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P0312: Cylinder 12 Misfire Detected

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Quick answer

P0312 means the engine computer counted too many misfires on cylinder 12, so this code only appears on twelve-cylinder engines. On these cars cylinder 12 is the last cylinder on the second bank, usually at the firewall end, and the usual causes are an ignition coil, a spark plug or an injector for that cylinder. A flashing light means stop; a misfire on a V12 can ruin one of two very expensive catalytic converters.

What it means

The computer watches the crankshaft sensor for the small acceleration each cylinder’s power stroke should produce. When cylinder 12 fails to deliver its pulse often enough within a set window, it stores P0312, turns on the light, and on most of these engines cuts fuel to that cylinder to protect the catalytic converter. Because the code carries the manufacturer’s cylinder number, P0312 exists only on engines with twelve cylinders: the V12s from Mercedes-Benz, BMW, Rolls-Royce, Aston Martin, Ferrari, Lamborghini and Jaguar, and the W12 from Audi, Volkswagen and Bentley.

Cylinder 12 is the last cylinder of the second bank. Mercedes and BMW number the right bank 1 to 6 and the left bank 7 to 12 from front to rear, so cylinder 12 is the rearmost cylinder on the driver’s side of a left-hand-drive car, against the firewall and often under the intake plenum. The W12 numbers its two narrow-angle banks the same way, 1 to 6 and 7 to 12, and its rear cylinders sit deep under the plenum and the turbo plumbing on the Bentley and Audi versions. Reaching cylinder 12 is usually the largest part of the labor.

The diagnosis is the same as on any engine, scaled up: plug, coil, injector, air, compression, in that order. Two things are specific to twelve cylinders. Many of these engines run two spark plugs and two coils per cylinder, so a misfire can be a weak coil rather than a dead one, and some older V12s use two engine computers, one per bank, so a fault confined to cylinders 7 through 12 can be a second-computer supply, ground or harness problem rather than anything in the cylinder. A misfire that stays put through a coil swap deserves a look at that bank’s wiring before the injector.

P0312 symptoms: what you'll notice

  • A fine, high-frequency shake at idle that is easy to miss on a twelve-cylinder engine because the other eleven cylinders smooth it over; it is most noticeable in gear at a stop.
  • A flashing check engine light under acceleration, with the engine cutting fuel to cylinder 12 and losing a noticeable amount of its usual effortless pull.
  • Reduced power and worse fuel consumption, and on twin-turbo versions a softer boost response from the bank that carries cylinder 12.
  • A fuel smell from the left-side exhaust at idle, and eventually a catalyst efficiency code for that bank (P0430 on most of these).
  • Companion codes: P0300, or P0307 through P0311 alongside P0312, which on a twin-computer V12 points at that bank’s computer or harness rather than six separate coils.

Common causes

Ordered from most to least likely.

  1. 1.

    Failing ignition coil (or one of two coils) on cylinder 12

    The most common cause on Mercedes, BMW and Bentley twelve-cylinders. Coils on the rear cylinders live in the hottest spot in the bay and fail first; on twin-plug engines a weak second coil produces a misfire the first coil almost covers.

  2. 2.

    Worn or fouled spark plug (or plugs) in cylinder 12

    Twin-spark engines carry 24 plugs and the rear ones are the ones most likely to be skipped. Oil in the plug well from a valve cover gasket is a common way to kill both the plug and its coil.

  3. 3.

    Clogged, sticking or leaking fuel injector on cylinder 12

    Port injectors on the older engines and direct injectors on the newer ones both fail this way. An injector balance test on the factory scan tool is the quickest check; swapping injectors is a full-day job on most of these.

  4. 4.

    Intake or vacuum leak at the rear of the second bank

    Aged plenum gaskets, cracked breather hoses and brittle vacuum lines at the firewall end lean out the rear cylinders first. Suspect it when the misfire is worst at idle and disappears under load.

  5. 5.

    Wiring, connector or second-engine-computer fault for the 7 to 12 bank

    Older V12s run two engine computers, one per bank. A supply, ground or harness fault on the second computer produces misfires on that bank only. Newer engines use one computer but still route the rear bank’s harness through the hottest, most chafe-prone path.

  6. 6.

    Low compression in cylinder 12

    A burnt valve, carbon-stuck rings or a lifter or valve-clearance problem. Suspect it when spark and fuel test good, or when the misfire is present from cold and never changes.

  7. 7.

    Carbon deposits on the intake valves of direct-injection twelve-cylinders

    The BMW N73 and N74, the newer Mercedes engines and the Bentley W12 build intake-valve carbon that disturbs airflow to individual cylinders. Cold-start misfires that clear as the engine warms are the signature.

How to fix it: diagnosis, step by step

Cheapest and most likely checks first.

  1. 1 Read the codes with a capable scan tool and study the pattern

    Use a tool that reads the manufacturer’s misfire counters and, on twin-computer engines, both computers. A lone P0312 points at that cylinder’s hardware. P0312 with codes from cylinders 7 through 11 points at the second bank’s computer, harness or air supply. Note whether the misfire is cold, hot, idle or load.

  2. 2 Locate cylinder 12 and plan the access

    Confirm the numbering for your engine, then look at what has to come off to reach the rear of the second bank: the plenum, a turbo inlet, the cowl cover. Do the plan before the test so the coil swap and plug inspection happen in one disassembly.

  3. 3 Swap the cylinder 12 coil (or coils) to a neighbor and clear the codes

    Move the coil to cylinder 11 or 10 and that cylinder’s coil to 12. Drive and read the counters. A misfire that moves with the coil is a coil. On twin-plug engines, swap both coils for the cylinder, then narrow it down with a second swap if the misfire moved.

  4. 4 Read the plug (or plugs) and look for oil

    Pull plug 12 and read it: oily, sooty, blistered or cracked. Oil in the well means the valve cover gasket or the plug-tube seal is leaking onto the coil; replace the gasket with the coil or the new coil fails too. Replace all plugs if they are due; on twin-spark engines that is 24.

  5. 5 Run an injector balance test and check the second bank’s supply

    Use the factory scan tool’s injector test to compare cylinder 12’s contribution with its neighbors, or listen for a steady click at the injector. If the misfire spans the whole bank, check power and ground to the second computer or the rear harness connectors before spending on injectors.

  6. 6 Check for a leak at the rear of the bank

    Smoke the intake or spray carb cleaner around the plenum gasket and the breather and vacuum lines at the firewall end of the second bank at idle. Look at the bank-two fuel trims: a lean trim on that bank that improves under load supports a leak.

  7. 7 Compression, leak-down and, on direct-injection engines, an intake-valve inspection

    Measure compression on cylinder 12 against the rest; more than about 10 to 15 percent low means valves, rings or clearance. On the direct-injection twelve-cylinders, a borescope through the intake port shows the carbon that causes cold misfires; walnut-shell blasting is the usual remedy.

Known P0312 patterns by make

Brand-specific failure patterns from the field: find your make before parts-shopping.

Mercedes-Benz 1992–2001 S600, SL600, CL600 (M120) and 2000–2012 S600, SL600, CL600, S65, SL65, CL65 (M137, M275)

The M120 runs two engine computers, one per bank, so a misfire pattern confined to cylinders 7 through 12 is a second-computer supply, ground or harness problem until proven otherwise. The M137 and M275 use two plugs and two coils per cylinder, 24 of each, and the rear coils on the left bank are the usual P0312 answer; replace the valve cover gaskets at the same time if the plug wells are oily, because oil is what kills those coils.

BMW / Rolls-Royce 2003–2015 760i and 760Li (N73, N74) and 2003 and newer Rolls-Royce Phantom, Ghost, Wraith and Dawn

These direct-injection V12s misfire for three reasons in rough order: a coil, an injector, and intake-valve carbon. A cold-start misfire on cylinder 12 that clears as the engine warms is carbon; one that appears hot is a coil; one that stays constant and shows in the injector test is the injector. The rear of the left bank sits under the plenum and the cowl, so combine every test into one disassembly and keep the diagnosis ahead of the parts.

Bentley / Audi / Volkswagen 2004–2018 Continental GT, Flying Spur and Bentayga, 2001–2018 Audi A8 W12, 2004–2016 Phaeton and Touareg W12

The 6.0L W12 packs its twelve cylinders into two narrow banks of six, and cylinder 12 is at the very back of the second bank under the intake and, on the turbocharged Bentleys, the turbo inlet plumbing. Coils and plugs are the usual causes and the rear ones are the hard ones; a plug service is a several-hour job and is often skipped, which is why the rear plugs are the ones that fail. Replace all twelve coils as a set if more than one has failed; they age together.

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Related codes

Frequently asked questions

What does code P0312 mean?
P0312 means the engine computer has detected repeated misfires on cylinder 12, so it only appears on twelve-cylinder engines. It is serious: a steady light means fix it soon, a flashing light means unburned fuel is overheating a catalytic converter, and on these cars that converter is one of the most expensive parts on the vehicle.
Can I drive with P0312?
With a steady light and the engine otherwise smooth, gentle trips to a specialist are fine. With a flashing light, no: a V12 or W12 has two or four catalytic converters, and each one costs more than most repairs. Most of these engines cut fuel to the misfiring cylinder to protect the converter, which makes the shake worse but the damage less.
Where is cylinder 12 on my engine?
On Mercedes-Benz, BMW and Rolls-Royce V12s, and the Audi, Volkswagen and Bentley W12, cylinders 7 to 12 run down the second bank and cylinder 12 is the rearmost, at the firewall end, on the driver’s side of a left-hand-drive car. Aston Martin and Ferrari follow their own diagrams. Confirm with the service documentation for your exact engine; on these cars the wrong bank is a very expensive mistake.
Why do some V12s have two engine computers?
Older twelve-cylinder engines, notably the Mercedes M120 of the 1990s and the BMW M73, used two engine control units, each running six cylinders. A power, ground or harness fault on the second unit produces misfires on cylinders 7 through 12 only. If P0312 arrives with codes from its bank-mates, check that computer’s supply before anything in the cylinders.
How do I know if it is the plug or the coil?
Swap the cylinder 12 coil (or both coils on twin-plug engines) with a neighbor and clear the codes. If the misfire follows the coil, replace it. If it stays on cylinder 12, read the plug, check for oil in the well, then test the injector. On these engines, do the plug inspection during the same disassembly as the coil swap.
How much does it cost to fix P0312?
An OEM coil is $60 to $200; plugs are $15 to $40 each and there may be 24 of them. Labor is the cost driver: reaching the rear of the second bank takes 2 to 5 hours on most twelve-cylinders, so a coil-and-plug repair runs $400 to $1,200 at a specialist, and a full plug service $800 to $2,000. An injector is $200 to $600 plus labor. Intake-valve carbon cleaning on the direct-injection engines is $800 to $2,000. Expect dealer diagnostic rates to be higher than on an ordinary car.
What is the difference between P0300 and P0312?
P0300 is a random or multiple misfire: misses spread across cylinders, which points at something shared, like fuel pressure, a large vacuum leak or, on twin-computer V12s, a whole bank’s computer. P0312 is a misfire counted on cylinder 12 specifically, which points at that cylinder’s coils, plugs, injector, runner or valves.
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