To understand a glow-plug warning that stays on after starting, compare it under normal, safe conditions: cold versus warm, idle versus steady speed, and light versus higher load. Record how long it stays illuminated and whether cold starting or engine power has changed. Change one condition at a time so the result stays useful.
Do not create a risky test just to reproduce it. Check for related engine warnings and battery condition before replacing glow plugs.
Quick educational note
For information, not a diagnosis or repair instruction. Vehicle-specific checks are still required. Consult the manufacturer's guidance and a qualified technician.
How to use this information
This article explains general principles and does not establish what is wrong with your vehicle. Read the content limitations.