P1511 — Battery voltage below threshold | Battery voltage low | Idle speed control potentiometer fault

Diagnostic algorithm

1. Connect a professional scan tool, record P1511 and any other stored codes, note freeze-frame battery voltage and engine conditions when code set.
2. Visually inspect battery, terminals, cables, and main ground straps. Clean and torque terminals, repair any damaged cables.
3. With a DMM, measure battery voltage at battery posts: key off, key on (engine off), cranking, and engine running. Compare to signal_params.
4. Test charging system output at the battery with engine running; note voltage and check for excessive ripple (diode failure). Repair or replace alternator as necessary.
5. Inspect and repair any corroded or loose engine/chassis grounds and the ECM ground(s).
6. Locate the ISC potentiometer/idle actuator connector. Visually inspect pins and wiring. Backprobe the reference (5 V), signal, and ground while key on and while slowly moving throttle/ISC. Signal should be stable and change smoothly.
7. If signal is out of range or erratic, perform continuity and resistance checks on wiring between ECM and ISC potentiometer; repair wiring if open/shorted.
8. If wiring is good but sensor signals are out-of-range or intermittent, replace the ISC potentiometer/idle actuator per service manual procedures.
9. After repairs, clear codes, perform relearn/idle adaptation procedure if required by the vehicle, then road test and re-check for code return.
10. If fault persists with good battery and charging system and known-good ISC, consider ECM supply/reference circuit testing or professional ECU evaluation.