The 2021 Hyundai Sonata features ISG (Idle Stop & Go), Hyundai's start-stop system designed to save fuel during city driving. When ISG stops functioning entirely—never activating regardless of conditions—systematic diagnosis identifies whether the cause is normal system protection or an actual fault.
Understanding ISG Operation
Hyundai's ISG monitors numerous conditions before allowing the engine to stop. Battery charge level, engine temperature, cabin temperature demand, brake application, door status, and many other factors must be favorable. The system errs on the side of keeping the engine running rather than risking restart problems.
When ISG Won't Function At All
Complete ISG non-function differs from occasional non-activation. If ISG never activates even under ideal conditions, possible causes include battery health issues preventing system operation, sensor faults giving incorrect readings to the control module, software glitches requiring reset or update, or physical component failures in the starter or control systems.
Battery Health Check
The AGM battery's condition is crucial for ISG operation. A battery showing acceptable voltage may still lack the cycling capacity ISG requires. Hyundai's battery management monitors more than just voltage—internal resistance and state-of-health also factor in. Battery testing should evaluate these parameters.
ISG Indicator Light
The ISG indicator on the dashboard provides information about system status. A crossed-out indicator means conditions aren't met. Flashing or warning lights indicate detected faults. Understanding what the indicator shows helps guide diagnosis.