How To Fix P0306 Code [Misfire Detected]

This diagnostic trouble code (DTC) is a generic powertrain code, which suggests that it applies to OBD-II equipped vehicles. All OBD II equipped vehicles use a distributor-less, coil-over-plug (COP), high-intensity spark, and ignition. We will look at the causes of this error code and steps to fix it.

A P0306 specifically applies to the six-cylinder. The P0306 Code is being stored in your OBD II vehicle is that the powertrain control module (PCM) has detected a private cylinder misfire.

In a realistic sense, the camshaft position sensor and crankshaft position sensor are vital to the operation of the OBD II ignition. Using input signals from these sensors, the PCM delivers a voltage signal that causes the high-intensity ignition coils (usually one for every cylinder) to fireside in sequential order. 

What Causes the P0306 Code?

This type of code could also be caused by a fuel delivery problem, an outsized vacuum leak, an exhaust gas recirculation (EGR) malfunction, or mechanical breakdown, but is most often the result of an ignition defect leading to a coffee or no spark condition

The PCM calculates input signals from the crankshaft position sensor, camshaft position sensor, and throttle position sensor (among others depending upon the vehicle) to configure an ignition spark timing strategy. 

P0306 code could mean the following: 

Defective ignition coil(s) 
Bad spark plug(s), spark plug wires, or spark plug boots 
Faulty fuel injector(s) 
Malfunctioning fuel delivery system ( fuel pump, fuel pump relay, fuel injectors, or fuel filter) 
Major engine vacuum leak 
EGR valve stuck in the wide-open position 
Clogged EGR ports 

What Are The Steps To Diagnose & Repair P0306 Code?

A reliable vehicle information source is required to diagnose a stored (or pending) code P0306.  Also required are diagnostic scanner, and digital volt/ohmmeter (DVOM)

Step1: Begin your diagnosis with a visual inspection of the affected ignition coil, spark plug and spark plug boot

Step 2: Components that are contaminated with liquid (oil, engine coolant, or water) must be cleaned or replaced 

Step 3: If the recommended maintenance interval demands (all) spark plug replacement, this is a good time to do it 

Step 4: Inspect primary wiring and connectors, for the ignition coil in question, and make repairs as necessary 

Step 5: Key-on-engine-running (KOER), listen for the presence of an outsized vacuum leak and make repairs if necessary 

Step 6: If lean exhaust codes or fuel supply codes accompany the misfire code, they ought to be diagnosed and repaired first 

Step 7: All EGR valve position codes should be rectified before diagnosing a misfire code 

Step 8: Insufficient EGR flow codes should be addressed before diagnosing this code 

Once these are done, connect the scanner to the vehicle diagnostic port and retrieve all stored codes and freeze frame data. I like to write this information down as it can be helpful later. Check again if the P0306 is reset during a lengthy test drive. 

If the code is reset, use your vehicle information source to search technical service bulletins (TSB) which pertain to the symptoms and code(s) in question.

Since TSB lists are compiled from many thousands of repairs, the information found in the appropriate one will likely aid you in making a correct diagnosis.

Pinpoint the exact cause of the malfunction. You may spend many hours testing individual components but I have a simple system to accomplish this task. 

The procedure described is for a vehicle equipped with an automatic transmission. Manual transmission-equipped vehicles can also be tested this way but it is more challenging. It is as follows: 

Determine within what RPM range the misfire most often occurs. 
With the RPM range determined, start the engine and permit it to succeed in normal operating temperature

Place a chock on both sides of the drive wheels of the vehicle. Have an assistant get into the driver’s seat and place the shifter in DRIVE with the parking brake set and their foot firmly pressing down on the pedal

Take up an edge alongside the front of the vehicle, so that you’ll reach the engine, with the hood open and secure 

Have the assistant gradually increase the RPM level by pressing the accelerator until the misfire is exhibited 

With the engine misfiring, CAREFULLY lift the ignition coil from its perch and note the degree of a high-intensity spark being produced 

High-intensity spark should be azure in color and stunning in intensity.
If it’s not, suspect a defective induction coil If you’re unsure about the extent of spark produced by the coil in question, lift a known working coil from its perch and observe the degree of spark

Replacement of the corresponding sparking plug and boot/wire is suggested if the induction coil must get replaced.

If the induction coil seems to be firing normally, turn the engine off and insert a known good sparking plug within the boot/wire 

Restart the engine and have the assistant repeat the procedure Observe high-intensity spark across the spark plug. It should even be azure and intense.

If not, a defective spark plug for the cylinder should be checked. If a high-intensity spark (for the affected cylinder) seems normal, you may perform a similar test for the fuel injector by carefully disconnecting it to ascertain if any difference in engine RPM is detected.

If the fuel injector is inoperative, use a node light to test for voltage and a ground signal (at the injector connector) while the engine is running 

Code Severity & Symptoms 

Conditions which promote storage of a P0306 are likely to cause converter and/or engine damage. This code should be classified as severe. 

Symptoms may include: 

Diminished engine performance 
Rough or unstable feeling from the engine (at idle or under light acceleration) 
Odd odor from engine exhaust 
MIL (malfunction indicator lamp) illumination 


In most cases, you’ll have found the explanation for the misfire by the time you finish testing for high-intensity spark.

EGR systems that use a system for injecting exhaust gases into individual cylinders are known to cause symptoms that simulate a misfire condition. Use caution when testing high-intensity spark. 50,000-volts are often harmful or may be fatal in extreme circumstances.

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