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

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.

A P0307 code means the car’s computer has detected that one of the engine’s cylinders is misfiring. In this case, it’s cylinder #7.

 

Symptoms

Symptoms may include:

Engine may be harder to start
Engine may stumble/stutter and/or hesitate
Other symptoms may also be present

Causes

A P0307 code could mean one or more of the following has occurred:

Faulty spark plug or wire
Faulty ignition coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve
Faulty catalytic converter(s)
Fuel shortage
Poor compression
Faulty computer

Possible Solutions

If there are no symptoms, the simplest thing to do is reset the code and see if it returns.

If there are symptoms such as the engine stumbling or hesitating, check all wiring and connectors leading to the cylinders (e.g., spark plugs). Depending on how long the ignition components have been in service, it may be a good idea to replace them as part of your regular maintenance schedule. I recommend spark plugs, spark plug wires, distributor cap and rotor (if applicable). Otherwise, check the ignition coils (e.g., coil packs). In some cases, the catalytic converter has failed. If you smell rotten eggs from the exhaust, your catalytic converter needs to be replaced. I’ve also heard in other cases that the issues were due to faulty fuel injectors.

What does it mean?

This diagnostic 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.

A P0309 code means the car’s computer has detected that one of the engine’s cylinders is not functioning properly. In this case, it’s cylinder #9.

Symptoms

Symptoms may include:

The engine may be harder to start
The engine may stumble/misfire and/or hesitate
Other symptoms may also be present

Causes

A P0309 code can mean one or more of the following has occurred:

Faulty spark plug or wire
Faulty ignition coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burnt exhaust valve
Faulty catalytic converter(s)
Fuel shortage
Poor compression
Faulty computer

Possible Solutions

If there are no symptoms, the simplest thing to do is reset the code and see if it returns.

If there are symptoms such as engine stumbling or hesitation, check all cables and connectors leading to the cylinders (i.e., spark plugs). Depending on the lifespan of the ignition components in the car, it may be wise to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap and rotor (if applicable). Otherwise, check the coils (e.g., coil packs). In some cases, the catalytic converter has failed. If you smell rotten eggs in the exhaust, your catalytic converter needs to be replaced. I’ve also heard in other cases that the issues were faulty fuel injectors.

P011E, P011F ISO/SAE Reserved

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.

A P0311 code means the vehicle’s computer has detected that one of the engine’s cylinders is not functioning properly. In this case, it’s cylinder #11.

Symptoms

Symptoms may include:

Engine may be harder to start
Engine may stumble/misfire and/or hesitate
Other symptoms may also be present

Causes

A P0311 code can mean one or more of the following has occurred:

Faulty spark plug or wire
Faulty ignition coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burnt exhaust valve
Faulty catalytic converter(s)
Fuel shortage
Poor compression
Faulty computer

Possible Solutions

If there are no symptoms, the simplest thing to do is reset the code and see if it returns.

If there are symptoms such as engine stumbling or hesitation, check all wires and connectors leading to the cylinders (i.e., spark plugs). Depending on the service life of ignition components in the vehicle, it may be wise to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap and rotor (if applicable). Otherwise, check the coils (e.g., coil packs). In some cases, the catalytic converter has failed. If you smell rotten eggs from the exhaust, your catalytic converter needs replacement. I’ve also heard in other cases the problems were faulty fuel injectors.

What does it mean?

This is a generic powertrain code, meaning it covers all makes/models from 1996 onward. However, specific troubleshooting steps vary by vehicle.

The P0313 code refers to an engine misfire detected under low fuel conditions. It is one of the few ambiguous vehicle codes that, taken at face value, diagnosed and fixed seems fairly straightforward.

The code is set when the computer—through signals from a range of sensors—determines that the engine failure is due to a lean condition (too much air and not enough fuel). If the fuel level is low enough to uncover the fuel pump, sporadic pressure increases due to the pump’s inability to pick up the remaining fuel will cause a “lean” condition.

In all likelihood, you have either let the fuel level drop to a low level before refilling, or you have a legitimate fuel delivery issue. If the fuel system proves to be functioning correctly, several other mechanical failures can cause this scenario.

Symptoms

Symptoms may include:

P0313 misfire detected with low fuel fault code
Rough running engine
Hard or no start
Hesitation on acceleration
Lack of power

Potential Causes

Causes of this DTC may include:

Likely:

Low fuel level exposing the fuel pump
Failing fuel pump
Clogged fuel filter
Failed fuel pressure regulator
Clogged or failed fuel injectors
Short or open circuit in the fuel pump harness
Poor electrical connectors

Additional Possibilities:

Spark plugs
Ignition wires
Faulty reluctor ring
Carbon-fouled valves
Mass airflow sensor
Faulty distributor cap
Faulty coil packs
Lack of compression
Large vacuum leak

Diagnosis and Repair

It is important to start by going online and checking all TSBs (Technical Service Bulletins) related to this code. If the fuel system is not the issue, several vehicles have a particular problem that tends to set this code.

For example, BMWs have a set of three oil separator hoses under the intake manifold that, when cracked, create a vacuum leak that sets this code.

Check your factory and extended warranty to see if it is still covered and for how long.

Purchase a code scanner from your local auto parts store or borrow one. They are relatively inexpensive and not only retrieve codes but have a cross-reference sheet for explanations and can reset the computer once finished.

Connect the scanner to the OBD port under the dashboard on the driver’s side. Turn the key to “On” and press the “Read” button. Note all codes and cross-reference them with the code sheet. Additional codes might be present that will direct you to a particular area, such as:

P0004 Fuel Volume Regulator Control Circuit High
P0091 Fuel Pressure Regulator 1 Control Circuit Low
P0103 Mass or Volume Air Flow Circuit High Input
P0267 Cylinder 3 Injector Circuit Low
P0304 Cylinder 4 Misfire Detected

Repair any additional code and retest by clearing the code with the scanner and test driving the vehicle.

If no codes are present, start with the fuel filter. The following diagnostic and repair procedures require some special tools:

Special wrenches to remove the fuel filter
Fuel pressure tester and adapters
Gas can
Volt/Ohm meter

Ensure there is at least half a tank of fuel.

Connect the fuel pressure tester to the fuel test port on the fuel rail.

Open the tester valve and let the fuel pressure flow into a gas can. Close the tester valve.
Lift the vehicle and replace the fuel filter.
Turn the key and check for leaks.
Remove the fuel pump module connector and check voltage at the fuel pump. To do this, an assistant will need to press the key for five seconds and turn it off for five. The computer powers the pump for two seconds. If the computer does not see the engine turning, it shuts off the fuel pump.
Probe the connector terminals for power. At the same time, listen for the pump to power up. If there is no sound or an unusual sound, the pump is failing. Ensure the wiring harness and connector are in good condition.
Lower the vehicle and start the engine. Note the fuel pressure at idle. If the engine runs better and the fuel pressure is within specifications according to the service manual, the problem is solved.
If this does not resolve the issue, check for possible vacuum leaks in the intake manifold.
Remove the vacuum hose from the fuel pressure regulator. Look for fuel inside the hose. Fuel means the diaphragm has failed.

If the fuel pump is faulty, take it to a service center for replacement. This makes technicians nervous when dropping the fuel tank. A spark could cause a disaster. Do not attempt at home, for fear of blowing up your house and surrounding homes if an unfortunate accident were to occur.

What does it mean?

This generic powertrain/drivetrain diagnostic trouble code generally applies to most manufacturers from 1996 onwards.

These manufacturers include, but are not limited to, GM, Ford, Dodge, Chrysler, Jeep, Hyundai, Kia, Mazda, Mitsubishi, Volvo, and VW.

This code focuses on the relationship between the crankshaft position sensor (CKP), camshaft position sensor (CMP), and the engine control module (ECM/PCM). The PCM monitors the relationship between these two sensors and compares it to an ideal reference stored in the PCM. If the variation exceeds a calibrated percentage, a trouble code is stored. It could be an electrical fault or a mechanical issue, depending on the reason for the misfire and the vehicle manufacturer.

Troubleshooting steps may vary depending on the manufacturer and the type of crankshaft position sensor (CKP), camshaft position sensor (CMP), and their wire colors.

Symptoms

Symptoms of a P0314 engine code may include:

    Malfunction Indicator Lamp (MIL) illuminated
    Misfires/rough running or surging
    Lower than normal fuel economy

Causes

Typically, the causes for this code to set are as follows:

Most common issues:

    Worn ignition system components, including the coil, spark plug wires, spark plugs, etc.
    Fuel system components – to include injectors

Less likely:

    Intermittent open in the power, ground, or signal circuit to the crankshaft position sensor
    Intermittent open in the power, ground, or signal circuit to the camshaft position sensor
    Failed PCM – rarely

Diagnostic Steps and Repair Information

A good starting point is always to check the Technical Service Bulletins (TSBs) for your specific vehicle. Your issue may be a known problem with a known fix published by the manufacturer and can save you time and money during diagnosis.

Next, note if there are any other diagnostic trouble codes. If any are related to the ignition system/fuel system, diagnose them first. Misdiagnosis is known to occur if a technician diagnoses this code before any ignition system/fuel system related codes have been thoroughly diagnosed and dismissed. Ensure there are no intake or exhaust leaks. An intake leak, or vacuum leak, causes the engine to idle roughly. An exhaust leak gives the impression of a lean running engine to the air/fuel ratio sensor/oxygen sensor (AFR/O2).

After that, clear all diagnostic codes and see if the P0314 returns. If not, the problem was intermittent or the P0314 was a memory code, meaning it was something from the past.

If the code resets, meaning it comes back immediately the next time you start the vehicle or on the very next road test, further inspection is needed. Locate the CKP sensor and CMP sensor on your specific vehicle. Once located, visually inspect the connectors and wiring. Look for scrapes, chafing, exposed wires, burnt spots, or melted plastic. Disconnect the connectors and carefully inspect the terminals (the metal parts) inside the plastic connectors. See if they look burnt or corroded instead of the normal metallic color you are probably used to seeing. You can get electrical contact cleaner and a plastic bristle brush at any auto parts store if you need to clean the terminals. If not, find some rubbing alcohol and a toothbrush to clean them, make sure not to put the toothbrush back in the bathroom! Then let them air dry, get some dielectric silicone compound (same stuff they use for light bulb sockets) and put some on where the terminals make contact.

Clear the diagnostic codes from memory again and see if this code returns. If not, the connections were likely your problem.

If the code returns, remove the CKP sensor and CMP sensor from the engine. Look at the end of the sensor where it comes close to the timing components/flexplate/flywheel on the engine. Note if there is any damage, where it might have been hit by any of these components. If it appears to have been hit/damaged, it will need to be replaced. At this point, it is wise to replace it with an original equipment manufacturer part or a dealer part. Repeatedly, technicians have replaced a faulty CKP sensor, only to find they have to replace it again because the aftermarket sensor did not meet the voltage specifications the PCM was looking for. With this code, relating to these two sensors, be more suspicious of the CMP sensor, as the CKP sensor would tend to cause the engine to stall. The CMP sensor might cause it to lose its ability to identify individual cylinders.

If the previous tests passed and the code is still present, monitor the air/fuel ratio sensor/oxygen sensor (AFR/O2). If it continuously indicates the engine is running lean, locate all possibilities that could cause a lean running/misfiring engine. These include:

    Intake or exhaust leaks
    Fuel system, including fuel pressure regulator/fuel pressure
    Fuel pressure sensor
    Fuel injectors
    Ignition system
    Post-catalytic converter O2 sensor
    EVAP system, to include the canister purge control valve.

Again, it cannot be overemphasized that all other codes should be diagnosed before this one, as issues causing other codes to set can also cause this one.

If the code still cannot be cleared, the only thing left to do is to seek the assistance of a driveability specialist diagnostician.