My engine has eight cylinders. Why is my scan tool showing P0291?

Because something has gone wrong before the code reached you. P0291 is generically defined as the cylinder 11 injector circuit, and only twelve-cylinder engines have an eleventh cylinder — BMW and Mercedes V12s, Jaguar and Aston Martin V12s, Ferrari and Lamborghini twelve-cylinder engines, and large industrial and marine diesels. On anything else, either the tool misread or mistranslated the code, so read it again with a different scanner, or your manufacturer has reassigned this number to something unrelated. Reassignment is common in the high P02xx range, and the manufacturer-specific definition always wins over the generic one. Look it up against your exact make and model before spending anything.

Does my V12 have one engine computer or two?

It depends on the engine, and the answer changes your whole approach. A number of twelve-cylinder engines are managed as two six-cylinder halves, with each bank running on its own control module or its own half of a split module, kept in step over the vehicle network. BMW's V12s and several Mercedes twelve-cylinder generations work this way. If yours does, cylinder 11 lives on the second bank and is driven by the second module — so the connector you back-probe and the module you would ultimately replace are not the same ones that control cylinder 1. Establish the architecture before you probe, because on a V12 nothing is quick to reach twice.

Can I just replace the injector?

You can, but on a twelve-cylinder engine that is an expensive guess. Test first: unplug the cylinder 11 injector, clear the code and start the engine. If P0291 comes straight back with the injector out of the circuit, the injector is not the problem and replacing it changes nothing. Then measure the coil resistance against its bank-mates — a shorted winding shows up as a reading well below the others. Those two tests take about twenty minutes and they routinely save a four-figure parts order, because V12 injectors are low-volume parts and shops often recommend doing a whole bank at once since the labor to get back in is the costly part.

Is it safe to drive with P0291?

No. A hard short on an injector circuit either kills the cylinder or holds the injector open, and both put unburned fuel into the exhaust — the quickest route to overheating a catalytic converter. There is a specific trap on a twelve-cylinder engine: one bad cylinder out of twelve is very hard to feel, because a V12 is the smoothest engine layout ever put in a production car. The vehicle can seem completely normal while a converter is being cooked, and a V12 has two of them. Trust the code rather than how the car feels, and if the check engine light is flashing, stop and have it towed.

OBD-II trouble code

P0291: Cylinder 11 Injector Circuit Low

The engine computer saw lower-than-expected voltage on the cylinder 11 injector control wire — a short to ground, a shorted injector coil, or a failed driver. Before any of that, confirm your engine has an eleventh cylinder, because only twelve-cylinder engines do.

Medium severityPowertrainFuel InjectorsDo not drive

Quick facts

System: Powertrain
Category: Fuel Injectors
Severity: Medium severity
Drivable: No — stop driving until repaired
Repair cost range: $50 – $2,500
DIY difficulty: Intermediate DIY

What does P0291 mean?

The engine control module (ECM) fires each injector by feeding battery voltage to one side and switching the ground on the other for a precise number of milliseconds. Between pulses the control wire should sit near battery voltage, because the injector coil ties it back to the feed. The ECM watches that line, and P0291 sets when the cylinder 11 injector control circuit reads low when it should be high.

This code narrows the vehicle population further than anything before it in the block. Cylinders 9 and 10 exist on both V10s and V12s. Cylinder 11 exists on V12s only, and that is a genuinely small list: the BMW N73 and N74 (760i, Rolls-Royce Phantom and Ghost derivatives), the Mercedes M120, M137, M275 and M279 (600-series, S600, SL600, CL600, Maybach, AMG 65 cars), the Jaguar and Aston Martin V12 families, Ferrari and Lamborghini twelve-cylinder engines, and large industrial, marine, locomotive and generator diesels. If your vehicle is a four, six, eight or even ten cylinder and a scan tool is reporting P0291, stop and re-read the code with a different tool. If it still reports, look the number up against your specific make — reassignment in the high P02xx range is common, and a manufacturer-specific definition always outranks the generic one.

Assuming the cylinder is real, three faults produce a low reading. The control wire is shorted to ground somewhere in the harness. The injector's own coil winding has shorted internally, dropping circuit resistance. Or the ECM's cylinder 11 injector driver has failed shorted-on. The first two are far more common than the third, and the third should only be concluded after both of the others have been cleared.

There is one fact about twelve-cylinder engines that changes how you diagnose this code, and it has no equivalent anywhere earlier in the block. Many V12s are not managed as one twelve-cylinder engine. They are managed as two six-cylinder halves, each bank running on its own control module or its own half of a split module, synchronised over the vehicle network. BMW's V12s and several Mercedes twelve-cylinder generations are built this way. If that describes your engine, cylinder 11 is on the second bank and is therefore driven by the second module — which means the module you would test, the connector you would back-probe, and the module you would ultimately condemn are all different from the ones that control cylinder 1. It also means a fault affecting only the higher-numbered cylinders may be a bank-module or bank-harness problem rather than a whole-engine one. Establish which architecture your engine uses before you probe anything, because probing the wrong module wastes a lot of time on a car where nothing is easy to reach.

A hard short means cylinder 11 is either dead or being flooded, which is why this is a do-not-drive condition even though the engine still runs. Unburned fuel dumped into the exhaust is the fastest way to destroy a catalytic converter, and a V12 has two of them — expensive ones.

Use the electrical nature of the fault to your advantage before you commit to disassembly. Unplug the cylinder 11 injector, clear the code, and start the engine. If P0291 returns with the injector disconnected, the fault is in the wiring or the ECM and no amount of intake removal will help. On a V12 that single test is worth more than on any other engine, because getting to a rear-of-bank injector often means removing an intake plenum, and on mid-engine cars sometimes far more than that.

Common causes

Cylinder 11 injector control wire shorted to ground from chafing or a pinched harness
Internally shorted cylinder 11 injector coil winding
Corroded, wet, or damaged connector at the cylinder 11 injector
Harness rubbed through where it passes a bracket, the intake plenum, or a heat source toward the rear of the second bank
Wiring damaged during recent intake, plenum, or valve cover service — routine work on a V12 requires removing most of the top of the engine
Failed cylinder 11 injector driver inside the ECM/PCM, or inside the bank-2 module on split-module V12s (uncommon — rule out everything else first)
Code misread or manufacturer-reassigned on an engine that has no eleventh cylinder

Symptoms

Check engine light on, often flashing if cylinder 11 is dead
Rough idle and vibration — subtle on a V12, which hides a single dead cylinder better than any other engine layout
Reduced power, most noticeable as the engine no longer pulling cleanly at the top of its range
Companion misfire codes, commonly P0300 for a random or multiple misfire
Raw fuel smell or black smoke if the injector is being held open
Hard starting or extended cranking
Poor fuel economy

Diagnostic steps

1.Confirm your engine actually has an eleventh cylinder. Only twelve-cylinder engines do. If yours is not one, re-read the code with a second tool and then look the number up against your specific make and model.
2.Determine whether your V12 runs on one control module or two. Many are managed as two six-cylinder halves, and on those the bank containing cylinder 11 has its own module, its own connector, and its own harness branch.
3.Record every stored code. A companion misfire code alongside P0291 confirms the cylinder is genuinely affected rather than the code being a stray electrical report.
4.Confirm which physical cylinder your engine calls number 11 using the manufacturer's numbering diagram. Do not derive it from the firing order — that tells you when a cylinder fires, not where it sits.
5.Unplug the cylinder 11 injector, clear the code, and start the engine. If P0291 returns with the injector disconnected, the fault is in the wiring or the module, not the injector.
6.With the injector unplugged and the key off, measure resistance from the control wire to ground. Near zero indicates the short to ground; an open reading clears the wire.
7.Measure the cylinder 11 injector's coil resistance and compare it against specification and against its bank-mates. A reading well below the others indicates shorted windings.
8.Inspect the harness along its whole run on that bank, paying attention to brackets, plenum edges, and anywhere heat or movement concentrates.
9.Inspect the connector for green corrosion, moisture, spread terminals, or a partially seated lock.
10.Wiggle-test the harness and connector with the engine running while watching live data, to catch an intermittent short.
11.Price the part and confirm availability before committing. Twelve-cylinder injectors are low-volume parts and are neither cheap nor always in stock.
12.If the wiring and the injector both test good and the drive signal is still wrong, suspect the injector driver in the module that controls that bank — but confirm the first two paths thoroughly, because module replacement requires programming.

Repair cost

$50 – $2,500

A wiring or connector repair is often $80-$400, and the upper end reflects the access on a twelve-cylinder engine rather than the difficulty of the repair itself. Injectors for V12 engines are low-volume parts: $150-$700 each is normal, direct-injection units cost more again, and many shops will recommend replacing a bank's worth at once because the labor to get back in is the expensive part. Labor to reach a rear-of-bank injector runs 1.5-5 hours on most front-engine V12s and more on mid-engine cars. A failed injector driver, or the bank module that contains it, runs $700-$2,500 including programming. Diagnosis alone is typically $120-$250 because the numbering and module architecture have to be established first.

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DIY vs shop

This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.