P0312 – Cylinder 12 Misfire Detected

Last updated: April 4, 2026

DTC P0312 is an ISO/SAE controlled, powertrain, General fault that indicates the engine control module has detected a misfire specifically attributed to cylinder 12. A “misfire” means cylinder 12 is not contributing the expected torque during its combustion event, which can make the engine run unevenly and may increase emissions. The module determines this by monitoring crankshaft speed changes and associating those changes with individual cylinder firing events. P0312 identifies the affected cylinder, but it does not identify the failed part. Correct diagnosis requires confirming the misfire pattern and then testing ignition, fuel delivery, air sealing, wiring integrity, and mechanical condition for cylinder 12.

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P0312 Quick Answer

P0312 – Cylinder 12 Misfire Detected means the ECM/PCM has identified misfire activity assigned to cylinder 12. Confirm cylinder 12 location, review freeze-frame and misfire data, then test cylinder 12 ignition, injector operation/circuits, air leaks affecting that cylinder, and engine mechanical condition.

What Does P0312 Mean?

P0312 – Cylinder 12 Misfire Detected is set when the engine controller determines that cylinder 12 is misfiring based on the OBD-II misfire monitoring strategy. In practical terms, cylinder 12 is not producing smooth, consistent power output compared to what the controller expects, which can lead to rough running, reduced power, and higher emissions.

Importantly, P0312 describes the event (a misfire) and the location (cylinder 12). It does not confirm whether the root cause is ignition-related, fuel-related, air-related, electrical, or mechanical. The same DTC can be produced by multiple fault types, so the correct next step is targeted testing on cylinder 12 using scan data and basic engine checks.

Theory of Operation

During normal operation, each cylinder’s power stroke slightly accelerates the crankshaft. The controller monitors crankshaft rotational speed changes using the crankshaft position sensor signal. By correlating crankshaft speed fluctuations with cylinder firing order (often using camshaft position synchronization), the controller can attribute abnormal speed drops to a specific cylinder event.

P0312 sets when the controller observes repeated crankshaft speed variations that match a misfire pattern during cylinder 12’s firing window. Because the algorithm is observing the result (reduced torque contribution), not the cause, anything that prevents proper combustion in cylinder 12—loss of spark, incorrect fueling, unmetered air, or low compression—can trigger the same code.

Symptoms

• Malfunction Indicator Lamp (MIL) illuminated; may flash during a severe misfire
• Rough idle or noticeable engine shake
• Hesitation or stumble during acceleration
• Reduced engine power
• Possible exhaust fuel smell if unburned fuel reaches the exhaust
• Hard start or extended cranking in some cases
• Decreased fuel economy

Common Causes

• Worn, fouled, damaged, or incorrectly gapped spark plug on cylinder 12
• Ignition coil or ignition coil boot issue affecting cylinder 12 spark output
• Fuel injector fault on cylinder 12 (electrical failure, restricted flow, or leakage)
• Open, short, high resistance, or intermittent connection in cylinder 12 ignition or injector wiring/connectors
• Vacuum leak or unmetered air affecting the intake runner/area feeding cylinder 12
• Fuel pressure or fuel volume problem that results in a cylinder-specific misfire under certain conditions
• Low compression, valve sealing issue, head gasket leak, or other mechanical condition affecting cylinder 12
• Fluid intrusion (oil/coolant/water) causing poor electrical contact or secondary ignition leakage near cylinder 12 components

Diagnosis Steps

Tools you’ll need: Scan tool capable of viewing misfire counters/Mode $06 and freeze-frame data, DVOM, basic hand tools, spark tester, and (as needed) a noid light or lab scope for injector control checks. For mechanical verification, use a compression gauge and ideally a leakdown tester. A smoke machine is helpful for finding intake air leaks.

1. Confirm the code and capture data. Verify P0312 is present (pending, stored, or confirmed). Record freeze-frame data and note engine speed, load, coolant temperature, vehicle speed, and fuel trim values at the time the DTC set.
2. Check for additional DTCs. Look for codes that could change the diagnostic direction, such as fuel system, sensor reference/ground, ignition primary, injector circuit, or multiple-cylinder misfire codes. Address codes that indicate a shared power/ground or system-wide issue first.
3. Verify cylinder 12 identification. Use accurate service information to confirm cylinder numbering and cylinder 12 location. Incorrect cylinder identification can lead to unnecessary parts replacement and missed root causes.
4. Review misfire data. Use the scan tool’s misfire counters and/or Mode $06 data to confirm cylinder 12 activity and determine whether misfires occur at idle, during a steady RPM hold, or under load.
5. Perform a focused visual inspection. Inspect cylinder 12 ignition and injector connectors, harness routing, terminal fit, corrosion, and signs of heat or abrasion damage. Look for fluid contamination around coil/plug areas (where applicable) and verify connectors are fully seated.
6. Verify power and ground integrity under load. Check the power feed(s) and ground paths relevant to ignition and injector operation. Use voltage-drop testing with the circuit operating; a connection can pass continuity checks yet fail under load.
7. Check ignition performance for cylinder 12. Inspect the spark plug condition and gap (where applicable). Use a spark tester to verify consistent spark output under conditions similar to those shown in freeze-frame data. If a secondary ignition issue is suspected, inspect for tracking, boot damage, or poor terminal contact.
8. Use a controlled swap test only after confirming the pattern. If practical and appropriate for the engine design, swap the cylinder 12 ignition coil and/or spark plug with another cylinder and recheck misfire counters. If the misfire moves with the component, that supports an ignition component fault.
9. Test injector operation and injector circuit. Confirm the injector is being commanded (noid light or scope) and compare electrical characteristics to a known-good cylinder on the same engine. If available, perform injector balance or flow testing to evaluate fueling contribution.
10. Verify air sealing and mechanical condition. Smoke test the intake for leaks that could affect cylinder 12. If ignition and injector checks do not resolve the misfire, perform a compression test and compare cylinder 12 to other cylinders; follow with a leakdown test if compression is low to identify where the leakage occurs.

Professional tip: Use Mode $06 misfire data (when available) alongside freeze-frame information to reproduce the same RPM/load conditions that triggered P0312. This helps verify whether the repair truly stopped cylinder 12 misfire activity rather than simply clearing the code.

Possible Fixes

• Replace or service the spark plug on cylinder 12 if inspection/testing confirms it is worn, fouled, damaged, or improperly gapped
• Replace the ignition coil/boot or repair the ignition connection for cylinder 12 if testing or a documented swap test confirms the fault
• Repair wiring, terminals, connector fitment, corrosion, or damaged harness sections affecting cylinder 12 ignition or injector circuits
• Repair or replace the cylinder 12 fuel injector if electrical/command/flow testing confirms an injector fault
• Repair intake air leaks or sealing issues that affect the air/fuel mixture delivered to cylinder 12
• Correct fuel supply problems (pressure/volume/power/ground) if testing shows they contribute to the misfire condition
• Address verified mechanical faults (compression loss, valve sealing problems, head gasket leakage) affecting cylinder 12

Can I Still Drive With P0312?

Driving with P0312 depends on how severe the cylinder 12 misfire is at that moment. If the MIL is flashing, the engine is running noticeably rough, power is reduced, or you detect fuel odor from the exhaust, continued driving can increase the risk of catalytic converter damage and should be avoided. If the MIL is steady and the vehicle appears to run normally, limited driving may be possible, but the misfire should be diagnosed promptly and heavy load operation should be avoided until the cause is confirmed and repaired.

How Serious Is This Code?

P0312 can range from moderate to serious. Any misfire increases emissions and can cause drivability concerns, but a sustained or severe misfire can overheat and damage the catalytic converter and may create unsafe conditions if the engine loses power unexpectedly. Because P0312 identifies a cylinder-specific combustion problem, it should be treated as a priority diagnostic item, especially if the MIL flashes or the misfire is easily felt.

Common Misdiagnoses

P0312 is often misdiagnosed by replacing parts without confirming the misfire pattern on cylinder 12 or without verifying cylinder 12 location using correct service information. Another error is assuming the DTC proves a specific component failure; the code indicates a cylinder 12 misfire event, not a guaranteed bad spark plug, coil, injector, or sensor. Overlooking wiring/connector faults, intake leaks affecting that cylinder, or mechanical compression issues can also lead to repeated repairs without resolving the underlying cause.

Most Likely Fix

The most likely fix is the repair that restores consistent combustion on cylinder 12 after testing confirms the actual root cause—most often by correcting an ignition, fuel injector, wiring/connection, air leak, or mechanical compression problem affecting cylinder 12. The repair should be verified by ensuring cylinder 12 misfire counters stop accumulating under the same operating conditions that originally set P0312.

Repair Costs

Repair cost depends on whether the confirmed root cause is wiring, connector condition, a sensor, a module, or the labor needed to diagnose the fault correctly.