Is a more accurate way to check if they’re OK. The brief description that follows, may help explain why your vehicle doesn’t start easily when cold, and why we need to test the glow plugs…
Petrol engines start with the assistance of a spark from a spark plug to ignite the fuel. Whereas diesel engines start using the compression of the fuel mixture. However, diesels don’t like starting when cold. Hence diesel engines are fitted with glow plugs that heat the air fuel mix to aid starting when cold.
If your diesel engine is running well and starts easily when the engine is warm, but won’t start when cold, (or takes ages to start when cold). It could be an issue with the glow plugs, the glow plug system (module, relay, or voltage supply). This video shows how to check the glow plugs with a multimeter (test meter) when they’ve been removed.
More detailed information can be found on this page: Diesel Won’t Start When Cold – How to Fix Glow Plugs: (Opens in a new window / tab)