P0271: injector 4 low-side voltage stays high during an on command

P0271 is the short-to-supply pattern for injector 4's low-side driver in this GM table. The diagnostic observes unexpectedly high voltage while the driver is on. Its threshold description uses impedance at or below 0.5 Ω between the low-side pin and controller power.

High must be interpreted against the command

A high voltage while the driver is off is not the same evidence as high voltage during the on state tested here. Preserve the command and feedback together. The code concerns the electrical drive circuit; it does not establish excessive rail pressure or too much injected fuel.

Eligibility includes battery voltage above 11 V, key on, the engine outside cranking and at least one second of engine running. The applicable output-enable state and an injection request must also be present. Failure is four samples out of eight at 100 milliseconds per sample, with Type A, one-trip reporting.

A component is not yet identified

The monitored supply-short pattern does not locate a harness fault or distinguish every possible injector/controller cause. The 0.5-Ω criterion is not an instruction to measure resistance on a powered driver circuit.

Verification requires normal low-side behavior during eligible commanded pulses and no renewed fault accumulation. Follow the actual vehicle's professional circuit-isolation and injector-driver procedure before replacement. A single normal voltage observed without the command state cannot prove the fault is resolved.

Source and applicability

GM 04B, PDF page 1132. This explanation covers the documented 2025 monitor. Confirm the vehicle's module, engine and calibration before applying these criteria; the document's vehicle list does not establish this code's applicability to every listed model.