Misfire in cylinder 4. If coolant level is dropping without visible leak, suspect head gasket failure.
SeverityCritical
CategoryEngine
Est. Repair Cost$50–$500
Code TypePowertrain
DIY RepairYes
DifficultyIntermediate
⚡ Common Causes
›Fouled spark plug
›Failed ignition coil
›Injector clog
›Cracked piston ring or head gasket failure
⚠️ Symptoms
›Rough running
›White or blue smoke from exhaust
›Poor fuel economy
›Overheating
✅ How to Fix — Step by Step
Step 1:Inspect and replace spark plug
Step 2:Swap and test coil
Step 3:Replace injector if no pulse
Step 4:Check for coolant in oil — possible head gasket
🔧 Diagnostic Tools Required
✔OBD-II Diagnostic Scanner
✔Digital Multimeter
✔Fuel Pressure Gauge
✔Compression Tester
✔Spark Tester
Using the correct diagnostic tools helps identify the root cause of P0304 before replacing parts unnecessarily. Always confirm the fault using live data and manufacturer-recommended diagnostic procedures.
✘Replacing ignition and fuel parts repeatedly without checking coolant level trends over time
✘Dismissing white exhaust smoke as normal cold-weather condensation
✘Not checking engine oil for a milky appearance (sign of coolant intrusion)
✘Skipping a cooling system pressure test when the cause isn't obviously electrical
📋 Technical Service Bulletins
Some engine families have documented head gasket failure patterns at specific mileage ranges — check your manufacturer's TSB database by VIN if coolant loss accompanies this code, since it can point straight to a known issue rather than a guessing game.
🏭 Manufacturer-Specific Notes
Subaru P0304
Certain Subaru boxer engine generations have documented head gasket weak points — a misfire combined with slow coolant loss is a classic early symptom worth investigating before it worsens.
Honda P0304
On most Honda four-cylinder engines, a cylinder 4 misfire is far more often a fouled plug or coil issue than a head gasket problem — rule out the simple causes first.
Jeep P0304
Jeep Cherokee 2.4L engines have had documented oil consumption issues that can foul spark plugs and trigger cylinder-specific misfires — check oil consumption history.
🎓 Expert Tip: A misfire on one cylinder that comes with a slow, unexplained coolant level drop and no visible external leak is worth pressure-testing the cooling system for early — catching a head gasket issue before it fully fails is far cheaper than after.
💡 Pro Tip: If coolant level drops without a visible leak, suspect head gasket failure alongside the misfire.
Repair Cost Estimate
Full Repair Estimate
$50–$500
DIY Possible?
YES
Severity
Critical
Diagnosis Time
30–60 minutes (longer if a head gasket test is needed)
Repair Time
1 hour for ignition/fuel parts; 6–12 hours if a head gasket replacement is required
Frequently Asked Questions
What does OBD2 code P0304 mean?+
P0304 stands for Cylinder 4 Misfire Detected. Misfire in cylinder 4. If coolant level is dropping without visible leak, suspect head gasket failure.
What causes code P0304?+
Fouled spark plug; Failed ignition coil; Injector clog; Cracked piston ring or head gasket failure
How do I fix code P0304?+
Inspect and replace spark plug; Swap and test coil; Replace injector if no pulse; Check for coolant in oil — possible head gasket
How much does it cost to fix P0304?+
Estimated repair cost for P0304 is $50–$500.
Is P0304 DIY-friendly?+
Yes — P0304 is generally DIY-friendly with basic tools.
References
SAE J1979 — E/E Diagnostic Test Modes (OBD-II communication standard)
ISO 15031 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics
Vehicle manufacturer service manual (consult your specific make and model)
