Error P0306 is an OBD-II DTC indicating a misfire in cylinder number 6. The PCM detects irregular crankshaft speed variation corresponding to the sixth cylinder, triggering the Check Engine light; the engine runs rough and loses power. The code P0306 specifically denotes a combustion problem in this cylinder. Causes range from spark plugs and ignition coils to injectors, air leaks, and mechanical issues. Ignoring this code increases the risk of catalytic converter and engine damage.
“P0306 is not a final diagnosis but a precise PCM clue about which cylinder has a combustion issue. Correct interpretation of this signal is key to quick diagnostics without replacing unnecessary parts by trial and error.” – Daniel Brooks, DecodeAuto
This information is general and does not replace professional consultation.
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Main symptoms of error P0306
When the PCM detects misfires in the sixth cylinder, symptoms appear almost immediately. The Check Engine light stays on or flashes when severe misfires occur. The engine runs rough at idle and under load, with vibrations transmitted to the steering wheel and body.
Power loss and poor acceleration response are noticeable when trying to accelerate—jerks and delayed throttle response especially occur at high RPM. Increased fuel consumption results from incomplete combustion in the sixth cylinder. A gasoline smell in the exhaust is a typical sign of unburned fuel entering the exhaust system.
Jerks when pressing the accelerator, unstable RPM, and backfires in the exhaust system complete the picture. These backfires result from fuel igniting in the exhaust tract, indicating a serious condition.
Is it safe to drive with error P0306?
In short: it is not recommended. Misfires in the sixth cylinder cause unburned fuel to enter the catalytic converter, leading to overheating and damage. A short drive to a repair shop at low RPM is technically possible if the engine does not stall and the Check Engine light does not flash.
If the indicator flashes, there is strong vibration, or a gasoline smell is present—stop and arrange towing. A flashing Check Engine light means the PCM has detected severe misfires with a risk of catalytic converter overheating. Continuing to drive in this state may cause converter damage.
Prolonged operation with P0306 increases risks to the engine and exhaust system. Unburned fuel can cause overheating of pistons and valves, damage to spark plug insulators, and catalytic converter failure.
“A flashing Check Engine light with P0306 is a signal to stop immediately. Operating with active misfires can lead to severe catalytic converter damage.”
This information is general and does not replace professional consultation.
What does error code P0306 mean in simple terms?
Code interpretation according to OBD-II standard
P0 is a generic powertrain code used by all manufacturers under ISO 15031. The digit 3 indicates ignition system or misfire issues. 06 is the cylinder number with misfires. The PCM detects irregular crankshaft speed variation corresponding specifically to cylinder number 6.
How does it work? The crankshaft position sensor (CKP) monitors crankshaft speed. During normal combustion, the piston receives a sharp downward acceleration. The PCM compares each cylinder’s acceleration with previous cycles and other cylinders. If the air-fuel mixture in the sixth cylinder fails to ignite or burns irregularly, the CKP detects a slowdown of the crankshaft at that stroke.
The camshaft position sensor (CMP) provides phase synchronization—it indicates when the intake valve of a specific cylinder opens. This allows the PCM to precisely identify that the misfire occurred in the sixth cylinder, not another.
What are misfires and why are they dangerous?
A misfire occurs when the air-fuel mixture fails to ignite in the cylinder or burns unstably. Instead of combustion pushing the piston down, incomplete or no combustion occurs. Unburned fuel is expelled into the exhaust system.
This is dangerous for several reasons. Unburned fuel can ignite in the exhaust manifold or catalytic converter, causing micro-explosions and overheating. The catalytic converter is designed to burn off trace fuel residues, not full unburned mixtures. Temperatures inside can exceed operating limits, causing ceramic substrates to melt and fuse.
Vibrations can damage engine mounts, sensors, and connectors. Power loss and increased fuel consumption make driving uncomfortable and inefficient.
Where is the 6th cylinder located on V6 / V8 / L6 engines?
The location of the sixth cylinder depends on the manufacturer and engine type. On inline six-cylinder engines (L6), cylinders are numbered sequentially from front to rear—cylinder six is usually closest to the transmission.
On V6 engines from General Motors and Chrysler, cylinders are numbered alternately along the crankshaft: 1, 3, 5 on one bank (usually the driver’s side), 2, 4, 6 on the opposite. The sixth cylinder is on the second bank in a middle-to-rear position.
On GM and Chrysler V8 engines, the same scheme applies: 1-3-5-7 on one bank, 2-4-6-8 on the other. Cylinder six is on the second bank, middle position.
Ford and Audi number cylinders sequentially by bank: 1-2-3-4 on one side, 5-6-7-8 on the other. In this case, cylinder six is on the second bank, second position.
Refer to the numbering diagram under the hood or in your owner’s manual.
Ignition system causes of P0306
Spark plug malfunction
A worn, fouled, or improperly gapped spark plug is the most common cause of misfires in a specific cylinder. A plug electrode covered with thick black soot or white deposits fails to generate a strong spark. Cracks in the insulator or ceramic cause spark leakage to ground—energy escapes through breakdown instead of across the electrode gap.
An incorrect electrode gap is another frequent issue. If the gap has widened due to wear or was set incorrectly during replacement, the spark becomes weak and unstable. Check the gap according to manufacturer specifications—typical values vary by engine type; consult the service manual.
Start by replacing the spark plug in cylinder 6 with a known good one. If the code disappears, the problem is resolved. If it remains, continue diagnostics.
Ignition coil failure
The ignition coil may fail due to an open in the primary or secondary winding, internal short circuits, or overheating of the housing. Signs of arcing on the coil housing indicate malfunction. Cracks in the insulator or burned plastic areas point to high-voltage leakage.
Test by swapping the coil from cylinder 6 with one from cylinder 2 or 4. Clear the codes and start the engine. If P0306 disappears and P0302 or P0304 appears instead, the coil is faulty and should be replaced. If the code remains on cylinder 6, the coil is not the cause.
Measure primary and secondary winding resistance with a multimeter according to manufacturer specs. Typical ranges vary widely by vehicle model; consult OEM service information for exact values. Deviations indicate opens or shorts.
High-voltage wire damage
A high-voltage wire with insulation breakdown, increased resistance, or a damaged boot causes weak or absent spark. Cracked insulation, especially at bends and clamps, allows spark leakage to engine ground.
Check wire resistance with a multimeter—typical values vary by manufacturer; consult specifications. Excessive resistance indicates internal breaks or aging conductors. Inspect wires in the dark with the engine running—bluish corona discharge around wires or boots indicates breakdown.
If in doubt, replace the high-voltage wires as a set. Denso or OEM wires are reliable choices for many vehicles.
Fuel delivery issues causing misfire in cylinder 6
The fuel system can also cause P0306. The injector for cylinder 6 may be clogged with deposits or sticking due to mechanical wear. If the injector does not open fully or sprays unevenly, the mixture in the cylinder becomes too lean or too rich—both conditions can disrupt ignition.
Perform injector cleaning on a test bench with flow and spray pattern measurement. A healthy injector sprays fuel in an even cone. A clogged one produces a stream or uneven spray.
Measure fuel pressure in the rail—it should meet manufacturer specifications (refer to OEM service information for exact values). Low fuel pressure indicates problems with the fuel pump, clogged filter, or leaks in the fuel line.
Assess fuel pump operation—it must maintain stable pressure at all RPM. Unstable spray and pressure pulsations can worsen mixture formation and cause misfires under load.
Features of GDI (direct injection) systems
Engines with direct fuel injection (GDI) operate at significantly higher fuel rail pressures than traditional port injection systems. GDI engines typically have higher operating pressures; consult factory specifications.
A common GDI issue is carbon buildup on intake valves due to the lack of fuel wash. Deposits on intake valves disrupt cylinder filling and can cause misfires, especially during cold starts. If P0306 appears mainly on a cold engine, check the condition of the intake valves.
Mechanical engine failures leading to P0306
Low compression in the cylinder
Low compression in cylinder six results from burned valves, piston ring wear, cylinder wall scoring, or head gasket failure. Compression measurement reveals the problem: if cylinder six pressure is lower than the others, the cause may be mechanical.
Burned valves cause consistently low compression without change after adding oil to the cylinder. Piston ring wear or scoring raises compression after oil addition. Perform a leak-down test to locate leaks—apply compressed air to the cylinder during the compression stroke and listen for escaping air: intake manifold (intake valve), exhaust system (exhaust valve), or crankcase breather (rings).
Air leaks
Air leaks through vacuum hoses, a leaking intake manifold, or a gasket can cause a lean mixture and misfires at idle. Lean mixtures ignite poorly; the PCM cannot compensate quickly enough via short-term fuel trim (STFT).
Check for leaks using a smoke machine—fill the intake tract with smoke and observe where it escapes. Listen for whistles and hissing at idle. Monitor short-term fuel trim with a scan tool—STFT values significantly above normal may indicate air leaks; refer to diagnostic thresholds for your model.
Common leak points near cylinder six include the intake manifold gasket, EGR vacuum hoses, injector seals, and throttle body gasket.
Sensor and PCM malfunctions
Rarely, false or secondary misfires relate to the crankshaft position sensor (CKP) and camshaft position sensor (CMP). Loss of synchronization between these sensors or signal noise confuses the PCM—it may detect misfires where none exist or miss real issues.
Oscilloscope traces of CKP and CMP signals help—overlay them on one screen and check phase synchronization. Areas of desynchronization, missing teeth, or noise spikes indicate sensor or wiring faults.
Check the gap between the sensor and trigger wheel—typical values vary by model; consult factory specs. Excessive gap weakens the signal. Inspect sensor wiring shielding—interference from high-voltage wires and coils distorts signals.
PCM firmware updates rarely fix P0306, but some manufacturers release technical service bulletins (TSBs) with software updates to eliminate false misfires. For example, Hyundai Service Campaign T5A (2020) recommends ECM updates to fix P030x codes on Sonata 1.6T-GDI; a Toyota TSB (2016) advises an ECM reflash on Prius PHV/V 2013–2016 to resolve P0300–P0304 after cold start.
PCM hardware failure is extremely rare. First exclude power supply, ground, and wiring issues to sensors and actuators.
This information is general and does not replace professional consultation.
Sources: SAE papers on misfire detection, Hyundai TSB T5A (2020), Toyota TSB (2016), ISO 15031.
Step-by-step diagnostics to find the cause of P0306
This information is general and does not replace professional consultation.
- Connect an OBD-II scanner and read the codes. Record freeze-frame data—the conditions when the code set: engine RPM, load, coolant temperature. These parameters help determine when the misfire happens.
- Visual inspection. Examine the spark plug, boot, high-voltage wire, and ignition coil of cylinder six. Check connectors for corrosion and damage. Verify engine ground quality.
- Test the coil by swapping. Exchange the coil of cylinder six with that of cylinder two or four. Clear the codes and start the engine. If the code “moves” to another cylinder, the coil is faulty.
- Check the spark plug. Replace it with a known good one. Assess electrode color and deposits: black oily soot indicates a rich mixture or oil contamination, white deposits indicate a lean mixture or overheating, light brown indicates normal operation.
- Check the fuel injector. Swap the injector from cylinder six to four. Listen to injector operation with a stethoscope—it should produce clear clicks. Perform an injector balance test with a scan tool—all injectors should show similar performance. Clean on a test bench if necessary.
- Measure compression in cylinder six and compare it with the others. Perform a leak-down test to locate leaks.
- Check for air leaks with a smoke machine. Fill the intake tract with smoke and observe leaks.
- Inspect PCM wiring and connectors. Check contacts for corrosion, insulation damage, and secure connections.
- If in doubt, record CKP and CMP oscilloscope traces. Verify phase synchronization and absence of signal noise.
- If the fault is not found, consult a specialized repair shop with diagnostic equipment.
Mode $06: how to view cylinder misfire counters
Mode $06 (OBD-II Mode 6) provides access to internal counters and PCM self-diagnostic results, including misfire counters for each cylinder. This is a powerful tool for quickly pinpointing the problematic cylinder without trial-and-error component replacement.
How to access Mode $06
Most modern OBD-II scanners support Mode $06. In the scanner menu, find “Mode 6,” “On-Board Monitoring,” or “Test Results.” Select the “Misfire” or “Catalyst” monitor.
For budget ELM327 adapters, use apps like Torque Pro (Android) or OBD Fusion (iOS), which include Mode 6 Test Results sections.
How to interpret data
In Mode $06 for misfire monitors, you will see parameters such as:
- TID (Test ID): test identifier
- CID (Component ID): component identifier (cylinder number)
- Value: current misfire count
- Min/Max: minimum/maximum allowed value
Example for cylinder 6:
- TID $01 (Misfire General Data)
- CID $06 (Cylinder 6)
- Value: 150 (number of recorded misfires)
- Max: 50 (threshold for code setting)
If the Value for CID $06 exceeds Max, the PCM sets P0306. Compare counters across all cylinders—a significant excess on cylinder 6 confirms the problem location.
Practical use
Before diagnostics, clear the codes and reset counters. Perform a 10–15 minute test drive in mixed conditions (idle, partial loads, accelerations). Recheck Mode $06—if the misfire counter for cylinder 6 rises again while others remain low, the problem is confirmed and localized.
Mode $06 is especially useful for intermittent misfires when P0306 appears sporadically and is hard to reproduce on a test bench.
Related codes and diagnostic priority
P0306 rarely appears alone. The PCM often records multiple related codes simultaneously. Proper prioritization saves time and helps address the root cause rather than symptoms.
Priority map
P0171 / P0174 (System too lean) + P0306
- Priority: fix the lean condition first.
- Logic: air leaks or low fuel pressure cause lean mixtures in all cylinders, but cylinder 6 may be most sensitive due to intake tract design.
- Actions: check vacuum hoses, intake manifold gaskets, fuel pressure, MAF/MAP sensors. After fixing leaks or restoring fuel pressure, rescan the system.
P0101 / P0102 (MAF sensor) + P0306
- Priority: clean or replace the MAF sensor first.
- Logic: a faulty MAF sends incorrect airflow data, causing incorrect fuel delivery and misfires in the most sensitive cylinder.
- Actions: clean the MAF with special cleaner or replace it. Reset fuel trim adaptations.
P0420 (Catalyst efficiency below threshold) + P0306
- Priority: fix P0306 first.
- Logic: misfires send unburned fuel to the catalyst, overheating and damaging it. P0420 is a consequence, not a cause.
- Actions: eliminate misfires, then retest catalyst efficiency after completing the drive cycle.
P0356 (Ignition coil circuit F / Cylinder 6) + P0306
- Priority: fix P0356 first.
- Logic: P0356 indicates an electrical fault in the ignition coil circuit of cylinder 6, the primary cause of misfires.
- Actions: check coil wiring, ground quality, coil resistance, and PCM control signal.
P0206 (Injector circuit 6 / Open circuit) + P0306
- Priority: fix P0206 first.
- Logic: an injector circuit fault for cylinder 6 directly points to fuel delivery issues in that cylinder.
- Actions: check injector connector, wiring, coil resistance, and PCM control signal.
General rule
If P0306 is accompanied by codes indicating systemic issues (air leaks, low fuel pressure, faulty MAF/MAP sensors), address the systemic cause first. If codes point to a specific component of cylinder 6 (coil, injector), start with that component.
Methods to fix error P0306
| Possible cause | Fix method | Approximate difficulty |
|---|---|---|
| Spark plug | Replace plug, set correct gap per OEM specs | Low |
| Ignition coil | Diagnose by swapping, replace faulty coil | Low–medium |
| High-voltage wire/boot | Replace set | Low |
| Injector of cylinder 6 | Clean/replace, code injector if needed | Medium |
| Low fuel pressure | Check pump/filter, repair as needed | Medium |
| Air leak | Fix leaks, replace gaskets | Medium |
| Low compression | Repair cylinder head/engine | High |
| CKP/CMP/PCM | Check, adjust gap, update software, replace | Medium–high |
Repairs involve replacing worn parts, cleaning the fuel system, and restoring intake tract sealing. Start with simple items—plugs and coils—and proceed to more complex issues like compression and cylinder head problems.
“After replacing ignition or fuel system components, always reset fuel trim adaptations and complete a full drive cycle to finalize all OBD-II monitors.” – Experienced diagnostician, 10+ years
How to clear error P0306 after repair
After fixing the cause, clear the code with an OBD-II scanner and perform a drive cycle: warm the engine to operating temperature, idle for 2–3 minutes, partial loads (40–60% throttle), full loads (80–100% throttle) followed by deceleration.
Ensure the misfire monitor status is “Ready”—this means the PCM completed the self-check and the code did not return. Check Ready status in Mode $01 PID $01 on the scanner—all monitors should show “Complete” or “Ready.”
If the code reappears, the issue is not fully resolved or was misdiagnosed.
Do not clear codes before recording freeze-frame data—this information is vital for verifying the conditions when the code set and for accurate diagnostics if the code returns.
Prevention of P0306 code appearance
Regular maintenance according to manufacturer schedules reduces the likelihood of P0306. Timely replacement of spark plugs, fuel and air filters, and use of quality fuel with the recommended octane rating help prevent problems.
Inspect ignition coils and high-voltage wires at every service—visual checks for cracks, arcing marks, and connector corrosion. Clean throttle body and intake manifold deposits to help prevent air leaks and unstable idle.
Regular diagnostics of fuel trims and adaptations with an OBD-II scanner help detect problems early—before codes appear.
Seasonal preventive checklist
- Spark plug replacement: follow manufacturer schedule (typically every 18,000–37,000 miles (30,000–60,000 km), depending on plug type)
- Fuel and air filter replacement: per schedule (typically every 9,000–12,000 miles (15,000–20,000 km))
- Ignition coil and high-voltage wire inspection: at every service
- Throttle body cleaning: per schedule (typically every 25,000–31,000 miles (40,000–50,000 km))
- Intake tract sealing check: if unstable idle occurs
- Fuel trim diagnostics (STFT/LTFT): every 6 months or if symptoms appear
Tools and safety measures
Basic toolset for P0306 diagnostics
- OBD-II scanner with Mode $06 support (e.g., BlueDriver, FIXD, Autel AL519)
- Multimeter for checking coil, plug, and wiring resistance
- Fuel pressure gauge for measuring rail pressure
- Compression tester for cylinder compression measurement
- Leak-down tester (optional, for leak localization)
- Smoke machine or alternative (propane torch with hose) for air leak detection
- Automotive stethoscope for listening to injectors and valves
- Set of wrenches and sockets for removing/installing plugs, coils, intake parts
Safety precautions
When working with the ignition system:
- Do not touch high-voltage components (plugs, coils, wires) with the engine running—voltages can reach tens of kilovolts.
- Use insulated tools and dielectric gloves when checking spark.
- Avoid open spark tests (lifting a coil above a plug) near fuel sources—risk of gasoline vapor ignition.
When working with the fuel system:
- Relieve fuel rail pressure before disconnecting fuel lines.
- Work in a well-ventilated area or outdoors.
- Keep a fire extinguisher nearby.
- Do not smoke or use open flames near the fuel system.
When performing compression and leak-down tests:
- Ensure the transmission is in neutral, the parking brake is engaged, and the wheels are blocked.
- Do not apply excessive pressure to the cylinder—follow the leak-down tester instructions.
DecodeAuto content is for educational purposes only and does not replace professional inspection or diagnostics at a specialized repair shop.
Frequently asked questions
What is the difference between P0306 and P0300?
P0306 indicates misfires specifically in the sixth cylinder, where the number 6 denotes the cylinder with the issue. P0300 signals random or multiple misfires across several cylinders without specifying a particular one. This distinction is critical for diagnostics. For P0306, start checking the sixth cylinder — spark plug, coil, injector, compression. For P0300, check general components — fuel pressure, fuel quality, crankshaft/camshaft position sensors, engine grounds, and overall intake system sealing. Swapping components between cylinders helps localize the problem.
Is it possible to swap ignition coils for diagnostics?
Yes, this is a quick and reliable method to pinpoint the fault. Swap the sixth cylinder’s coil with the coil from the second or fourth cylinder. Clear the codes with a scanner and start the engine. Let it idle for 2–3 minutes, then take a short drive. If the P0306 code disappears and is replaced by P0302 or P0304, the coil is faulty and needs replacement. If the code remains on the sixth cylinder, the coil is not the issue; continue diagnostics.
Why does P0306 occur more often during cold starts?
Engines with direct injection (GDI) commonly experience carbon buildup on intake valves. Unlike port injection systems where fuel washes over the valves and removes deposits, in GDI engines the valves contact only air and crankcase gases. Over time, carbon deposits accumulate on the valve faces and can affect sealing. During cold starts, this can reduce compression and disrupt cylinder filling, causing misfires. As the engine warms up, the symptoms may improve. If P0306 appears mainly on cold start and vanishes after warming up, inspect the intake valves with a borescope and clean them if necessary (with walnut shell blasting or chemical methods).
P0306 appeared after rain/washing — what is the cause?
Water entering the spark plug wells can cause spark leakage to ground. High voltage finds the path of least resistance — through water to the engine block, bypassing the spark plug. The result is no spark in the cylinder and a misfire. A typical scenario: water accumulates in spark plug wells due to a leaking valve cover, damaged well seals, or careless engine bay washing. On some models (especially V engines), the wells of the 5th and 6th cylinders are located lower on the cylinder head and are more prone to water accumulation. What to do: Open the hood and inspect spark plug wells for water. If water is present, remove it with a syringe, blow out the wells with compressed air, and let them dry. Check the condition of spark plug well seals and the valve cover gasket — replace if worn. After drying, start the engine and check if the code disappears. To prevent this, avoid directing high-pressure water jets at the valve cover during engine bay washing.