P0304
Cylinder 4 Misfire DetectedP0304 is a generic OBD-II powertrain diagnostic trouble code: Cylinder 4 Misfire Detected. It is logged by the engine control unit when the misfire monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0304 means
Code P0304 is set when the engine control module (ECM/PCM) detects a misfire condition in cylinder number 4. The PCM monitors crankshaft rotational velocity via the crankshaft position sensor (CKP) and camshaft position sensor (CMP): each time a cylinder fires, a slight acceleration spike is expected. When cylinder 4 fails to contribute its share of rotational force, the PCM detects the resulting velocity drop and logs P0304. If the misfire rate exceeds the threshold for catalytic converter damage (typically 1–2% of firing events within 200 crankshaft revolutions), the MIL will flash rather than illuminate steadily — a warning that raw fuel is entering and overheating the catalyst.
Because P0304 is cylinder-specific rather than a global misfire (P0300), the fault almost always points to a component exclusive to cylinder 4: the spark plug, ignition coil, or fuel injector. However, low compression due to a burnt exhaust valve, worn piston rings, or a leaking head gasket can also prevent the air-fuel charge from igniting properly and must be ruled out when ignition and fuel components test good. Left unrepaired, a sustained misfire will wash cylinder walls with raw fuel, damage the oxygen sensors, overheat and destroy the catalytic converter, and can cause engine oil dilution.
Common causes
Most-frequently reported root causes when P0304 is logged.
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1
Faulty or fouled spark plug on cylinder 4
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2
Defective ignition coil (or coil-on-plug) on cylinder 4
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3
Failed or clogged fuel injector on cylinder 4
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4
Damaged, shorted, or open wiring/connector to the ignition coil or injector for cylinder 4
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5
Low compression on cylinder 4 (worn rings, burnt valve, or damaged piston)
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6
Leaking cylinder 4 head gasket causing combustion pressure loss
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7
Vacuum leak near cylinder 4 causing an overly lean air-fuel mixture
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8
Carbon tracking or cracked spark plug boot causing secondary ignition leak
Symptoms drivers notice
How to diagnose P0304
A typical diagnostic flow when this code is present.
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1
Connect a scan tool and confirm P0304 is stored; note freeze-frame data (RPM, load, coolant temp) and check for companion codes such as P0300, P030X on other cylinders, or fuel/ignition system codes
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2
Perform a visual inspection of the cylinder 4 spark plug, coil-on-plug boot, and injector wiring harness for obvious damage, corrosion, or carbon tracking
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3
Swap the cylinder 4 spark plug with a known-good cylinder's plug, clear codes, and test-drive — if the misfire follows the plug, replace it; if it stays on cylinder 4, the plug is not the cause
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4
Swap the cylinder 4 ignition coil to a different cylinder, clear codes, and retest — if the misfire moves, replace the coil; if it stays on cylinder 4, move to the injector
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5
Use a fuel injector test kit or perform a balance test with the scan tool to verify injector pulse and flow for cylinder 4; inspect the injector connector for corrosion
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6
Perform a cylinder compression test on cylinder 4 (and compare to adjacent cylinders); follow with a leak-down test if compression is low to identify whether the fault is rings, valves, or gasket
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7
Check for vacuum leaks near the cylinder 4 intake port using smoke testing or carb cleaner with the engine running, and inspect the PCV system
Related powertrain codes
Frequently asked questions
Can I drive with a P0304 code?
Short distances at low load may be possible, but driving with an active misfire is not recommended. A flashing MIL indicates the misfire rate is high enough to damage the catalytic converter within minutes of driving. Even a steady MIL misfire will eventually foul oxygen sensors, dilute engine oil with raw fuel, and cause further engine wear. Have the vehicle diagnosed as soon as possible.
Is P0304 always caused by the spark plug or coil?
The spark plug and ignition coil are the most common causes and should be checked first, but a clogged or failed fuel injector, low cylinder compression, a burnt exhaust valve, or a leaking head gasket can all trigger P0304. If swapping the plug and coil does not move the misfire off cylinder 4, a compression and leak-down test is the logical next step.
Why does P0304 sometimes appear alongside P0300?
P0300 is a random or multiple-cylinder misfire code that the PCM sets when misfires are detected across more than one cylinder or are not consistently isolated to one. If a P0300 appears with P0304, it may mean cylinder 4 is the primary offender driving overall misfire counts, or it may indicate a system-wide issue such as a vacuum leak, fuel pressure problem, or failing mass airflow sensor that is worsening an already-marginal cylinder 4.
How much does it cost to fix P0304?
Cost varies significantly by root cause. A single spark plug replacement is typically $20–$100 including labour. Replacing a coil-on-plug unit runs $80–$200. A fuel injector replacement is $150–$400. If low compression points to a valve job or head gasket repair, costs can range from $500 to over $2,000 depending on the engine design and labour rates in your area. Always confirm the exact cause before replacing parts.
Disabling P0304 in software
RaceTune can permanently disable P0304 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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