P0312 code: Cylinder 12 Misfire Detected
P0312 means cylinder 12 Misfire Detected. The code says where to look, not what failed — the fix depends on which cause your car actually has.
What P0312 means
P0312 flags a misfire in cylinder 12, the last cylinder in a V12's firing order — so this code appears exclusively on twelve-cylinder engines in luxury and performance vehicles. The cylinder isn't burning its mixture reliably, and the most likely causes are its spark plug or ignition coil, followed by a weak or clogged fuel injector and, least often, low compression from a valve or ring problem. The proven test is swapping the suspect coil or plug to a neighboring cylinder and seeing whether the misfire code moves with it. Because a V12 runs almost smoothly on eleven cylinders, the symptom can be nothing more than a check-engine light — but the unburned fuel from the dead cylinder still cooks the catalytic converter on that bank. Cylinder 12's physical location varies by manufacturer, and on some engines the rearmost cylinders are the hardest to reach.
P0312 symptoms
- Mild roughness or vibration, often barely perceptible
- Reduced power or hesitation under load
- Flashing check-engine light during an active misfire
- Reduced fuel economy
- Fuel smell from the exhaust in severe cases
What it costs to fix P0312
Varies by vehicle — get it diagnosed.
The repair cost for this code depends too much on your exact vehicle and the underlying cause to quote a meaningful range. Confirm the cause first so you only pay for the fix you actually need.
Diagnose P0312 for my vehicle →How to fix P0312
Advanced — usually a shop job unless you’re well equipped
- Locate cylinder 12 and judge the access
Cylinder 12 belongs only to twelve-cylinder engines, and its physical spot comes from the maker's numbering diagram for your engine. If that diagram puts it at the back of a bank near the firewall, expect to remove covers, cowl panels, or intake pieces before you can even see the coil.
- Prove the miss with live data before tearing in
A V12 runs almost smoothly on eleven cylinders, so verify the problem before taking the rear of the engine apart. Watch cylinder 12's misfire counter at idle and on a drive; a count that keeps climbing confirms an active miss, while one stored event with nothing since may be a one-off worth rechecking after a few drive cycles.
- Move coil 12 to a cylinder you can reach easily
Once it's out, fit coil 12 on an accessible cylinder and put that cylinder's coil on 12, then drive and recheck the counters. The miss jumping to the coil's new home means replace coil 12; the miss staying on 12 shifts attention to the plug and fuel side.
- Free the boot gently and inspect plug 12
Boots on rear cylinders bake onto the plug, so twist them free rather than yanking, and look for carbon tracking that lets spark short to the well. If the plug set is at or beyond the maintenance-schedule interval, replace them all, and start each new plug by hand to protect the aluminum threads in the head.
- Escalate to injector and compression testing
If coil and plug don't move the misfire, check injector 12's electrical side and pulse, then have compression and leak-down tested on cylinder 12. Direct-injection removal and internal diagnosis on a twelve-cylinder engine is specialist work, and a firm verdict should come before any major spend.
DIY or shop? This sits at the advanced end mainly because of where cylinder 12 tends to live: reaching its coil and plug can mean pulling cowl panels, intake components, or covers on a tightly packed V12. A careful home mechanic with a service manual can still read the misfire data and do the coil swap if the coil is reachable. Leave intake-off work, injector removal, and compression diagnosis to a shop familiar with twelve-cylinder engines, and stop driving on a flashing light — the catalysts on these cars are costly.
Tools for the DIY steps: Scan tool with per-cylinder misfire counts · Service manual or cylinder numbering diagram · Coil boot puller · Thin-wall spark plug socket · Torque wrench · Mechanic's stethoscope
Not sure which step applies to your car? Get the likely cause for your exact vehicle before buying anything for P0312.
Start a case with P0312 and your car. Redline weighs these causes against what your car shows, and names the check that would settle it before you buy parts.
Can I drive with P0312?
Redline can’t determine from a symptom or code alone whether your car is safe to drive.
P0312: common questions
Which cars can even set P0312?
Only twelve-cylinder engines — V12s in high-end luxury and performance cars. On anything smaller, this code would be spurious: re-scan and verify before spending any diagnostic effort on it.
Can I drive with P0312?
That's a judgment the code can't make for you. Even with a steady light and no noticeable change, book the repair promptly. If the light flashes, the misfire is actively damaging the catalytic converter — stop driving and get it seen.
Why is fixing one cylinder on a V12 sometimes expensive?
The parts are ordinary — a plug, a coil — but V12 engine bays are packed, and reaching some cylinders means removing covers, ducting, or other components. Labor to access the cylinder, not the part, is usually the bulk of the bill.
How do I confirm whether it's the coil or something else?
Swap cylinder 12's coil with a neighbor, clear the codes, and drive. If the misfire code follows the coil to the new cylinder, replace the coil; if P0312 stays put, test the plug, then the injector, then compression — in that order of likelihood.
Related trouble codes
Don’t have a scanner yet? A ~$25 Bluetooth adapter reads codes like P0312 from your phone. See our guide to the best OBD2 Bluetooth adapters. Run a repair shop? See Redline for shops.
Generic OBD-II reference. Manufacturer-specific behavior varies. Confirm with a scan tool and, for safety-related codes, a professional inspection. Cost ranges are reference ranges, not quotes.
Seeing P0312 on your car?
Every problem becomes a Case. Tell Redline what the car is doing: it shows what the evidence supports, what’s still missing, and what to check next, before you approve a repair.
“Inner pad ~1 mm. Outer pad 3 mm.”
Next: check the slide pins move freely before buying parts.