What does it mean?
This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to vehicles equipped with OBD-II. Although generic, specific repair steps may vary by make/model.
The reason a P0306 code is stored in your OBD II vehicle is that the Powertrain Control Module (PCM) has detected an individual cylinder misfire. A P0306 specifically applies to cylinder number six. Consult a reliable vehicle information source to locate cylinder number six for the vehicle in question.
This type of code can be caused by a fuel supply problem, a significant vacuum leak, a malfunctioning Exhaust Gas Recirculation (EGR) system, or an engine mechanical failure, but is most often the result of a fault in the ignition system causing a weak or no spark condition.
Virtually all OBD II-equipped vehicles use a distributorless, coil-on-plug (COP), high-intensity spark ignition system. It is controlled by the PCM to perform precise spark and ignition timing.
The PCM calculates input signals from the crankshaft position sensor, camshaft position sensor, and throttle position sensor (among others depending on the vehicle) to configure an ignition spark timing strategy.
In a practical sense, the camshaft position sensor and crankshaft position sensor are essential to the operation of the OBD II ignition system. Using the input signals from these sensors, the PCM delivers a voltage signal that causes the high-intensity ignition coils (usually one per cylinder) to fire in a sequential order.
Since the crankshaft rotates at approximately twice the speed of the camshaft(s), it is crucial that the PCM knows their exact position; both overall and relative to each other. A simple way to explain this aspect of engine operation is as follows:
Top Dead Center (TDC) is the point where the crankshaft and camshafts align with the piston (for cylinder number one) at its highest point and the intake valve(s) (for cylinder number one) open. This is called the compression stroke.
During the compression stroke, air and fuel are drawn into the combustion chamber. At this point, an ignition spark is required to cause combustion. The PCM recognizes the position of the crankshaft and camshaft and triggers the required voltage signal to cause a high-intensity spark from the ignition coil.
Combustion in the cylinder propels the piston downward. As the engine rolls through the compression stroke and piston number one begins to retract toward the crankshaft, the intake valve(s) are closed. This begins the exhaust stroke. As the crankshaft completes another revolution, piston number one reaches its highest point again. The camshaft(s) having only completed half a turn, the intake valve remains closed and the exhaust valve is open. At the top of the exhaust stroke, no ignition spark is required, as this stroke is used to push the spent exhaust gases out of the cylinder, through the opening created by the open exhaust valve(s), and into the exhaust manifold.
Typical operation of the high-intensity ignition coil is accomplished with a constant supply of fused battery voltage, switched (only present with the ignition switch in the ON position), and a ground pulse provided (at the appropriate time) by the PCM. When the ground pulse is applied to the ignition coil’s (primary) circuit, the coil emits a high-intensity spark (up to 50,000 volts) for a fraction of a second. This high-intensity spark is transferred through a spark plug wire or boot and a spark plug, which is threaded into the cylinder head or intake manifold, where it comes into contact with a precise air/fuel mixture. The result is a controlled explosion. If this explosion does not occur, the engine’s RPM level is affected and the PCM detects this. Subsequently, the PCM monitors the camshaft position, crankshaft position, and individual coil feedback voltage inputs to determine which cylinder has misfired or is currently misfiring.
If the cylinder misfire is not consistent or severe enough, the code may appear as pending and the Malfunction Indicator Lamp (MIL) may only flash when the PCM actually detects a misfire (then turn off when it does not). The system is designed this way to alert the driver that a misfire of this degree can be harmful to the catalytic converter and other engine components. Once the misfire becomes more constant and severe, a P0306 will be stored and a steady MIL illumination will occur.
Code Severity and Symptoms
Conditions that promote the storage of a P0306 are likely to cause damage to the catalytic converter and/or engine. This code should be classified as severe.
Symptoms may include:
Decreased engine performance
Rough or unstable engine feel (at idle or under light acceleration)
Strange smell from the engine exhaust
Flashing or steady MIL (Malfunction Indicator Lamp)
Causes
A P0306 code can mean that one or more of the following events has occurred:
Faulty ignition coil(s)
Faulty spark plug(s), spark plug wires, or spark plug boots
Faulty fuel injector(s)
Faulty fuel delivery system (fuel pump, fuel pump relay, fuel injectors, or fuel filter)
Significant engine vacuum leak
EGR valve stuck in the wide-open position
Clogged EGR ports
Diagnostic and Repair Steps
A diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reliable source of vehicle information will be needed to diagnose a stored (or pending) P0306 code.
Start your diagnosis with a visual inspection of the relevant ignition coil, spark plug, and spark plug boot
Components contaminated with fluid (oil, engine coolant, or water) should be cleaned or replaced
If the recommended maintenance interval requires (all) spark plug replacement, now is a good time to do it
Inspect the main wiring and connectors for the ignition coil in question, and perform necessary repairs
Key On Engine Running (KOER), check for the presence of a significant vacuum leak and perform repairs if necessary
If lean exhaust codes or fuel supply codes accompany the misfire code, they should be diagnosed and repaired first
Any EGR valve position codes must be rectified before diagnosing a misfire code
Insufficient EGR flow codes should be addressed before diagnosing this code
Once all the issues listed above are resolved, connect the scanner to the vehicle’s diagnostic port and retrieve all stored codes and freeze frame data. I recommend noting this information as it may be useful later. Now, clear the codes and see if the P0306 resets during a long road test.
If the code resets, use your vehicle information source to look for Technical Service Bulletins (TSBs) that relate to the symptoms and code(s) in question. Since TSB listings are compiled from thousands of repairs, the information found in the appropriate listing will likely help you make a correct diagnosis.
Be sure to locate the misfiring cylinder. Once this is done, you need to identify the exact cause of the malfunction. You can spend many hours testing individual components, but I have a simple system for accomplishing this task. The procedure described is for a vehicle with an automatic transmission. Vehicles with manual transmissions can also be tested this way, but it is more difficult.
It is as follows:
Determine in which RPM range the misfire occurs most frequently. This can be done by testing or checking freeze frame data
With the RPM range determined, start the engine and let it reach its normal operating temperature
Place wheel chocks on both sides of the vehicle’s drive wheels
Have an assistant sit in the driver’s seat and place the gear selector in DRIVE with the parking brake applied and foot firmly on the brake pedal
Position yourself along the front of the vehicle so you can reach the engine with the hood open and secured
Have the assistant gradually increase the RPM level by pressing the accelerator pedal until the misfire manifests
With the engine misfiring, CAREFULLY lift the ignition coil from its seat and note the degree of high-intensity spark produced
The high-intensity spark should be bright blue in color and of surprising intensity. If not, suspect a faulty ignition coil
If you are unsure of the spark level produced by the coil in question, lift a known working coil from its seat and observe the degree of spark
Replacement of the corresponding spark plug and boot/wire is recommended if the ignition coil is to be replaced
If the ignition coil appears to be functioning normally, stop the engine and insert a known good spark plug into the boot/wire
Restart the engine and have the assistant repeat the procedure
Observe the high-intensity spark through the spark plug. It should also be bright blue and intense in nature. If not, suspect a faulty spark plug for the cylinder in question
If the high-intensity spark (for the affected cylinder) appears normal, you can perform a similar test for the fuel injector by carefully disconnecting it to see if an engine RPM difference is detected. A functioning fuel injector will also emit an audible ticking sound
If the fuel injector is not operating, use a test light to check for voltage and a ground signal (at the injector connector) while the engine is running
In most cases, you will have found the cause of the misfire by the time you finish testing the high-intensity spark.
EGR systems that use a method to inject exhaust gas into individual cylinders are known to cause symptoms mimicking a misfire. EGR cylinder ports become clogged and cause all EGR gas to be dumped into a single cylinder, causing a misfire
Be cautious when testing a high-intensity spark. 50,000 volts can be harmful, even fatal under extreme circumstances
When testing a high-intensity spark, keep it away from fuel sources to avoid catastrophe