OBD-II Code P0291 — What It Means & How to Fix It

What Does This Error Mean?

P0291 means the engine computer detected that cylinder 9's injector circuit has too low a voltage. The injector isn't getting enough electrical signal to open and spray fuel properly. This is the opposite problem from P0290, but just as serious. Cylinder 9 may be starved of fuel, causing a misfire and rough idle. This code only appears on large engines — V10 or V12 vehicles.

Affected Models

All vehicles 1996+ with 9+ cylinders
Common in Ford Excursion V10
Common in Dodge Viper V10
Common in BMW V12 models
Common in large diesel trucks

Common Causes

Open circuit in the injector wiring — a broken wire that has lost continuity
Faulty fuel injector with an open coil — it won't respond to electrical signals
Corroded or loose connector at the cylinder 9 injector plug creating high resistance
Blown fuse or relay in the injector power supply circuit
ECM output driver failure that can no longer send a proper signal to the injector

How to Fix It

Scan for all stored codes with an OBD-II scanner. Note whether you also have misfire codes for cylinder 9 (P0309). This confirms the injector circuit issue is causing an actual misfire, which helps prioritize the repair.

Write down all codes before clearing anything — the pattern can guide diagnosis.
Check the fuse box for any blown fuses related to the fuel injectors. Your owner's manual will show which fuse covers the injector circuit. A blown fuse is the simplest possible fix.

Fuses cost less than $1. Always check them first before any other diagnosis.
Locate the cylinder 9 injector connector and inspect it closely. Unplug it and look for pushed-back pins, corrosion, or damage. Clean with electrical contact cleaner, ensure the pins lock properly when you reconnect.

A connector that looks fine visually can still have high resistance — wiggle it while running the engine to see if the code clears.
Test the fuel injector's resistance with a multimeter. Unplug the connector, set the meter to ohms, and probe both terminals on the injector itself. An open coil inside the injector will read infinite (OL) resistance and means the injector must be replaced.

Typical injector resistance is 10-17 ohms for high-impedance types.
If the injector tests good, trace the wiring from the injector connector back toward the ECM. Look for any breaks, rodent damage, or pinched wires. Use a multimeter in continuity mode to confirm the wire is intact from end to end.

Rodent damage is more common than most people realize and is a frequent cause of open circuits in injector wiring.

When to Call a Professional

If you've checked fuses, the connector, and visible wiring without finding the problem, a mechanic with a wiring diagram and multimeter can trace the open circuit efficiently. Don't replace the ECM without professional confirmation — the injector or its wiring is almost always the real cause. Diagnosis typically costs $100-$200, and injector replacement runs $150-$400 installed.

Frequently Asked Questions

What is the difference between P0291 and P0290?

P0290 means the injector circuit voltage is too HIGH — a short to voltage. P0291 means the injector circuit voltage is too LOW — an open circuit or short to ground. Both codes affect cylinder 9's injector. The repair approach is similar, but P0291 often points to a broken wire or dead injector coil.

Will my engine run with P0291?

The engine will likely run, but roughly. Cylinder 9 won't receive fuel if the injector circuit is open, causing that cylinder to misfire continuously. On a V10, losing one cylinder is noticeable — expect vibration, poor acceleration, and higher fuel consumption. Don't drive long distances until this is repaired.

Can a bad ground cause P0291?

Yes, absolutely. The injector circuit relies on a clean ground return path. A corroded or loose engine ground strap can cause low voltage readings across multiple injectors. Check all engine ground connections — especially the strap from the engine block to the chassis.