P0307 – Cylinder 7 Misfire Detected

Last updated: April 4, 2026

P0307 means your engine computer has detected a misfire on cylinder 7. You will often feel a shake at idle, a stumble on acceleration, or a lack of power. The check engine light may flash during the misfire, which means you must stop driving to prevent catalyst damage. According to factory diagnostic data used across OBD-II applications, this code indicates “Cylinder 7 Misfire Detected.” P0307 does not prove a bad spark plug or coil. It only tells you which cylinder the misfire monitor flagged so you can test that cylinder’s ignition, fuel, air, and mechanical condition.

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P0307 Quick Answer

P0307 points to a misfire happening on cylinder 7. Start by confirming which cylinder is #7, then check ignition and injector operation on that cylinder before replacing parts.

What Does P0307 Mean?

P0307’s official definition is “Cylinder 7 Misfire Detected.” In plain terms, the PCM/ECM saw cylinder 7 contribute less power than expected. That imbalance shows up as rough running, hesitation, or a flashing MIL under load. The code identifies a suspected trouble area (cylinder 7), not a confirmed failed component.

Technically, the PCM detects misfires by watching crankshaft speed changes and comparing them to expected firing events. When cylinder 7 should accelerate the crankshaft, the PCM instead sees a weak or inconsistent acceleration pattern. That matters because the root cause can be ignition, fuel delivery, air/fuel mixture control, or cylinder sealing. You must confirm the failure with testing, not guesses.

Theory of Operation

Under normal operation, each cylinder fires and produces a predictable crankshaft speed increase. The PCM uses the crankshaft position sensor signal to track those small speed changes. It assigns each change to a specific cylinder based on engine position. At the same time, the PCM controls spark timing and injector pulse width to keep combustion stable.

P0307 sets when cylinder 7 repeatedly fails to deliver expected torque. Spark can drop out, fuel can go missing, or the cylinder can lose compression. Mixture problems can also lean out one cylinder more than the others. The misfire monitor then records the event and stores P0307 when it meets the enable conditions.

Symptoms

P0307 symptoms usually show up as a cylinder-specific drivability complaint that worsens under load.

• Flashing MIL during acceleration or climbing hills, indicating an active misfire risk to the catalyst
• Rough idle that feels like a shake or “lope,” often worse in gear
• Hesitation or stumble on tip-in throttle, especially at low RPM
• Reduced power with a noticeable dead spot as cylinder 7 drops out
• Fuel smell from the exhaust if the cylinder misfires with fuel still delivered
• Poor fuel economy caused by incomplete combustion and correction fuel trims
• Hard start or extended crank if the misfire relates to weak ignition or poor cylinder sealing

Common Causes

• Ignition coil output weak on cylinder 7: A weak coil cannot deliver enough spark energy under load, so the crankshaft speed change pattern flags a cylinder 7 misfire.
• Coil boot or plug well arcing (COP systems): Carbon tracking or oil in the plug well leaks spark to ground, which skips combustion events and sets P0307.
• Spark plug fault on cylinder 7 (fouled, worn, or incorrect gap): A plug that cannot fire consistently causes incomplete combustion, which the misfire monitor identifies as cylinder 7.
• Fuel injector 7 restricted or electrically failing: Low fuel delivery or an intermittent injector solenoid reduces cylinder 7 torque contribution and triggers the misfire detection logic.
• Injector 7 wiring or connector high resistance: Corrosion or poor pin tension drops injector current under load, which leans that cylinder and produces a repeatable misfire count.
• Vacuum leak local to cylinder 7 runner or intake gasket leak: Unmetered air near that port leans only cylinder 7, so the engine misfires even if overall trims look normal.
• Low compression or mechanical fault on cylinder 7: A burned valve, weak rings, or a valve train issue reduces trapped air charge, so combustion becomes unstable at idle and during acceleration.
• Fuel quality or pressure problem that shows up first on cylinder 7: Marginal pressure or contaminated fuel can push the “weakest” cylinder into misfire before others show codes.
• ECM misfire detection influenced by crank/cam signal noise: Poor sensor wiring routing or a failing crank/cam sensor can distort speed calculations and falsely attribute misfires to cylinder 7.

Diagnosis Steps

Tools you need: a scan tool with Mode $06 misfire data, live data, and freeze frame; a digital multimeter; a test light; and basic hand tools. A spark tester, noid light, and fuel pressure gauge speed up pinpointing ignition versus fuel faults. Use a compression gauge or leak-down tester when you suspect mechanical issues.

1. Confirm the P0307 code and note whether it shows as pending or confirmed/stored. Record freeze frame data, especially RPM, engine load, coolant temperature, fuel system status (open/closed loop), STFT/LTFT, and vehicle speed. Freeze frame shows the exact conditions when P0307 set.
2. Check for related DTCs before touching parts. Pay attention to other misfire codes (P0300, P030X), fuel trim codes (P0171/P0174), and crank/cam codes. A pending-only P0307 often points to an intermittent issue that needs a road test and data capture.
3. Inspect fuses and power distribution that feed the ignition coils and injectors. Verify the ignition feed circuit stays powered with key ON and engine running. A partial voltage loss can misfire one cylinder first if its coil or injector load is highest.
4. Verify ECM and engine grounds with a voltage-drop test under load. Run the engine and turn on electrical loads. Measure ground drop from the ECM ground pin or engine ground point to battery negative, and keep it under 0.1V with the circuit operating.
5. Do a quick visual inspection around cylinder 7 before meter work. Check the coil connector, injector connector, harness routing, and any obvious rub-through near brackets. Look for oil in the plug well, water intrusion, broken locks, or green corrosion.
6. Use the scan tool to review misfire counters and Mode $06 data for cylinder 7. Compare cylinder 7 counts to neighboring cylinders at idle and at 1500–2500 RPM. If the counts climb only under load, focus on coil output, plug gap, and fuel delivery.
7. Perform an ignition swap test in a controlled way. Swap the cylinder 7 coil with another cylinder, clear codes, and retest under the same freeze frame conditions. If the misfire follows the coil, you confirmed an ignition component direction without guessing.
8. If the coil does not move the misfire, inspect and test the spark plug from cylinder 7. Check for fouling, cracks, incorrect gap, or signs of detonation. Verify the plug type matches the application, since incorrect heat range can cause repeat misfires.
9. Test injector 7 operation next. Use a stethoscope for a consistent click, then verify injector power and ground control. If available, use a noid light to confirm pulse and a current ramp to spot a weak solenoid.
10. Check injector 7 circuit integrity with loaded testing, not just continuity. Backprobe for voltage drop on the power feed to the injector with it operating. Then check the ECM-controlled ground side for excessive drop, which points to high resistance in the connector or harness.
11. Rule out air leaks and mixture problems that can bias one cylinder. Smoke test the intake, focusing on the runner and gasket area near cylinder 7. Also review STFT/LTFT behavior at idle and cruise to see if the engine runs lean under the same conditions as the misfire.
12. If ignition and fuel tests pass, verify mechanical health on cylinder 7. Perform a compression test and compare across cylinders, then confirm with a leak-down test if needed. Low compression or high leakage explains a persistent P0307 that does not respond to ignition or injector swaps.
13. After repairs, clear codes and road test under similar enable conditions to the freeze frame. Use a scan tool snapshot during the drive to capture live misfire counters, trims, load, and RPM if the concern is intermittent. Confirm the misfire monitor and related OBD-II readiness monitors run to completion before calling the P0307 repair verified.

Professional tip: When P0307 shows only at idle in closed loop, do a targeted smoke test at the cylinder 7 intake runner first. When it shows only under load, put your effort into coil output, plug condition, and injector delivery. Use freeze frame to pick the correct test, not guesswork.

Possible Fixes

• Repair coil/plug well insulation issues: Clean oil intrusion, replace a damaged coil boot, and correct plug well sealing after you confirm arcing or tracking.
• Replace the spark plug (and coil if proven by swap testing): Install the correct plug type and gap, and only replace the coil when misfire movement or output testing confirms it.
• Restore injector 7 operation: Clean or replace a restricted injector after you confirm low contribution, poor spray, or electrical failure with pulse and current checks.
• Repair wiring, terminals, or connectors for coil/injector 7: Fix spread pins, corrosion, or chafed wiring after voltage-drop testing proves a high-resistance feed or control circuit.
• Fix a localized intake air leak: Replace a leaking intake gasket or repair a cracked hose/runner once smoke testing confirms unmetered air near cylinder 7.
• Address mechanical engine faults on cylinder 7: Repair valves, rings, or valve train issues only after compression or leak-down results confirm a mechanical cause for the P0307 misfire.

Can I Still Drive With P0307?

You can sometimes drive short distances with a P0307 code, but you should not treat it as “normal.” A cylinder 7 misfire can cause bucking, poor power, and stalling at stops. More importantly, a continuing misfire can overheat the catalytic converter and damage it fast, especially under load or at highway speed. If the engine shakes hard, the MIL flashes, or you smell raw fuel, stop driving and tow it. Drive only to a safe location or a repair shop if the misfire stays mild and steady.

How Serious Is This Code?

P0307 ranges from an inconvenience to an expensive failure. If the misfire happens only on cold start and clears quickly, you may only notice a brief rough idle. If the misfire persists, drivability drops and fuel economy suffers. A constant misfire raises exhaust temperatures and can melt the catalytic converter core. That turns a simple ignition or fuel issue into a major repair. Safety becomes a concern when the engine loses power during merges, hills, or passing, or when it stalls in traffic.

Common Misdiagnoses

Technicians often replace the #7 spark plug or coil first, then chase the same P0307 code again. That happens when the real issue sits in the coil power feed, coil ground control, or a connector with spread terminals. Another common miss: ignoring fuel trim and misfire counters and skipping a load test on the ignition feed. DIY owners also confuse “cylinder 7” with a firing order guess. Always confirm cylinder numbering for the engine. Finally, many shops call a bad injector without verifying injector command, fuel pressure, or a contribution test.

Most Likely Fix

The most common confirmed repair direction for P0307 involves restoring consistent spark or fuel delivery on cylinder 7 after basic circuit checks. Many engines end up needing a worn plug replaced or a coil/boot fault corrected, but only after you verify coil power, ground integrity, and connector tension under load. The next frequent direction involves fuel delivery on that cylinder. That includes an injector fault, an injector connector issue, or a mechanical problem that shows up during a compression or leak-down check.

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.