P0304 – Cylinder 4 Misfire Detected

Last updated: April 2, 2026

P0304 means your engine control module (ECM/PCM) has detected a misfire on cylinder 4. You will usually feel a rough idle, shaking on acceleration, or a loss of power, and the check engine light may flash under load. The code does not prove a bad spark plug or coil. It only points you toward cylinder 4 as the problem area. According to many OEM factory diagnostic strategies, this code sets when the PCM sees crankshaft speed changes consistent with a misfire event on cylinder 4. You must confirm ignition, fuel, air, and mechanical integrity before replacing parts.

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P0304 Quick Answer

P0304 points to a misfire affecting cylinder 4. Start by confirming which cylinder is #4, then check cylinder 4 ignition output (spark), injector operation, and compression before replacing anything.

What Does P0304 Mean?

P0304 code means “Cylinder 4 Misfire Detected.” In plain terms, the PCM sees cylinder 4 not contributing normally, so the engine runs unevenly. That shows up as a shake at idle, a stumble on tip-in, or a flashing MIL during hard acceleration. The code identifies the affected cylinder number, not the failed part. Your next job is to determine whether the misfire comes from ignition, fuel delivery, air/fuel mixture, or engine mechanical condition on that cylinder.

Technically, the PCM does not “watch” spark directly on most vehicles. It infers misfire from crankshaft speed fluctuations using the crankshaft position sensor signal. The PCM then assigns the event to a specific cylinder based on firing order and crank angle. That matters because a bad coil, injector circuit issue, vacuum leak near one runner, EGR distribution issue, or low compression can all produce the same crank speed pattern. Diagnosis must verify inputs and outputs at cylinder 4 before you call any component failed.

Theory of Operation

Under normal operation, each cylinder produces a predictable torque pulse after ignition. The crankshaft accelerates slightly on each power stroke and decelerates between strokes. The PCM monitors crankshaft position sensor timing to track those tiny speed changes. When cylinder 4 fires correctly, the PCM sees a consistent contribution that matches expected patterns for load and RPM.

P0304 sets when cylinder 4’s contribution drops enough for the PCM to recognize repeated misfire events. Ignition problems often cause a sharp, load-sensitive miss. Fuel problems often cause a lean miss that worsens with load, or a rich miss that smells like fuel. Mechanical faults usually create a steady miss that persists at idle and does not “swap” with parts. The same code can also appear when a mixture problem affects one intake runner more than the others.

Symptoms

P0304 symptoms usually show up first as a drivability complaint, then as a code on the scan tool.

• Flashing MIL: the check engine light may flash during acceleration, indicating active misfire that can damage the catalytic converter
• Rough idle: the engine shakes at stops, especially in gear with A/C on
• Hesitation: stumble or bucking on tip-in or under load, often most noticeable climbing a hill
• Loss of power: reduced acceleration and poor throttle response, sometimes accompanied by a “dead cylinder” feel
• Fuel smell: raw fuel odor from the exhaust if cylinder 4 does not burn the mixture consistently
• Poor fuel economy: increased consumption as the PCM adds fuel or misfire wastes the charge
• Secondary codes: you may also see P0300, fuel trim codes, or catalyst efficiency codes depending on duration and severity

Common Causes

• Worn or fouled spark plug on cylinder 4: A weak spark or plug fouling lets the air-fuel mixture misfire under load or at idle.
• Ignition coil or coil-on-plug boot issue (cylinder 4): Internal coil breakdown or carbon tracking leaks spark to ground instead of firing the plug.
• Cylinder 4 fuel injector fault (electrical or mechanical): An injector that sticks, clogs, or loses electrical control delivers the wrong fuel amount and causes a dead or weak cylinder.
• Injector or coil harness/connector problem near cylinder 4: Heat, oil intrusion, or connector tension creates intermittents that show up as a single-cylinder misfire.
• Vacuum leak or intake runner leak feeding cylinder 4: Extra unmetered air leans that cylinder and misfires most at idle and light throttle.
• Low compression or mechanical fault in cylinder 4: Burned valve, worn rings, or head gasket leakage reduces cylinder sealing and combustion stability.
• Fuel quality or fuel delivery problem that “shows” first on cylinder 4: Low fuel pressure or contaminated fuel can misfire one cylinder sooner if its injector flow is marginal.
• EGR or PCV distribution issue affecting that cylinder: Improper exhaust gas or crankcase vapor routing dilutes mixture unevenly and triggers a cylinder-specific misfire.

Diagnosis Steps

Use a scan tool with Mode $06 misfire data if available, plus bidirectional controls when supported. Keep a digital multimeter, a wiring diagram, and basic back-probing tools ready. A spark tester, noid light, and a fuel pressure gauge help confirm ignition and injector control. Use a compression gauge and, ideally, a leakdown tester for mechanical confirmation.

1. Confirm the P0304 code and record freeze-frame data before clearing anything. Focus on RPM, engine load, coolant temperature, fuel system status (open/closed loop), STFT/LTFT, and vehicle speed. Compare pending versus confirmed P0304 since many misfire detections set pending first, then confirm on a second drive cycle.
2. Check related DTCs and address them in order. Look for fuel trim codes (P0171/P0172), crank/cam sensor codes, injector circuit codes, or catalyst damage codes. Misfire codes often act as “symptoms” of another fault.
3. Check fuses and power distribution that feed ignition coils, injectors, and the ECM. Use a test light or loaded circuit check, not continuity alone. A fuse can look good and still fail under load at the terminal.
4. Verify ECM and engine ground integrity with voltage-drop testing under load. Measure from battery negative to the engine block and to the ECM ground pins while cranking or with key-on loads active. Keep ground drop under 0.1V with the circuit operating, since high resistance can pass a no-load test and fail in real conditions.
5. Perform a targeted visual inspection around cylinder 4. Inspect the coil connector, injector connector, harness routing, and any rubbed-through loom near brackets. Look for oil in plug wells, water intrusion, loose terminals, and signs of arcing on the boot.
6. Use misfire counters to verify the problem stays on cylinder 4. Check Mode $06 or enhanced data for cylinder-specific misfire counts at idle and at 1500–2500 RPM. If the misfire moves with RPM or load, note the pattern for later testing.
7. Isolate ignition by swapping known-good parts when practical and safe. Swap the cylinder 4 coil with another cylinder, then repeat the same operating conditions and recheck misfire counts. If the misfire follows the coil or boot, you found a likely ignition-side cause and can confirm with a spark output test.
8. Verify injector control and injector operation on cylinder 4 before replacing parts. Check for injector power supply on the feed side with key on, then confirm ECM pulse on the control side with a noid light or scope during crank/run. If power or pulse drops out intermittently, perform a wiggle test on the harness while watching the signal.
9. Check for air leaks that can bias cylinder 4 lean. Inspect the intake gasket area near that runner, PCV hoses, and any vacuum ports close to cylinder 4. Use smoke testing or carefully applied propane/brake-clean method per shop safety rules, then watch STFT response and idle quality.
10. Confirm mechanical integrity if ignition and fuel checks pass. Run a compression test on all cylinders and compare results, then follow with a leakdown test to locate leakage through intake, exhaust, crankcase, or cooling system. Mechanical faults often create a steady misfire that will not respond to coil or injector swaps.
11. Confirm the repair with a road test under the same freeze-frame conditions. Use a scan tool snapshot (manual recording) during the drive to capture misfire counts, fuel trims, and load when the concern would occur. Freeze frame shows conditions when the code set, while a snapshot helps catch intermittents in real time.
12. After repairs, clear codes only when needed and verify the misfire monitor completes. Remember that clearing codes resets OBD-II readiness to Not Ready, so the misfire and catalyst monitors must run and show Ready/Complete under the correct enable conditions.

Professional tip: If P0304 appears only at idle with high positive STFT, chase vacuum leaks and injector flow first. If it appears under load with normal trims, prioritize ignition breakdown and cylinder compression checks. Always prove power, ground, and signal integrity before you condemn a coil, injector, or ECM.

Possible Fixes

• Repair connector pin fit, corrosion, or harness damage at the cylinder 4 coil or injector, then re-secure routing to prevent repeat chafing.
• Replace worn spark plugs and correct plug gap issues, and replace the cylinder 4 coil/boot only after swap testing or spark verification supports it.
• Restore injector operation by correcting power/pulse faults, cleaning restricted injectors where appropriate, or replacing an injector after control testing confirms the fault stays with that injector.
• Repair intake/vacuum leaks near cylinder 4, including intake manifold gasket issues, cracked hoses, or PCV system faults that skew mixture.
• Correct fuel delivery problems such as low fuel pressure or contamination after confirming with a pressure/volume test and fuel quality inspection.
• Repair mechanical issues found during compression or leakdown testing, such as valve sealing problems or head gasket leakage, then verify misfire counters stay at zero.

Can I Still Drive With P0304?

You can sometimes limp a vehicle with a P0304 code, but you should not treat it as “safe.” A cylinder 4 misfire can cause sudden power loss, shaking, or stalling at idle. That matters most in traffic, during passing, or when merging. If the misfire flashes the MIL, stop driving and tow it. A flashing light usually means the misfire rate can overheat and damage the catalytic converter. If the light stays steady and the engine runs smooth, drive only as needed to a repair location. Avoid heavy throttle, high RPM, and long hills until you confirm the cause.

How Serious Is This Code?

P0304 ranges from a mild annoyance to a converter-killing problem. A slight intermittent miss may only show up at idle or light cruise, so it feels like a nuisance. Under load, that same fault can turn into bucking, hesitation, or loss of power. Those symptoms create a real drivability risk. Fuel that does not burn in cylinder 4 can overheat the catalytic converter and increase emissions. Extended misfire can also foul spark plugs and dilute engine oil with fuel. Treat a flashing MIL or harsh misfire as urgent. Treat a steady MIL with minor symptoms as “repair soon,” not “ignore.”

Common Misdiagnoses

Techs waste the most money on P0304 by guessing parts before they prove the misfire source. Swapping a coil or plug without verifying spark quality under load can hide a wiring fault. Another common miss involves labeling a fuel injector “bad” without checking injector power, driver control, and connector tension. Vacuum leaks near the cylinder 4 runner also fool people, especially on V engines with split plenums. Some engines show P0304 from low compression, not ignition. A quick coil swap will not fix a burned valve. Use Mode $06 misfire data, a cylinder balance test, and a scope or spark tester before you buy parts.

Most Likely Fix

The most common confirmed repair direction for a P0304 misfire is restoring consistent ignition on cylinder 4. That often means correcting a plug problem, coil output problem, or coil/injector connector issue at that cylinder. The second frequent direction involves a cylinder-specific fueling issue, such as poor injector electrical connection, restricted injector flow, or an intake air leak affecting that runner. Prove the fault with a cylinder swap test, injector command test, and voltage-drop checks on the power and ground paths. After the repair, road test under the same load and RPM from freeze-frame data to confirm the misfire does not return.

Repair Costs

Repair cost depends on whether the confirmed root cause is wiring, connector condition, a sensor, a module, or the labor needed to diagnose the fault correctly.