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P0312 Code: Cylinder 12 Misfire Diagnostic Procedure

P0312 code identifies a combustion misfire associated with Cylinder 12 but does not identify the failed component.

The P0312 OBD-II trouble code means the engine control module has detected a combustion misfire associated with Cylinder 12.

Unlike P0300, which indicates random or multiple-cylinder misfires, P0312 identifies one specific cylinder. That provides a focused starting point, but it does not prove the Cylinder 12 ignition coil has failed.

A worn spark plug, weak ignition coil, defective fuel injector, damaged electrical circuit, intake leak, low compression, valve problem, or incorrect mechanical timing can all trigger P0312.

The engine computer knows Cylinder 12 produced less power than expected. It does not know which component deserves replacement. For that, unfortunately, we still need testing instead of interpretive dance around an open hood.

P0312 belongs to the misfire-code family covered in the complete Pro Street P-code OBD-II guide.

P0312 Quick Answer

ItemInformation
Trouble codeP0312
DefinitionCylinder 12 Misfire Detected
SystemIgnition and combustion
SeverityHigh
Safe to drive?Only briefly if the engine runs smoothly and the Check Engine Light is not flashing
Most common causesSpark plug, ignition coil, injector, wiring, intake leak, or low compression
Common applicationsV12 and W12 engines
Related codesP0212, P0362 and P0300
Typical repair rangeApproximately $100 to $15,000+, depending on the vehicle and cause

What Does the P0312 Code Mean?

P0312 breaks down as follows:

The engine control module monitors slight changes in crankshaft speed during each combustion event. Every properly operating cylinder accelerates the crankshaft when combustion pushes its piston downward.

When Cylinder 12 repeatedly contributes less power than expected, the crankshaft momentarily slows. The control module recognizes that pattern and stores P0312.

Depending on the frequency and severity of the failure, the Check Engine Light may remain steadily illuminated or begin flashing.

P0312 identifies the cylinder associated with the combustion problem. It does not identify whether the failure involves:

For broader information about combustion failures, visit our engine misfire symptoms, causes and diagnosis guide.

Which Vehicles Can Store P0312?

P0312 only applies to engines with at least 12 cylinders.

Common configurations include:

Applications may include some vehicles from:

The basic definition remains consistent across OBD-II vehicles, but cylinder location, ignition design, injector control, access procedures, and specifications vary considerably.

A generic code reader may identify Cylinder 12 in seconds. Actually reaching Cylinder 12 may require intake removal, several pages of service instructions, and a quiet moment to reconsider some life choices.

Where Is Cylinder 12 Located?

Cylinder 12 location depends on the manufacturer and engine family.

On a V12 engine, Cylinder 12 may be:

Some manufacturers place odd-numbered cylinders on one bank and even-numbered cylinders on the opposite bank. Others number cylinders sequentially along each bank.

W12 engines use a more complicated cylinder arrangement that should be identified only with manufacturer-specific service information.

Before removing a coil, plug, or injector, identify Cylinder 12 using:

Never assume Cylinder 12 is simply the last ignition coil at the rear of the engine.

Testing the wrong cylinder wastes time and can create another misfire if an unrelated connector, coil boot, or wiring harness is disturbed.

Common Conditions Associated With P0312

A Cylinder 12 combustion problem may cause:

A single-cylinder misfire may be difficult to feel on a V12 because the other 11 cylinders can partially mask the loss of contribution.

That does not make P0312 harmless. Unburned fuel from one cylinder can still overheat its catalytic converter, contaminate the engine oil, and increase emissions.

How Serious Is P0312?

P0312 should be treated as a high-priority trouble code.

An active misfire may send unburned fuel into the exhaust. That fuel can ignite inside the catalytic converter, causing it to overheat or melt.

Continued driving may contribute to:

Stop Driving Immediately If:

A flashing Check Engine Light generally indicates a catalyst-damaging misfire. Reduce engine load, pull over safely, and shut off the engine.

If the light remains steady and the engine runs reasonably smoothly, driving a short distance for diagnosis may be possible. Avoid heavy throttle, high RPM, towing, and boost.

Most Common Causes of P0312

Cylinder 12 can misfire because of an ignition, fuel, electrical, airflow, compression, or mechanical problem.

Possible causeRelative likelihood
Worn or fouled spark plugVery high
Weak or failed ignition coilVery high
Damaged coil boot or spark plug wireHigh
Clogged or failed fuel injectorHigh
Coil or injector wiring faultMedium
Intake leak near Cylinder 12Medium
Low Cylinder 12 compressionMedium
Oil or coolant in the spark plug wellApplication dependent
Carbon buildup on an intake valveMedium on direct-injected engines
Burned or leaking valveMedium
Worn camshaft lobe or failed followerApplication dependent
Damaged piston ringLower
Head gasket failureLower
Incorrect valve timingLower
PCM driver failureRare

The engine family, mileage, maintenance history, related codes, and freeze-frame conditions should determine the testing order.

How to Diagnose P0312

Step 1: Confirm P0312

Connect an OBD-II scanner and record:

Do not erase P0312 before saving the freeze-frame information.

Record:

The operating conditions may provide important clues.

When P0312 occursPossible diagnostic direction
Cold startInjector leak, deposits, coolant intrusion, or weak compression
Warm idleIntake leak, injector imbalance, or mechanical problem
Heavy accelerationWeak coil, worn plug, excessive plug gap, or insufficient fuel
Wet weatherCoil boot, connector, or insulation damage
After heat soakIgnition coil, injector, connector, or wiring fault
ConstantlyDead coil, failed injector, circuit problem, or low compression
After recent serviceLoose connector, wrong plug, damaged boot, or installation error

Step 2: Check for Related Trouble Codes

P0312 may appear with other misfire, ignition, injector, or timing codes.

Related codes include:

Several cylinder-specific codes may point toward a shared problem such as:

Step 3: Determine Whether the Misfire Is Bank-Specific

A V12 or W12 engine may store P0312 with other codes affecting the same cylinder bank.

Compare:

If several cylinders on the same bank are misfiring, investigate:

P0312 alone supports a Cylinder 12 investigation. P0312 combined with multiple same-bank faults requires a wider view.

Step 4: Verify Cylinder 12 Location

Use vehicle-specific service information to identify Cylinder 12.

Inspect the surrounding area for:

Access may require removal of:

Confirm the service procedure before beginning major disassembly.

Step 5: Inspect the Cylinder 12 Spark Plug

Remove the Cylinder 12 spark plug and compare it with a plug from a healthy cylinder.

Check for:

A fuel-wet spark plug may indicate that fuel is entering Cylinder 12 but the mixture is not igniting.

A dry plug may suggest inadequate fuel delivery.

Oil deposits may indicate:

If the spark plugs are beyond their service interval, replace the complete set using the correct type and gap.

On a V12, that may involve replacing 12 plugs and removing enough surrounding hardware to briefly wonder whether the spark plugs were installed before the rest of the car.

Step 6: Swap the Cylinder 12 Ignition Coil

If the engine uses accessible coil-on-plug ignition, move the Cylinder 12 coil to another cylinder.

For example, swap the Cylinder 12 and Cylinder 10 coils. Clear the codes and operate the engine under conditions similar to those recorded in the freeze-frame data.

If the code changes from:

P0312 to P0310

the ignition coil is likely defective.

If P0312 returns on Cylinder 12, the coil probably is not the cause.

Inspect the coil and boot for:

Review our bad ignition coil guide for general ignition-system information.

Also see our ignition coil replacement guide before installing replacement components.

If moving the coil requires extensive disassembly, electrical and waveform testing may be more practical.

Step 7: Understand P0312 Versus P0362

P0312 and P0362 describe related but different faults.

CodeMeaning
P0312The PCM detected a combustion misfire associated with Cylinder 12
P0362The PCM detected an electrical malfunction involving Ignition Coil L’s primary or secondary circuit

P0362 provides stronger evidence of an ignition-coil circuit problem.

Possible P0362 causes include:

P0312 without P0362 may still be ignition-related, but fuel delivery, airflow, compression, and valve operation must also be tested.

Coil letters frequently correspond with cylinder numbers, but manufacturer information should be checked before assuming Coil L controls Cylinder 12.

Step 8: Test the Cylinder 12 Fuel Injector

A clogged, leaking, stuck, electrically open, or unpowered injector can cause P0312.

Listen to the Cylinder 12 injector with a mechanic’s stethoscope and compare it with the other injectors.

Further testing may include:

Use manufacturer specifications for injector resistance and circuit testing.

Pro Street injector-testing resources include:

Never open a gasoline direct-injection fuel system without following the manufacturer’s pressure-relief procedure.

Step 9: Understand P0312 Versus P0212

P0212 identifies an electrical fault involving the Cylinder 12 injector circuit.

CodeMeaning
P0312Cylinder 12 combustion misfire
P0212Cylinder 12 injector circuit malfunction

If P0312 and P0212 appear together, concentrate on:

P0212 does not automatically prove the injector has failed. A damaged wire or loose connector can prevent a functional injector from operating.

Step 10: Inspect the Coil and Injector Circuits

If the ignition coil and injector test correctly, inspect their circuits.

Check for:

Use voltage-drop testing while the circuit is operating whenever possible.

A circuit may pass a continuity test while failing under load. Wiring frequently observes the difference between “technically connected” and “actually useful.”

PCM failure is possible but uncommon. Verify the component, wiring, voltage supply, grounds, and mechanical condition before replacing the control module.

Step 11: Check for an Intake Leak

An intake leak near the Cylinder 12 runner can create a localized lean condition.

Inspect:

A smoke test is generally the safest and most effective way to locate an intake leak.

Compare fuel trims between the two cylinder banks. A significantly more positive correction on Cylinder 12’s bank may support a bank-specific intake or fuel-delivery problem.

Step 12: Check Bank-Specific Fuel Delivery

Some V12 and W12 engines use separate fuel rails, pressure sensors, pumps, or control circuits.

Review:

A bank-specific fuel-pressure problem can make Cylinder 12 appear to be the primary failure while affecting several nearby cylinders to a lesser degree.

Step 13: Perform a Compression Test

If Cylinder 12 has spark, fuel, and correct electrical control, test its compression.

Compare Cylinder 12 with the remaining cylinders and use the manufacturer’s minimum pressure and allowable variation.

Low compression may be caused by:

If Cylinder 12 compression is low, continue with a cylinder leak-down test.

Step 14: Perform a Cylinder Leak-Down Test

Position Cylinder 12 at top dead center on its compression stroke and introduce regulated compressed air.

Air heard or observed atLikely fault
Throttle body or intakeLeaking intake valve
Exhaust outletLeaking exhaust valve
Oil filler openingPiston rings or cylinder damage
Cooling systemHead gasket, cylinder head, or block damage
Adjacent cylinderHead gasket failure between cylinders

Leak-down testing helps determine whether P0312 originates from internal engine damage.

Step 15: Inspect Cylinder 12 Valve Operation

If compression is low or valvetrain noise is present, inspect valve movement.

Depending on the engine, testing may require:

Reduced valve movement may indicate a failed follower, broken valve spring, collapsed lifter, or worn camshaft lobe.

Step 16: Check for Intake-Valve Deposits

Direct-injected engines can develop deposits on their intake valves because fuel no longer flows across the back of each valve.

Uneven deposits near Cylinder 12 may cause:

A borescope inspection can help confirm deposit buildup. Approved cleaning procedures may include media blasting or professional chemical cleaning.

Step 17: Clear P0312 and Verify the Repair

After repairing the confirmed fault:

  1. Clear stored and pending codes.
  2. Start the engine.
  3. Verify a stable idle.
  4. Monitor Cylinder 12 misfire counters.
  5. Recreate the original freeze-frame conditions when safe.
  6. Complete the required drive cycle.
  7. Scan for pending codes.
  8. Confirm that readiness monitors complete normally.

The repair is confirmed when Cylinder 12 misfire counts remain normal and P0312 does not return.

Common P0312 Repairs and Costs

Repair costs can be high on V12 and W12 engines because component access may require considerable disassembly.

RepairApproximate installed cost
Replace spark plugs$300–$2,500
Replace one ignition coil$150–$1,200
Replace coil boot$100–$500
Clean or replace one injector$400–$3,000
Repair wiring or connector$200–$1,500
Repair intake or vacuum leak$300–$3,000
Intake-valve carbon cleaning$1,000–$5,000
Replace intake manifold gasket$1,500–$6,000
Replace failed follower or camshaft$3,000–$15,000+
Cylinder-head or valve repair$5,000–$20,000+
Engine replacement$15,000–$75,000+

These are broad estimates. Parts availability, labor rates, vehicle design, and access can dramatically change the final price.

The failed part may be inexpensive. Unfortunately, the manufacturer may have protected it behind every other component produced during that model year.

Common P0312 Diagnostic Mistakes

Replacing Coil 12 Without Testing It

Swap the coil with another cylinder when practical. If the fault follows, replace it. If P0312 remains, continue testing.

Testing the Wrong Cylinder

Cylinder numbering differs between V12 and W12 engines. Confirm Cylinder 12 before removing components.

Confusing P0312 With P0362

P0312 identifies a combustion misfire. P0362 identifies an electrical problem involving Ignition Coil L’s circuit.

Ignoring P0212

P0312 combined with P0212 points toward the Cylinder 12 injector or its circuit and should change the diagnostic priority.

Ignoring Same-Bank Codes

Several codes on the same bank may indicate a shared intake, fuel-delivery, timing, wiring, or exhaust problem.

Replacing the Injector Without Testing It

Confirm injector power, control, resistance, and fuel delivery before replacement.

Clearing Freeze-Frame Data Too Early

Freeze-frame information may be the best evidence of the conditions that triggered P0312.

Skipping Compression Testing

If ignition and fuel tests pass, verify the mechanical condition of Cylinder 12.

Replacing the Catalytic Converter First

The catalytic converter may have been damaged by P0312. Repair the misfire before evaluating catalyst performance.

P0312 Frequently Asked Questions

Does P0312 Mean Ignition Coil 12 Is Bad?

No. A failed coil is common, but P0312 may also result from a spark plug, injector, wiring, intake leak, compression problem, or valvetrain failure.

Can P0362 Cause P0312?

Yes. P0362 identifies an electrical malfunction involving Ignition Coil L, which commonly corresponds with Cylinder 12. Verify the manufacturer’s coil assignment first.

Can P0212 Cause P0312?

Yes. A Cylinder 12 injector-circuit failure can prevent the cylinder from receiving the correct amount of fuel.

Can a Spark Plug Cause P0312?

Yes. A worn, cracked, fouled, overheated, or incorrectly gapped Cylinder 12 spark plug can trigger P0312.

Can a Fuel Injector Cause P0312?

Yes. A clogged, leaking, stuck, electrically failed, or unpowered Cylinder 12 injector can create a cylinder-specific misfire.

Can an Intake Leak Affect Only Cylinder 12?

Yes. An intake manifold or injector-seal leak near the Cylinder 12 runner may create a localized lean condition.

Can a Bank-Specific Fuel Problem Trigger P0312?

Yes. Separate fuel rails, pumps, sensors, or control circuits may cause several cylinders on one bank to run incorrectly, with Cylinder 12 recording the highest misfire count.

Can Low Compression Cause P0312?

Yes. Burned valves, worn rings, piston damage, valvetrain faults, or head gasket failure can reduce Cylinder 12 compression.

Why Does P0312 Only Appear Under Acceleration?

A weak coil, worn plug, excessive plug gap, restricted injector, or fuel-pressure problem may only become noticeable as engine load increases.

Can I Drive With P0312?

Only briefly if the engine runs smoothly and the Check Engine Light remains steady. Stop driving if the light flashes, the engine shakes severely, power drops, or raw fuel can be smelled.

Can P0312 Damage the Catalytic Converter?

Yes. Unburned fuel can overheat and destroy the catalytic converter.

Will P0312 Clear Itself?

The Check Engine Light may turn off after several successful drive cycles, but stored or permanent code data may remain until the required self-tests are completed.

Related Misfire, Injector and Ignition Codes

Explore the Pro Street OBD-II Code Libraries

Use the Pro Street cornerstone pages to find additional diagnostic trouble codes:

P0312 identifies a combustion misfire associated with Cylinder 12. Save the freeze-frame information and verify the correct cylinder before removing components.

Inspect the spark plug, swap-test the ignition coil where practical, and test the injector and related circuits. If P0312 remains, inspect for bank-specific intake or fuel problems, low compression, and abnormal valve operation.

The correct repair is the one supported by testing—not whichever V12 component costs enough to sound convincing.

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