P0312 OBD-II Trouble Code: Cylinder 12 Misfire Detected

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Cost of diagnosing the P0312 code

P0312 is a diagnostic trouble code (DTC) for "Cylinder 12 Misfire Detected". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.

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P0312 code definition

The definition of trouble code P0312 is that cylinder #12 has misfired.

What the P0312 code means

Every vehicle’s engine relies on the use of cylinders to operate. Each of these cylinders needs a spark plug that is located at its top. These spark plugs fire one after the next to ignite the mixture of air and fuel that is necessary for a vehicle’s engine to work.

Once ignited, the mixture produces energy that then pushes the crankshaft. Cylinders take turns to regulate the crankshaft’s rotation. Firing in sequence like this allows the crankshaft to rotate seamlessly and the vehicle to traverse the road smoothly.

Sometimes, a cylinder will misfire. When this happens, the RPM (revolutions per minute) of the crankshaft may increase or decrease. There is a predetermined range of RPM the manufacturer has deemed as appropriate for the vehicle. If a misfire causes the crankshaft to reach an RPM that is 2% above or below this range, the PCM (powertrain control module) will store a trouble code. In a 12-cylinder engine, when the cylinder responsible for the misfire is #12, the code that is logged is P0312.

What causes the P0312 code?

The easiest diagnosis and by far the most common reason for P0312 to be stored is that something is wrong with its spark plug. It could be completely broken or it might have become dirty somehow.

There could also be a lack of compression in this cylinder. That would lead to a lack of RPMs because there wouldn’t be as much power coming out.

Other common causes are:

  • Burned valves
  • Faulty fuel injectors
  • Bad ignition timing
  • A vacuum leak
  • Faulty crankshaft sensor
  • Faulty camshaft sensor
  • Faulty mass airflow (MAF) sensor
  • Problems with the catalytic converter

Although it’s hardly ever the case, it is possible that the PCM is malfunctioning. If that were the case, though, you most likely wouldn’t experience the following symptoms.

What are the symptoms of the P0312 code?

As with a lot of trouble codes, P0312 will cause the Check Engine Light to come on. If the RPMs go above or below the margin by up to 10%, the light is activated. However, one big difference is that with this code, the light can actually be made to blink. This is what happens if RPMs go outside 10% of the acceptable margin. That’s because it means the #12 cylinder is misfiring because it isn’t getting a spark from its spark plug – a much more serious problem.

Other symptoms include:

How does a mechanic diagnose the P0312 code?

A mechanic will begin by using an OBD-II scanner to find out which problem codes have been stored by the PCM. They will then inspect the engine components. This would include cylinder #12, its spark plug, wiring and connectors. After making necessary repairs and replacements, a test drive would be necessary while the scanner is plugged in and live.

Common mistakes when diagnosing the P0312 code

The most common mistake is when a mechanic just replaces the spark plug and assumes that will be sufficient. As you can probably tell from the above, a P0312 code can represent a number of other problems too so even if the spark plug was faulty a comprehensive diagnosis must still be carried out.

How serious is the P0312 code?

This is definitely a code to take seriously. Some of the potential symptoms we mentioned above could actually make it dangerous to operate the vehicle. Even if the dangerous symptoms aren’t present, it will still be difficult to drive an automobile with this code.

What repairs can fix the P0312 code?

The following repairs are usually sufficient:

The worst case scenario would be having to replace the engine because cylinder damage had become so extensive.

Additional comments for consideration regarding the P0312 code

Always replace spark plugs in pairs. However, P0312 usually means that the rest of the vehicle could do with a tune-up too, so it’s probably best to have one carried out in the near future.

Need help with a P0312 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

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