P0271 — Cylinder 4 Injector Circuit High

What does P0271 mean exactly?

P0271 means the PCM detected abnormally high voltage in the cylinder 4 injector circuit, usually because the circuit is open, shorted to battery voltage, or the injector or driver cannot pull it down as commanded.

Is replacing the cylinder 4 injector always the fix?

Not necessarily. P0271 can also result from a bad connector, damaged wiring, a short to B+, or a failed PCM output, so voltage and resistance tests should identify the fault before replacement parts are purchased.

Can I drive my vehicle with P0271?

Limited cautious driving may be possible if the engine runs smoothly and there is no fuel odor, overheating, or severe misfire. Otherwise, stop driving because continued operation with an inactive cylinder 4 can damage the exhaust system.

Why does P0271 return after the code is cleared?

If P0271 returns immediately, the underlying open, high-resistance connection, short, failed injector, or PCM driver is still present. Clearing the code only resets the PCM and does not repair the electrical fault.

When your check engine light pops up and a scan tool shows P0271, your engine's computer has identified a problem with how cylinder 4 is being fed fuel. The powertrain control module constantly monitors the voltage in each injector's electrical circuit, and P0271 tells you that the signal going to the fourth cylinder's injector is sitting higher than it should—essentially, the module is seeing something wrong with the electrical feedback from that specific injector.

The underlying system here is the electronic fuel injection system. Modern engines deliver fuel through electrically-controlled injectors that spray precise amounts of fuel directly into each cylinder. The ECU fires each injector by grounding the circuit momentarily; when something interrupts this precise electrical conversation—either a short to power, an open ground, or a failed component—the module detects the abnormal voltage and logs P0271. The engine needs to see this fault condition repeat under specific operating conditions before the check engine light actually illuminates and the code becomes permanent.

One practical note: cylinder numbering follows a standard convention. On inline engines, cylinder 4 is the fourth one from the front, typically the end closest to the timing belt or chain. On V-type engines, the numbering varies by manufacturer, so consult your vehicle's firing order diagram to confirm the location. This matters because some cylinders sit deep in the valley of the engine, requiring intake manifold removal to access them—adding significant labor time and cost to the repair.

Symptoms of P0271

• Noticeable increase in fuel consumption
• Rough running at idle with occasional hesitation
• Check Engine Light illuminated
• Difficulty starting the engine or extended cranking time
• Raw fuel odor present, particularly when the problem is pronounced
• Reduced engine power, especially when under acceleration

Common Causes of P0271

The most frequent culprit is a failed injector for cylinder 4 itself—the internal electronics can short and pull the signal circuit high. A broken or corroded ground wire returning from that injector creates the same voltage anomaly in the ECU's eyes. The signal wire to the injector may have chafed and made contact with a constant power source, directly causing the elevated reading. Fuel system pressure problems upstream can also trigger this code: a faulty rail pressure sensor, a malfunctioning pressure regulator, or a clogged filter forces the injectors to compensate in ways that affect the electrical characteristics the module monitors. Contaminated or wrong-spec fuel, vacuum leaks allowing unmetered air into the intake, and a weak fuel pump struggling to maintain proper rail pressure round out the less common but still possible causes.

Diagnosing P0271

Start by connecting an OBD2 scanner and confirming P0271 is stored, then capture and save the freeze frame data—this snapshot shows engine load, RPM, coolant temperature, and vehicle speed at the exact moment the fault occurred. Recreating those conditions during testing makes diagnosis far more reliable.

Look for related diagnostic trouble codes before proceeding. A fuel pressure sensor code or mass air flow code often accompanies P0271 and points directly to the root cause. Also check manufacturer technical service bulletins for your specific make and model; many P0xxx codes have documented factory fixes or software calibrations that resolve the issue without any parts replacement.

Perform a thorough visual inspection of the cylinder 4 injector wiring and connector. Look for damaged insulation, corroded terminals, loose connections, or harness rubbing against engine components. Electrical issues here are common and inexpensive to fix if caught early.

With the cylinder 4 injector connector unplugged, measure voltage at the harness side. If you still see battery voltage on the signal wire with the injector disconnected, the harness itself has a short to power and needs repair. If the voltage drops to normal operating range, the injector itself has failed internally and requires replacement.

Measure fuel pressure at the rail and compare against factory specifications at idle and under load. Low or fluctuating pressure explains many intermittent injector codes. Check short-term and long-term fuel trim values in the scanner's live data stream to see which direction the ECU is pulling the mixture to compensate.

Finally, clear the code, complete a drive cycle that reproduces the freeze frame conditions, and verify P0271 does not return. If the code stays cleared, the underlying cause was likely intermittent; if it returns, continue testing the fuel system components.

Repair Options and Costs

The fix depends entirely on which component triggered the fault. Common repair scenarios with typical parts and labor costs at independent shops:

• Clean or replace injectors: $200–$800
• Replace the fuel pressure regulator or sensor: $150–$450
• Replace the fuel filter: $80–$200
• Replace the fuel pump: $400–$900

The full repair bill for P0271 typically falls between $80 and $900 depending on which part is at fault. Always diagnose before buying parts—jumping straight to the most expensive repair rarely saves money, and the culprit is often the fuel filter or a wiring issue rather than the fuel pump.

Can You Drive with P0271?

Drive with caution. A cylinder 4 injector circuit issue can cause your engine to run lean or rich, leading to rough idling, stalling, hesitation under acceleration, and increased fuel consumption. If the mixture disturbance is severe or prolonged, unburned fuel can enter the exhaust and damage catalytic converters—replacing those costs far more than fixing the injector circuit. Avoid long trips until the code is diagnosed and repaired. A short drive to a trusted repair shop is acceptable, but monitor for worsening symptoms like misfiring, strong fuel odor, or power loss.

Related OBD2 Codes

• P0200: Injector Circuit Malfunction
• [P0201: Injector Circuit Malfunction - Cylinder 1](https://