OBD-II code P0291 indicates that the engine control module (PCM/ECM) has detected a low-voltage condition in the electrical circuit controlling the fuel injector for cylinder 11. This usually means the injector is not receiving the expected voltage or current, pointing to a wiring, connector, or injector problem rather than a fueling or mechanical issue. Cylinder 11 is only present on engines with 11 or more cylinders (most commonly V12 configurations), so this code is relatively uncommon.
An internally shorted or open-coil injector is the single most common cause. Injectors live in a hot, vibration-heavy environment and their solenoid windings degrade over time.
Chafed insulation, heat damage, or spread/corroded pins in the injector connector create high resistance or intermittent open circuits that the PCM interprets as low voltage.
A break in the conductor or a short to ground between the PCM and the cylinder 11 injector will drop circuit voltage below the expected range.
Corroded engine grounds or a weak chassis ground can pull the low-side driver voltage low enough to trigger the code.
Occasionally an output driver inside the PCM that switches cylinder 11 fails or becomes weak. This is plausible but uncommon and should only be considered after wiring and injector are ruled out.
Total control-module failure is rare for this code. Most vehicles diagnosed with P0291 and a 'replaced PCM' outcome actually had a wiring or injector fault that was missed.
Short trips are usually possible, but the cylinder may be running lean or dead, which can overheat and damage the catalytic converter over time. Diagnose and repair sooner rather than later.
Connector and wiring repairs are within reach of a competent DIYer with a multimeter and basic soldering/terminal tools. Injector replacement on some engines is straightforward; on others, intake manifold removal is required and a professional may be more practical.
Almost never. The PCM is the least likely cause of P0291, yet it is frequently the first part suggested. In the great majority of cases, the fault is the injector itself, its connector, or its wiring. Replace or repair those first and verify the fault before ever considering the control module.
A misfire code indicates the cylinder's combustion is not contributing properly, which can have many root causes. P0291 specifically points at the electrical injector circuit being low, so diagnosis is focused on voltage, resistance, and driver signal rather than spark, compression, or fuel pressure.