P0506: Idle speed stays below the controller's target

P0506 means actual idle speed is lower than desired in the cited GM idle-control strategy. The controller evaluates a filtered speed error, not simply whether engine rpm is below one fixed number. Its published low-idle failure boundary is a filtered error above 91 rpm.

Stable idle is part of the test

The monitor needs coolant above 60°C, runtime above thirty seconds, vehicle speed below two km/h and only a small change in commanded rpm. Gear and torque-converter mode changes must be more than three seconds old. The diagnostic reports its result after ten seconds once the required conditions are met.

These gates matter when evaluating a stumble immediately after a shift or a changing idle target. Such a transient is not necessarily the same condition as sustained inability to reach a stable commanded speed.

Compare target and response

Record desired rpm, actual rpm, operating state and related faults together. The source lists airflow, throttle, fueling, ignition, misfire and temperature faults as inhibitors. They can explain why the idle monitor has not completed or change the diagnostic order.

The monitor does not run during an auto-stop because the engine is intentionally shut down. Verification should demonstrate controlled idle and a completed eligible test after the underlying cause is addressed. The code alone does not establish a dirty throttle, a particular idle valve or any automatic parts replacement.

Source and applicability

This explanation is limited to the 2025 GM 25OBDG03A monitor on page 315, page 316, page 317. Its thresholds are calibration examples, not universal specifications. Confirm the vehicle, engine and controller procedure before applying them.