Your 6th Gen (EK/EM), 7th Gen (EM2/ES1), 8th Gen (FA5/FG2), 9th Gen (FB6/FG4), 10th Gen (FC1/FK7), or 11th Gen (FL1/FL5) Civic just threw a P0304 — and Cylinder 4 is experiencing a Misfire.
P0304 is the code for “Cylinder 4 Misfire Detected”. The ECU is indicating that Cylinder 4 is not firing properly. The Crankshaft position (CKP) sensor detected a reduction in RPM momentum following the Cylinder 4 spark event.
Honda Cylinder Numbering: Honda counts cylinders 1-2-3-4 from left to right (passenger side/drive belt side to transmission side). Cylinder 4 is the far right cylinder — closest to the transmission.
If the misfire is intermittent, the check engine light may remain steady. If it is a continuous misfire that is sending unburned fuel into the Catalytic Converter, the light may flash. Continued driving with a flashing CEL is not recommended — catalytic converter damage may occur.
Here is a garage-tested breakdown of what P0304 actually means on these cars, and how to repair it without unnecessary expense AT the dealership.
P0304 Breakdown: Understanding Cylinder 4 Misfire
P0304 (“Cylinder 4 Misfire Detected”) indicates that misfires are being recorded specifically on Cylinder 4.
Cylinder 4 Considerations: Cylinder 4 is located in the hottest zone of the engine bay, closest to the transmission. On K-Series engines, it is at the end of the fuel rail, making it more susceptible to lean conditions if fuel pressure drops.
The ECU monitors crankshaft speed variations through the crankshaft position sensor (CKP). Each cylinder firing produces a slight acceleration of the crankshaft. When Cylinder 4 fails to fire properly, the crank slows down more than expected, and the ECU stores P0304.
The Symptoms:
- Rough, unstable idle
- Hesitation and loss of power during acceleration
- Reduced performance when VTEC engages
- Fuel odor or sulfur smell from the exhaust
- Flashing check engine light — continued driving is not recommended
The Diagnostic Swap Test
Before replacing components, use the following diagnostic procedure to identify the cause.
Step 1: The Ignition Coil Swap Test.
Turn the engine OFF. Remove the plastic engine cover. Remove the ignition coil from Cylinder 4 (far right, closest to the transmission). Swap it with the ignition coil from Cylinder 2 (second from the passenger side).
Clear the codes with an OBD2 scanner, start the vehicle, and drive until the check engine light returns.
The Verdict:
- If the code changes to P0302 (Cylinder 2 Misfire): The ignition coil is faulty. Replace it.
- If the code remains P0304: The coil is likely functional. Proceed to Step 2.
Step 2: The Spark Plug Swap Test.
Remove the spark plug from Cylinder 4. Inspect the tip. Is it oil-fouled? Is the electrode gap excessive or damaged?
Swap the plug into Cylinder 1. Clear codes and drive.
The Verdict:
- If the code follows the plug to P0301: Replace the spark plug(s). Use NGK Laser Iridiums or equivalent quality plugs.
- If the code remains P0304: The issue may be mechanical (valve clearance) or fuel-related (injector).
Component Quality: Use OEM Denso, Hitachi, or NGK components. Aftermarket coils are known to cause recurring misfire issues.
Common Causes of P0304
1. Ignition Coil Failure.
Honda coil packs (factory Denso or Hitachi) can fail from age and heat cycles. Internal insulation may break down, causing spark to arc against the spark plug tube.
⚠️ Moisture Intrusion (Type R Specific). On the 10th Gen Civic Type R (FK8), the Hood scoop design may allow water to reach the Cylinder 4 area. This can lead to corrosion on ignition coils and moisture in the spark plug tube. If P0304 is present on a Type R, inspect the coil boot for rust first.
2. Tight Exhaust Valve Clearance.
Mechanical K-series (K20Z3, K24Z7) and R-series (R18) engines do not have hydraulic lifters. Over time, exhaust valve seats may wear, and valve clearance may decrease. When the engine warms up, valve stem expansion can prevent the exhaust valves from closing completely. This can cause compression loss in Cylinder 4, resulting in rough idle and a static P0304 code that may diminish when the engine is revved.
3. Oil Contamination from Spark Plug Tube Seals.
If the Valve Cover spark plug tube seals are leaking, Cylinder 4’s spark plug well may fill with engine oil. The oil can affect the ignition coil boot, preventing proper spark delivery.
4. Clogged or Leaking Fuel Injector (10th/11th Gen Direct Injection).
On direct-injected L15B7 and K20C1 engines, fuel injectors can become restricted or fail. Cylinder 4 running lean or rich can cause a misfire that does not respond to coil or plug swaps.
Generation-Specific Issues
8th & 9th Gen Base Models (R18A1 / R18Z1):
- If coils and plugs are functional but rough idle and P0304 occur on a cold start, a valve adjustment may be required.
- The exhaust valves on the R18 may tighten every 40k–50k miles, affecting compression on Cylinder 4.
- Specifications: Intake: 0.008–0.010 in (0.20–0.25 mm), Exhaust: 0.010–0.012 in (0.25–0.30 mm).
8th & 9th Gen Si (K20Z3 / K24Z7):
- ⚠️ Fuel Starvation & Heat Soak. The stock fuel rail feeds fuel from the driver’s side (Cylinder 1) and terminates at Cylinder 4. On modified engines, Cylinder 4 is the first to experience lean conditions if fuel pressure drops.
- Cylinder 4 is also located near the transmission bellhousing, which can contribute to localized heat soak that may affect the ignition coil and spark plug.
- Consideration: For high-power builds, upgrading to an AEM or Hybrid Racing fuel rail ($140–$180) may help address fuel delivery issues.
10th & 11th Gen 1.5T (L15B7 / Direct Injection):
- ⚠️ TSB A19-033 (2016–2018 1.5T). Before replacing components, check with a Honda dealer regarding Technical Service Bulletin A19-033, which addresses P0304, P0300, P0301, P0302, P0303, and P0172 on 2016–2018 Civics with the 1.5T. The fix may be a PGM-FI and TCM software update, which may be covered.
- P0304 + P0172 (System Too Rich). If both codes are present, Fuel Injector failure or fuel dilution may be the cause. A fuel odor on the dipstick is a common indicator.
- Fuel Injector Failure. If the TSB does not apply, the cause may be a leaking fuel injector on Cylinder 4. Replacement of all four injectors with updated OEM part 16010-59B-315 and the fuel joint pipe is recommended. When one injector fails on the 1.5T, replacing all four as a set is recommended.
- Injector Wire Loom Chafing. On early 10th Gen Civics (FC1/FK7), the Wiring Harness near the high-pressure fuel pump and Cylinder 4 injector may rub against structural components, causing intermittent grounds and P0304 under load. Inspect the harness for chafing.
- Carbon Buildup. Direct-injected engines may experience carbon buildup on the intake valves. A walnut blasting service may be required by 80k miles.
- LSPI Warning. A static Cylinder 4 misfire under boost may indicate a bent connecting rod or cracked piston ring land due to LSPI (Low-Speed Pre-Ignition).
10th & 11th Gen Type R (K20C1, FK8/FL5):
- ⚠️ Moisture Intrusion (FK8 Specific). On the Civic Type R (FK8), the hood scoop design may allow water to reach the Cylinder 4 area, leading to corrosion on ignition coils and moisture in the spark plug tube. If P0304 is present, inspect the coil boot for rust first.
- If running high boost with an aftermarket tune, a misfire on Cylinder 4 may cause the ECU to reduce boost targets and adjust timing.
- Inspect the fuel injector on Cylinder 4 — high-pressure injectors can fail under heavy load.
OEM Part Supersession Information
Honda has updated both injectors and coils on the 1.5T engine. Use the updated part numbers when replacing components.
Fuel Injector Update:
- Old part: 16010-59B-305
- New part: 16010-59B-315 (updated, improved reliability)
- Recommendation: Use the updated 16010-59B-315 part.
Ignition Coil Update:
- Old part: 30520-59B-013
- New part: 30520-59B-023 (updated, improved moisture resistance)
- Recommendation: Use the updated 30520-59B-023 part, particularly for FK8 Type R applications.
Parts, Cost Breakdown & US Sourcing
| Part | Fits | OEM Part Number | Online Cost | Dealer Cost |
|---|---|---|---|---|
| Ignition Coil (Hitachi) | 8th/9th Gen K-series | 30520-R40-007 (Hitachi IGC0071) | $35–$55 | $80–$120 |
| Ignition Coil (Denso, Updated) | 10th Gen L15B7 1.5T | 30520-59B-023 (supersedes -013) | $45–$65 | $90–$140 |
| Ignition Coil (CTR FK8/FL5) | 10th/11th Gen K20C1 | Verify by VIN | $50–$75 | $100–$160 |
| NGK Laser Iridium Plug | 8th/9th Gen K-series | ILZKAR8H8S | $12–$15 each | $20–$25 each |
| NGK Laser Iridium Plug | 8th Gen R18 | IZFR6K11S (Stock No. 5266) | $10–$14 each | $18–$22 each |
| NGK Laser Iridium Plug | 10th Gen 1.5T | ILZKAR8H8S | $14–$18 each | $22–$28 each |
| Fuel Injector Set (4, Updated) | 10th Gen L15B7 1.5T | 16010-59B-315 (supersedes -305) | $280–$450 | $400–$600 |
| Fuel Rail (AEM / Hybrid Racing) | 8th/9th Gen K-series | Aftermarket | $140–$180 | N/A |
| Valve Cover Gasket Set | 8th/9th/10th Gen | Verify by VIN | $25–$45 | $60–$100 |
Repair Cost Breakdown
| Repair | DIY Cost | Independent Shop | Honda Dealer |
|---|---|---|---|
| Ignition Coil Replacement (1) | $35–$65 | $100–$180 | $180–$280 |
| Spark Plug Replacement (4) | $40–$70 | $150–$250 | $250–$380 |
| Valve Adjustment (8th/9th Gen) | $25–$45 (gaskets) | $250–$400 | $350–$550 |
| Walnut Blasting (10th/11th Gen) | N/A (special equipment) | $400–$600 | $500–$800 |
| Fuel Injector Replacement (Set of 4) | $280–$450 | $600–$1,000 | $800–$1,400 |
| Fuel Rail Upgrade (K-series) | $140–$180 | $250–$400 | N/A |
| TSB A19-033 Software Update | $0 (if covered) | N/A | $0 (if covered) |
| Dealer Diagnostic Fee | N/A | N/A | $120–$200 |
Sourcing Tip: We track live market rates and cross-reference part numbers directly from US OEM catalogs such as HondaPartsNow and popular distributors like RockAuto and Fitment Industries.
Used Buyer Warning
If you are test-driving a Civic Si, R18, or 10th Gen Turbo, and the vehicle is idling rough with a P0304 code, proceed with caution.
If the seller claims it is a minor issue — perform the coil swap test or have the vehicle inspected.
Why this matters:
- 8th/9th Gen K-series: Cylinder 4 fuel starvation is a known issue on modified engines. If the vehicle has aftermarket intake and exhaust but no upgraded fuel rail, the engine may have been running lean on Cylinder 4 for an extended period. This can lead to cylinder wall scoring or piston damage — $3,000+ rebuild.
- 10th/11th Gen 1.5T: A static Cylinder 4 misfire under boost may indicate a bent connecting rod or cracked piston ring land due to LSPI. This can require a $3,000+ engine rebuild.
- FK8 Type R: If the seller mentions “it just needs a coil” but the coil boots are rusted, water may have been present in the spark plug tube. This can lead to cylinder head corrosion — $1,500+ repair.
- 10th Gen TSB Eligibility: Verify if the vehicle is eligible for TSB A19-033. If the seller did not address a free software update, other maintenance may have been neglected.
- If the seller cannot clear the code and demonstrate smooth operation, consider this a warning sign.
Used buyer rule: P0304 with rough idle should be treated with caution — perform a compression test or have the vehicle evaluated by a professional.
Quick Reference Checklist
☐ P0304 = Cylinder 4 Misfire Detected — ECU detected crankshaft deceleration on Cylinder 4
☐ Flashing Check Engine Light? Unburned fuel may be entering the catalytic converter. Continued driving is not recommended.
☐ Cylinder 4 location: Far right (closest to the transmission)
☐ Perform the Ignition Coil Swap Test — swap coil 4 to cylinder 2. Code follows to P0302? Bad coil. Replace it.
☐ Perform the Spark Plug Swap Test — swap plug 4 to cylinder 1. Code follows to P0301? Bad plug.
☐ 8th/9th Gen R18/K-series: Cold start misfire that diminishes when warm = tight exhaust valves — valve adjustment may be required
☐ 8th/9th Gen Si (K20Z3/K24Z7): Fuel starvation on Cylinder 4 on modified builds — consider upgrading to an AEM/Hybrid Racing fuel rail
☐ 2016–2018 1.5T Owners: Check TSB A19-033 first — a software update may resolve the issue
☐ 10th/11th Gen L15B7: Fuel odor on dipstick + P0304 = leaking fuel injector — replace all four with updated part 16010-59B-315
☐ 10th/11th Gen L15B7: P0304 + P0172 may indicate fuel dilution or injector failure
☐ 10th/11th Gen L15B7: Check wiring harness near Cylinder 4 injector for chafing — intermittent grounds can cause P0304 under load
☐ FK8 Type R (CTR): Inspect Cylinder 4 coil boot for rust — moisture intrusion through hood scoop is a known consideration
☐ 10th/11th Gen L15B7: Carbon buildup on intake valves = walnut blasting may be required at 60k–90k miles
☐ 10th/11th Gen Turbo: Misfire under boost may indicate possible bent rod or cracked ring land from LSPI
☐ OEM Part Supersession: Injectors updated from 16010-59B-305 to 16010-59B-315. Coils updated from 30520-59B-013 to 30520-59B-023. Use the updated parts.
☐ Avoid low-cost aftermarket coils or plugs — use Hitachi, Denso, or NGK
☐ Used Buyer Rule — P0304 on 8th/9th Gen K-series may indicate fuel starvation damage; on 10th/11th Gen, possible bent rod, Head Gasket, or neglected TSB. Thorough inspection is recommended before purchase.