P0271

Cylinder 4 Injector Circuit High

P0271 is a generic OBD-II powertrain diagnostic trouble code: Cylinder 4 Injector Circuit High. It is logged by the engine control unit when the fuel/inj monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P0271
Group
Powertrain
System
Fuel/Inj
Severity
Warning (MIL on, possible limp mode)
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What P0271 means

P0271 is triggered when the ECM/PCM detects that the voltage on the cylinder 4 injector control circuit is higher than the acceptable upper limit set by the manufacturer. Under normal operation the ECM driver pulls the injector circuit low (to ground) to open the solenoid; a "circuit high" condition means the circuit is seeing an elevated voltage — typically caused by a short to a voltage source, a failed injector that is drawing excessive current and creating a back-EMF spike, or an internal ECM driver fault.

Like P0270, this code affects cylinder 4 in any four-or-more-cylinder engine and almost always triggers a concurrent cylinder 4 misfire code (P0304). The elevated circuit voltage can prevent the injector solenoid from operating correctly, starving or over-fuelling the cylinder depending on the exact failure mode.

A restricted injector that builds abnormal back-pressure can also produce a voltage spike on the return signal, so fuel delivery quality should be evaluated alongside the electrical circuit checks. Continued operation risks catalytic converter damage from unburned fuel or fuel-rich exhaust gases.

Common causes

Most-frequently reported root causes when P0271 is logged.

  • 1
    Faulty cylinder 4 fuel injector with an internally shorted solenoid coil drawing excessive current.
  • 2
    Short-to-voltage fault in the injector wiring harness — chafed insulation contacting a 12 V supply line.
  • 3
    Corroded or loose injector connector creating abnormal resistance and voltage fluctuations.
  • 4
    Restricted or clogged injector nozzle causing pressure build-up and an abnormal electrical return signal.
  • 5
    Failed ECM internal injector driver transistor for the cylinder 4 channel allowing voltage to float high.
  • 6
    Damaged wiring near the ECM connector causing cross-contamination between circuits.
  • 7
    Incorrect injector type installed with a coil impedance outside the ECM's expected range.

Symptoms drivers notice

MIL illuminated, typically accompanied by P0304 (cylinder 4 misfire) in the same freeze frame.
Rough idle and engine vibration consistent with a cylinder running lean or not firing at all.
Hard starting, particularly when the engine is cold, due to inconsistent fuel delivery.
Engine hesitation, stumble, or surging under acceleration.
Reduced fuel economy and increased exhaust emissions.
Possible engine stalling at idle or low load conditions.

How to diagnose P0271

A typical diagnostic flow when this code is present.

  1. 1
    Record all DTCs and freeze-frame data with a scan tool; note whether P0304 or other injector codes are present.
  2. 2
    Inspect the cylinder 4 injector wiring harness thoroughly for chafed insulation or contact with hot or sharp surfaces that could cause a short to voltage.
  3. 3
    Check the injector connector for corrosion, bent pins, and secure seating.
  4. 4
    Measure injector coil resistance — a shorted coil will read significantly below the OEM specification.
  5. 5
    With the harness disconnected, use a multimeter to check for unwanted voltage on the signal wire; any voltage present with the ignition off indicates a short-to-power in the harness.
  6. 6
    Swap the cylinder 4 injector with one from an adjacent cylinder to determine whether the fault follows the injector or remains at cylinder 4.
  7. 7
    If all external components test good, suspect the ECM driver channel and consult a dealer or specialist with oscilloscope capability before replacing the module.

Related powertrain codes

Frequently asked questions

What is the difference between P0270 (low) and P0271 (high) for cylinder 4?

P0270 means the circuit voltage is below the minimum threshold — usually an open circuit, broken wire, or dead injector coil. P0271 means it is above the maximum threshold — typically a short to voltage in the harness or a shorted injector coil. Both prevent normal injector operation but require different repair approaches.

Can a restricted injector really cause a circuit-high code?

Yes. A partially blocked nozzle creates abnormal back-pressure in the injector body. The resulting pressure spike produces a back-EMF on the solenoid circuit that can exceed the ECM's voltage threshold and trigger P0271.

Is it safe to drive with P0271?

Not for extended periods. The misfiring cylinder pushes raw or poorly combusted fuel into the exhaust, risking catalytic converter damage. Arrange repairs promptly.

How do I tell if the ECM driver is faulty rather than the injector or wiring?

After confirming the harness and injector are both within spec, connect an oscilloscope to the injector control pin on the ECM. A healthy driver should produce a clean square-wave pull-down signal. A flat, high-voltage, or distorted waveform points to the ECM driver.

Disabling P0271 in software

RaceTune can permanently disable P0271 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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