P0306 – Cylinder 6 Misfire Detected

P0306 means the engine computer has detected a misfire on cylinder 6. You will usually feel it as a rough idle, a shake under load, or a loss of power, and fuel economy often drops. Misfires can also damage the catalytic converter if you keep driving. The P0306 code does not prove a bad spark plug or coil. It only tells you which cylinder the misfire monitor flagged. According to manufacturer OBD-II diagnostic logic, this code indicates cylinder 6 is not contributing power consistently during combustion.

P0306 Quick Answer

P0306 points to a misfire on cylinder 6. Check cylinder 6 ignition first (plug, coil, coil boot, connector) and confirm with misfire counters and a swap test before replacing parts.

What Does P0306 Mean?

P0306 meaning: Cylinder 6 Misfire Detected. In plain terms, the PCM/ECM sees cylinder 6 slow the crankshaft more than expected during its power stroke. That loss of contribution can come from ignition, fuel delivery, air leakage, compression, or a control issue. The code names the affected cylinder, not the failed component, so you must test what cylinder 6 needs to burn correctly.

Technically, the misfire monitor tracks crankshaft speed changes using the crankshaft position sensor signal. The module assigns those speed changes to specific cylinders based on engine position. When the pattern matches a cylinder 6 event often enough in a drive cycle, the PCM stores P0306 as pending or confirmed. That matters because any fault that weakens combustion on cylinder 6 can trigger the same code.

Theory of Operation

During normal operation, each cylinder accelerates the crankshaft on its power stroke. The PCM expects a consistent acceleration pattern across all cylinders. It uses CKP timing data to calculate small speed changes and compares them to learned values. The PCM also uses load, RPM, and temperature to decide when the monitor can run.

P0306 sets when cylinder 6 produces less torque than the others. Ignition faults reduce spark energy or timing accuracy. Fuel faults lean out the mixture or stop injection. Air leaks near that runner dilute the mixture at that cylinder. Mechanical issues reduce compression or disrupt valve timing, so the cylinder cannot burn efficiently.

Symptoms

P0306 symptoms usually show up as a cylinder-specific roughness that gets worse under load.

• Warning light: Check Engine Light on; it may flash during a severe misfire
• Rough idle: noticeable shake or uneven idle, especially in gear
• Hesitation: stumble on tip-in or a miss during steady cruise
• Power loss: weak acceleration, sometimes worse climbing hills
• Fuel smell: raw fuel odor from the exhaust after extended misfiring
• Cold start: misfire worse when cold, then improves as it warms
• Scan tool data: cylinder 6 misfire counter climbs faster than other cylinders

Common Causes

• Worn or fouled spark plug on cylinder 6: A plug with excessive gap, deposits, or oil fouling weakens the spark and leaves the air-fuel charge unburned.
• Ignition coil (coil-on-plug or coil pack) output weak on cylinder 6: A marginal coil may fire at idle yet break down under load, creating a consistent misfire count on that cylinder.
• Fuel injector for cylinder 6 restricted or electrically faulty: Low flow, a sticking pintle, or a driver/control issue leans that cylinder and triggers misfire detection.
• Injector/coil wiring or connector problem at cylinder 6: Corrosion, terminal spread, or chafed wiring can interrupt coil or injector control and cause an intermittent misfire.
• Vacuum leak or intake runner leak affecting cylinder 6: Unmetered air near that port leans one cylinder more than the others, which the ECM interprets as a cylinder-specific misfire.
• Mechanical issue in cylinder 6 (compression or valve sealing): Low compression from a burnt valve, ring issue, or cam timing problem reduces burn quality and elevates misfire counts.
• Fuel quality or pressure/volume problem that shows up on one cylinder first: Low pump volume, a weak regulator, or contaminated fuel can push a marginal cylinder over the edge under load.
• EGR or PCV distribution fault influencing cylinder balance: Improper dilution at certain intake runners can destabilize combustion and cause a repeatable cylinder 6 misfire pattern.

Diagnosis Steps

Tools: a capable scan tool with Mode $06 misfire data, a DVOM, and the correct wiring diagrams. Use a spark tester or scope if available. A noid light helps verify injector command. Plan on a compression gauge and, ideally, a leakdown tester. Have basic hand tools for plug and coil access.

1. Confirm the P0306 code and note whether it shows as pending, stored, or confirmed. Record freeze-frame data, focusing on RPM, engine load, coolant temperature, vehicle speed, fuel system status (open/closed loop), and STFT/LTFT. Those values tell you whether the misfire happened cold, hot, idle, or under load.
2. Check for related DTCs that change the diagnostic path. Pay attention to fuel trim codes (P0171/P0174), crank/cam correlation codes, and injector or ignition primary circuit codes. Use the scan tool misfire counters to confirm cylinder 6 leads the counts.
3. Verify basic power distribution before deep testing. Check ignition and injector fuses, and confirm the main relay outputs if the vehicle uses one. A weak feed can create multiple “random” misfires, so validate the supply first.
4. Verify ECM and engine grounds with voltage-drop testing under load. Run the engine and load electrical consumers, then measure ground drop from the component ground to battery negative. Keep ground drop under 0.1V with the circuit operating, or fix the high-resistance ground before chasing parts.
5. Perform a close visual inspection of cylinder 6 ignition and injector connectors and harness routing. Look for oil intrusion in coil boots, broken locks, terminal push-outs, and harness rub near valve covers and engine brackets. Repair any connector tension issues before swapping components.
6. Use live data to narrow the operating condition. Watch misfire counters at idle, snap throttle, and steady cruise. If the misfire rises under load, suspect spark breakdown, lean fueling, or compression issues more than a simple idle-only problem.
7. Use Mode $06 data (if supported) to see how strongly cylinder 6 fails compared to others. Then create a scan tool snapshot during a road test to capture fuel trims, load, and misfire counts at the moment the concern occurs. Freeze frame shows what happened when the DTC set, while a snapshot captures intermittent conditions you trigger during diagnosis.
8. Test the ignition side at cylinder 6. Inspect the spark plug for wear, deposits, and oil or coolant contamination. If access allows, swap the cylinder 6 coil with another cylinder and recheck misfire counts to see if the misfire follows the coil. Do the same with the plug if needed, but document each change.
9. Verify injector operation on cylinder 6 before replacing it. Listen for consistent injector clicking, then confirm injector command with a noid light or scope at the connector. Measure injector resistance only as a quick screen, and then perform a cylinder balance test or controlled fuel cut test if the scan tool supports it.
10. Check for a localized air leak affecting cylinder 6. Inspect the intake manifold gasket area near that runner, PCV plumbing, and any vacuum ports. Use smoke testing if available, and watch for a trim shift or misfire change when you direct smoke or a controlled enrichment source near the suspected leak.
11. If ignition and fueling checks pass, verify mechanical integrity. Perform a compression test and follow with a leakdown test on cylinder 6 if compression looks low or unstable. Compare results to neighboring cylinders, and investigate valve sealing or cam timing concerns when the numbers do not match.
12. Confirm the repair with a drive cycle under similar conditions to the freeze frame. Recheck for pending codes, review misfire counters, and ensure the MIL stays off. After clearing codes, verify the appropriate OBD-II readiness monitors complete before emissions testing, since clearing resets monitors to Not Ready.

Professional tip: Don’t rely on continuity checks alone for power and ground. A connection can pass continuity and still fail under load. Use voltage-drop testing at the coil and injector feeds while the engine runs. That test finds high resistance that creates misfires under acceleration.

Possible Fixes

• Repair corroded terminals, broken connector locks, or chafed wiring at the cylinder 6 coil or injector circuit.
• Replace the spark plug(s) if inspection shows wear, incorrect gap, or fouling that matches the misfire pattern.
• Replace a weak ignition coil only after confirming the misfire follows the coil or testing proves poor output.
• Clean, service, or replace a restricted or non-responsive fuel injector after verifying command and fuel delivery concerns.
• Repair intake/vacuum leaks near the cylinder 6 runner and verify fuel trims return to normal.
• Address mechanical faults found on compression or leakdown testing, such as valve sealing or timing-related issues.

Can I Still Drive With P0306?

You can sometimes limp a vehicle with a P0306 code, but you should not treat it as “safe.” A cylinder 6 misfire can turn into a flashing MIL, which signals a catalyst-damaging misfire. Stop driving if the MIL flashes, the engine shakes hard, power drops sharply, or the transmission starts hunting. Light, occasional misfires may feel like a small stumble, yet they can still overheat the catalytic converter and dilute the oil with fuel. If you must drive, keep trips short, avoid heavy throttle, and avoid towing. Plan a diagnostic check as soon as possible, especially if raw fuel smell or popping in the exhaust appears.

How Serious Is This Code?

P0306 ranges from an inconvenience to an expensive failure, depending on misfire severity and duration. A mild misfire at idle may only cause a rough idle and a steady check engine light. A misfire under load can create hesitation during merges and passing, which becomes a safety issue. Continued driving with an active misfire can damage the catalytic converter and oxygen sensors from overheating and excess oxygen in the exhaust stream. It can also foul spark plugs and wash down cylinder walls, which accelerates engine wear. Treat a flashing MIL as urgent. That condition means the PCM sees a misfire rate high enough to risk catalyst damage.

Common Misdiagnoses

Many P0306 repairs fail because the tech skips verification and “parts darts” the ignition system. Swapping a coil or plug can hide a wiring fault for a short time. It also misses load-related failures that only show up under cylinder pressure. Another common mistake involves blaming the fuel injector without checking injector power, ground control, and connector tension. Vacuum leaks near one intake runner also get overlooked, especially on V6 and V8 engines with split manifolds. Technicians also misread fuel trim data and replace oxygen sensors. O2 sensors report the misfire’s exhaust result, not the cause. Avoid wasted spending by confirming cylinder 6 ignition output, injector command, compression, and air leaks before replacing parts.

Most Likely Fix

The most common confirmed P0306 repair paths involve ignition secondary faults on cylinder 6 or a cylinder-specific fueling problem. Start with evidence. Use Mode $06 misfire data, a cylinder contribution test, and a short road test under the freeze-frame conditions. If you find a consistent cylinder 6 miss, inspect the plug and coil boot for carbon tracking, oil intrusion, or an incorrect plug gap. Next, verify injector electrical integrity at the connector and check for harness rub or poor terminal tension. If ignition and injector checks pass, confirm engine mechanical health with a compression or relative compression test. Then verify no intake leak targets cylinder 6.

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.

Last updated: April 1, 2026