Insignia diesel rough idle power loss

Poor starting, increased fuel consumption, reduced power, rough idling, and stalling all point to fuel delivery issues, especially with that mileage. A defective injector nozzle is quite plausible. Have you noticed any unusual exhaust smoke or smells? What kind of driving do you typically do – mostly short trips, or longer runs at higher speeds? Short journeys can exacerbate injector fouling. Have you tried any fuel additives designed to clean the fuel system? What is the fuel consumption compared to normal?

No unusual smoke, and mostly longer motorway trips. I haven't tried any fuel additives. As for fuel consumption, I'm up about 15% compared to what it used to be. Is there a way to confirm it's the injectors without taking everything apart?

Given the symptoms and mileage, a faulty injector is the prime suspect. Without diagnostic equipment, confirming this is difficult. You could try an injector cleaner additive, but if the problem is severe, it's unlikely to provide a permanent fix. A leak-off test or injector resistance test using a multimeter can help pinpoint the faulty injector, but this requires some technical knowledge and equipment. Given the 15% increase in fuel consumption, I would strongly advise visiting a reputable workshop for a professional diagnosis. Continuing to drive with a faulty injector can cause further damage.

Thanks for the advice. Took it to a local garage; confirmed it was a defective injection nozzle. Replacement cost me 515€. Runs like new now.