How do I fix OBD fault code P0304?

Remove and inspect cylinder 3 spark plug for wear or fouling; replace if gap is incorrect or electrode is damaged.. Test or replace the ignition coil assigned to cylinder 3 if spark plug is good.. Clean or replace cylinder 3 fuel injector if fuel delivery is weak or clogged.

OBD Code P0304: Cylinder 3 Misfire Detected

Q: What does OBD code P0304 mean?

Cylinder 3 Misfire Detected. Cylinder 3 is misfiring, meaning its spark plug isn't igniting the fuel-air mixture reliably. It's like one piston in a 4-cylinder engine occasionally refusing to fire, causing rough running and lost power.

Q: What are the symptoms of fault code P0304?

Rough idle or engine vibration, Check Engine Light illuminated, Loss of power and fuel economy

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What This Actually Means

In plain language — no jargon

Cylinder 3 is misfiring, meaning its spark plug isn't igniting the fuel-air mixture reliably. It's like one piston in a 4-cylinder engine occasionally refusing to fire, causing rough running and lost power.

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Symptoms You May Notice

3 known symptoms for this code

Rough idle or engine vibration

Check Engine Light illuminated

Loss of power and fuel economy

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How Your ECU Detects This

Technical sensor logic and voltage thresholds

The ECU monitors crankshaft speed variation using the crank sensor to detect when a cylinder fails to combust. When cylinder 3 misfires, it creates a detectable dip in engine speed. The threshold is typically exceeded after a certain number of misfire events within a driving cycle.

Voltage & Parameter Thresholds

Parameter	Normal Range	Fault Condition
Crankshaft RPM Variation	Smooth acceleration without drops	Intermittent speed loss on cylinder 3 firing stroke
Misfire Count	0-2 misfires per 1000 revolutions	3+ misfires per 1000 revolutions

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Diagnostic & DIY Fix Guide

Check these in order — from cheapest to most complex

1

Spark Plug

Remove and inspect cylinder 3 spark plug for wear or fouling; replace if gap is incorrect or electrode is damaged.

2

Ignition Coil Pack

Test or replace the ignition coil assigned to cylinder 3 if spark plug is good.

3

Fuel Injector

Clean or replace cylinder 3 fuel injector if fuel delivery is weak or clogged.

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When to See a Professional Mechanic

Not all fault codes are safe to DIY

Code P0304 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.

Safety note: OBD-II codes identify the system or circuit where a fault was detected — they do not always identify the exact failed component. A professional mechanic using live sensor data will diagnose the root cause more accurately than replacing parts based on the code alone.

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How to Clear Code P0304

What happens after you fix the fault

Once the fault is repaired, P0304 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.

The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.

✅ Safe to Clear When

Fault has been diagnosed and repaired
You want to confirm the repair worked
Code appeared after a sensor was cleaned

⚠️ Do Not Clear When

Preparing for an emissions/PUC test
Root cause is still undiagnosed
Check engine light is flashing

Emissions test note: Clearing codes resets OBD readiness monitors. Most vehicles need 50–100 km of mixed driving before monitors complete. Do not clear codes immediately before an emissions or PUC inspection.