P0306: Cylinder 6 Misfire Detected

What is P0306?

P0306 is a generic diagnostic trouble code (DTC) defined as “Cylinder 6 Misfire Detected.”

Your car’s Engine Control Module (ECM) or Powertrain Control Module (PCM) registers this code when it detects a misfire on cylinder number 6 of your engine. 

Vehicles with internal combustion engines move when they ignite the air and fuel mixture inside the engine cylinder. The power generated through fuel combustion makes the cylinder pistons move up and down and rotate the crankshaft, which helps drive the wheels. 

A misfire typically occurs when the ignition timing of your vehicle is off, causing the combustion reaction in the misfiring cylinder to fail. When that happens, the engine speed fluctuates, and the crankshaft position sensor (crank sensor) signals vary. 

When your ECM detects this signal fluctuation, it logs the respective misfire code and activates the Check Engine Light.  

Most modern-day vehicles have 4, 6, or 8 cylinder engines. However, the P0306 code only applies to vehicles with more than six cylinders and explicitly denotes a misfire on cylinder number 6.

Common symptoms

Typically, the P0306 code is accompanied by a couple of telltale signs. These include: 

• An illuminated or flashing Check Engine Light
• The engine runs rough, hesitates, or jerks on acceleration
• Rough idling
• Lack of engine power or engine speed
• Decreased fuel economy
• Trouble starting the engine
• Fuel smell from the exhaust

Can I still drive?

If your vehicle’s ECM has registered the P0306 misfire code and you can see an illuminated or flashing Check Engine Light, you should stop driving immediately. 

A misfire is a severe car malfunction. If ignored, a misfiring cylinder could lead to ignition failure, catalytic converter damage, and several drivability issues. Some of these repairs can be pretty expensive. 

A misfire can also cause your engine to run rich, thereby decreasing your vehicle’s fuel efficiency.

P0306 causes

A cylinder 6 misfire could be caused by a wide range of reasons, including a faulty ignition system, a worn spark plug failing to generate a high intensity spark or low engine compression. 

Here are the common reasons for a misfire on cylinder number 6: 

• Worn out spark plugs, spark plug wire, or spark plug boot
• A damaged ignition coil or coil pack 
• Distributor failure 
• Clogged or faulty fuel rail, fuel injector, or fuel injector connector 
• Low fuel pressure 
• Old or contaminated fuel 
• Vacuum leak 
• Defective crankshaft position sensor (crank sensor) 
• Failing camshaft position sensor 
• Bad mass air flow sensor 
• Failing throttle position sensor (attached to the throttle body)
• Engine timing off
• Low engine compression 
• Leaking valve cover 
• Leaking head gasket

Diagnosis

Several reasons could trigger a cylinder 6 misfire. So, an accurate diagnosis is essential to find the root cause and avoid expensive future repairs. 

Here’s how your mechanic will diagnose the P0306 code: 

• Your mechanic will start the misfire diagnosis using an OBD-II scan tool to ensure no other engine code is present. If your ECM has registered an additional code, they’ll address it first. 

• They’ll note the freeze frame data to examine the exact misfire condition.

• Next, they’ll clear the code and test drive your vehicle to see if the code returns. 

• If the code reappears and the Check Engine Light comes on, they’ll check the cylinder 6 ignition coil or spark plug wire for any loose wire connection. They’ll check if any plug wire leaks ignition spark to the ground. 

• They visually inspect the spark plug, plug wire, and the spark plug boot on cylinder 6 for signs of damage. 

• They’ll then move the spark plug or ignition coil to a different cylinder to check if the misfire moves to another cylinder. In that case, the ignition coil or the spark plug is faulty and should be replaced. 

• Next, your mechanic will ensure that your engine is getting sufficient fuel. They’ll check the fuel pressure as a low fuel pressure could cause intermittent misfires. If the pressure is low, they’ll inspect the fuel injector for any clogging. They’ll also check the injector connector and wiring. 

• If they don’t spot any faults in the ignition system, your mechanic will run a compression and leak-down test. This will help diagnose any mechanical fault causing the misfire.

Possible repairs for P0306 & Costs

Fixing this P0306 cylinder 6 misfire usually begins with an hour of diagnostic time, for which a mechanic may charge you $75-$150.

Based on the diagnosis, here are some of the repairs or replacements that your mechanic may suggest: 

• Replace the faulty ignition coil, spark plug, or plug wire (ideally, you should replace all the spark plugs together)
• Replace the leaking valve cover 
• Fix the vacuum leak 
• Replace the fuel injector
• Replace the fuel pump or fuel pressure regulator 
• Replace a faulty PCM 

Here are the average cost estimates for some of the DTC P0306 repairs (including the labor charges): 

• Spark plugs: $66-$250
• Vacuum leak: $100-$200
• Spark plug wire: $180-$240
• Fuel pressure regulator: $200-$400
• Ignition coil: $230-$640 
• Engine control module: $200-$1,200
• Fuel pump: $1300-$1700
• Fuel injector: $1500-$1900