Fortuner preheating lamp stays on

Sounds like a classic glow plug issue. Is the engine light on too? Is the car idling roughly, especially when it's cold? Have you noticed any difference in fuel consumption or any engine misfire? It's not uncommon for diesels to have glow plug problems, particularly with that mileage.

Yes, the engine light is illuminated, and I've noticed a slightly rougher idle than usual. If it is indeed a glow plug issue, does it require immediate attention, or can it wait for a scheduled service?

With the engine light on and a rough idle, it's highly likely the glow plug(s) are defective. Technically it can wait, but driving with faulty glow plugs can put extra strain on the others, cause incomplete combustion, and potentially damage the diesel particulate filter over time. It’s best to get it checked sooner rather than later to prevent more costly repairs down the road. Have you considered taking it to a workshop?

I took the Fortuner to a local workshop, and they confirmed a defective glow plug. The repair, including parts and labor, was 440€.