Seal Slider Set (Tool Number: 07AAG-RPYA100)

For Repair Procedure B (includes all parts from A, plus):

Nut, Self-Lock (10 mm) (Part Number: 90012-59B-004)
Gasket Kit, Cyl. Head (Part Number: 06110-RPY-G02)
Gasket, Turbocharger Pipe (Part Number: 19524-RPY-G01)
Pipe Set, Fuel Joint (Part Number: 16012-RPY-315)
Bolt, Socket (6 x 20) (Part Number: 96600-06020-18)
Pipe, Fuel Feed (Part Number: 16013-RPY-305)
Clip, Injector (Part Number: 16451-RPY-G01)
O-ring (Part Number: 91310-RPY-G01)
Disk, Supporting (Part Number: 91312-RPY-G01)
Washer, Sealing (20 mm) (Part Number: 12209-PR3-000)
General Assy., Cylinder Head (Part Number: 10003-RPY-G01)
Honda Long-Life Antifreeze/Coolant (Type-2) (Part Number: 08CLA-P99-0F0A8)

Important Warnings

For Repair Procedure B, technicians must carefully read the "Precaution for Handling Sodium-Filled Valves" and follow all instructions, as the exhaust valves contain metal sodium. Additionally, before proceeding with Repair Procedure B, it is crucial to photograph the spark plug tube if corrosion measurement exceeds 12 mm.

Owner Action

If your 2021 Honda Civic Type R is experiencing a Honda Civic Type R P0304 misfire or displaying the "Misfire No. 4 Cylinder" message, it is recommended to contact an authorized Honda automobile dealer. They have the specialized tools, equipment, and training required to properly diagnose and address this specific issue according to Service Bulletin 25-062. Do-it-yourself repairs are not advised for this complex procedure.

NHTSA ID: 11024893

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Frequently Asked Questions

What does DTC P0304 mean for my Honda Civic Type R?: DTC P0304 indicates a 'Misfire No. 4 Cylinder' has been detected in your 2021 Honda Civic Type R. This often triggers the Malfunction Indicator Lamp (MIL) on your dashboard.
What causes the P0304 misfire in the 2021 Honda Civic Type R?: The misfire is typically caused by water entering through the hood scoop and accumulating in the no. 4 spark plug hole. This leads to corrosion of the ignition coil, plug tube, and spark plug, resulting in the Honda Civic Type R P0304 misfire.
Is there a fix for the Honda Civic Type R P0304 misfire?: Yes, Honda Service Bulletin 25-062 outlines two repair procedures. Depending on the severity of corrosion, it involves either replacing the cylinder head cover, spark plug, and ignition coil, or a more extensive cylinder head replacement. A redesigned cylinder head cover is used to prevent future water intrusion.
Should I attempt to fix the P0304 misfire myself?: No, Honda explicitly states that the procedures in Service Bulletin 25-062 are intended for skilled technicians with proper tools and training. It is recommended to contact an authorized Honda dealer to diagnose and repair the issue.

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