OBD-II trouble code
P0294: Cylinder 12 Injector Circuit Low
Lower-than-expected voltage on the cylinder 12 injector control wire — a short to ground, a shorted coil, or a failed driver. Cylinder 12 exists only on twelve-cylinder engines, and on those, the single most useful question is whether anyone has had the top of the engine apart recently.
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 P0294 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. P0294 sets when the cylinder 12 injector control circuit reads low when it should be high — a short to ground in the harness, a shorted injector coil winding, or an ECM driver failed shorted-on. The first two are far more common than the third.
Cylinder 12 exists only on twelve-cylinder engines: the BMW N73 and N74, the Mercedes M120, M137, M275 and M279, Jaguar and Aston Martin V12s, Ferrari and Lamborghini twelve-cylinder engines, and large industrial, marine and generator diesels. If a scan tool reports P0294 on anything smaller, read it again with a different tool, and if it persists, look the number up against your specific make — reassignment in the high P02xx range is common and the manufacturer's own definition outranks the generic one.
The question that solves this code more often than any other is one that almost nobody asks first: has anyone worked on the top of this engine recently? On most engines that would be a minor consideration. On a V12 it is the leading candidate, and the reason is structural. Routine service on a twelve-cylinder engine — spark plugs, coils, valve cover gaskets, a coolant hose, almost anything — requires removing intake plenums, covers, and frequently the wiring harness itself has to be unclipped and moved aside. Every one of those operations puts hands on the injector harness. Pinched wires under a plenum edge, connectors that were unplugged and never fully re-latched, a harness re-routed against a bracket it never touched before, a bolt run down through a wire loom: these are ordinary, forgivable outcomes of a big job, and on a V12 the big job is how you change a spark plug. If this code appeared within a few weeks of any service, retrace the work before testing anything else. It is free and it is right often enough to be the first move.
Water is the second physical suspect, and cylinder 12 is where it lands. On a front-engine V12, cylinder 12 is the rearmost cylinder of the second bank, sitting against the firewall beneath the cowl. The cowl at the base of the windshield drains through channels that fill with leaves and dirt over the years. When they block, water goes where it should not, and one of the places it goes is directly onto the rearmost injector connectors — corroding terminals and bridging circuits, which is a textbook route to exactly the short to ground this code reports. Checking the cowl drains takes five minutes, regularly ends the diagnosis, and clearing them is what stops the repair from happening again next spring. On mid-engine cars the geometry differs but the principle holds: find out where water can collect above the rearmost connectors.
A hard short means cylinder 12 is either dead or flooded, and unburned fuel dumped into the exhaust destroys catalytic converters quickly. Treat the vehicle as not driveable until the fault is found, however normal it feels — and it will feel normal, because a V12 hides one bad cylinder better than any other engine layout.
One more piece of leverage before you commit to disassembly: a short to ground is visible from either end of the wire. You can test it at the module connector without ever reaching the back of the engine. On the hardest injector on the hardest engine to work on, that is the highest-value shortcut available.
Common causes
- Wiring damaged, pinched, or re-routed during recent service — the single most common cause on a V12, because routine work requires removing the top of the engine
- Connector unplugged during service and not fully re-latched
- Cylinder 12 injector control wire shorted to ground from chafing against a bracket or plenum edge
- Water intrusion from blocked cowl drains reaching the rearmost injector connectors
- Corroded or green terminals at the cylinder 12 injector connector
- Internally shorted cylinder 12 injector coil winding
- Heat damage to harness insulation near the rear of the bank
- Failed cylinder 12 injector driver inside the module controlling that bank (uncommon — rule out everything else first)
Symptoms
- Check engine light on, often flashing if cylinder 12 is dead
- Rough idle and vibration, though unusually subtle on a twelve-cylinder engine
- Reduced power and hesitation under load
- Companion misfire codes, commonly P0300
- Raw fuel smell or black smoke if the injector is being held open
- Hard starting or extended cranking
- Poor fuel economy
- Symptoms that began shortly after other work was done on the engine
Diagnostic steps
- 1.Ask what was done to this engine recently. On a V12, almost every service job requires disturbing the injector harness, and post-service wiring damage or an unlatched connector is the leading cause of this code. Retrace the work before testing anything.
- 2.Confirm your engine actually has a twelfth cylinder and confirm which physical cylinder the manufacturer calls number 12 from its own numbering diagram.
- 3.Check the cowl drains at the base of the windshield. Blocked drains put water straight onto the rearmost injector connectors, and clearing them is what stops the repair from repeating.
- 4.Record every stored code. A companion misfire code alongside P0294 confirms the cylinder is genuinely affected.
- 5.Test from the module end first. A short to ground is visible from either end of the wire, so you can identify it at the module connector without reaching the back of the engine.
- 6.Unplug the cylinder 12 injector, clear the code, and start the engine. If P0294 returns with the injector disconnected, the fault is in the wiring or the module and no teardown will help.
- 7.With the injector unplugged and the key off, measure resistance from the control wire to ground. Near zero confirms the short.
- 8.Measure the cylinder 12 injector's coil resistance and compare it against the injector in the mirror position on the opposite bank — same part, same age, same engine.
- 9.Inspect the connector for corrosion, moisture, spread terminals, or a partially seated lock, and inspect the harness where it passes brackets and plenum edges.
- 10.Wiggle-test the harness and connector with the engine running while watching live data, to catch an intermittent short.
- 11.Photograph the harness routing before reassembly so the next person who opens this engine puts it back where it belongs.
- 12.Only suspect the module driver after the wiring and injector have both been cleared, since module replacement requires programming.
Repair cost
$50 – $2,500
If the cause turns out to be a connector left unlatched after other work, the fix costs nothing but the labor to find it. A wiring or connector repair generally runs $80-$400 on a twelve-cylinder engine, mostly access. Clearing blocked cowl drains is a $0-$150 job that is worth doing regardless. A V12 injector is a low-volume part at $150-$700 each plus 1.5-5 hours of labor to reach the rearmost cylinder, and more on mid-engine cars; many shops recommend doing a bank at once because getting back in is the expensive part. A failed driver or bank control module runs $700-$2,500 including programming. Diagnosis alone is typically $120-$250.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with fuel injector replacement preselected. Adjust labor rate and vehicle category to fit your situation.
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.