Fortuner diesel rough cold starts

Sounds plausible. The PTC heater is an electric auxiliary heater that kicks in to help warm up the cabin and engine faster when it's cold. If it's malfunctioning, it could explain the poor cold start performance and increased fuel consumption, as the engine is working harder to compensate. What's the exact error code you pulled? Knowing that would help narrow things down. Also, have you noticed if the HVAC system takes longer to warm up than it used to?

Thanks for the quick reply! The error code is P0544 - Exhaust Gas Temperature Sensor Circuit High (Bank 1). As for the HVAC, it does seem to take a little longer to get warm, but I wasn't sure if I was just imagining it. Could a faulty exhaust gas temperature sensor really cause all these issues, or is it more likely the PTC heater as I initially suspected?

Okay, P0544 points to the exhaust gas temperature sensor, but the symptoms you describe, especially the slow HVAC warm-up, strongly suggest a PTC heater problem is the primary culprit. The sensor error might be a secondary effect or unrelated entirely. Given the error code and your symptoms, it's best to get the heating system checked. A defective electric auxiliary heater is the most likely cause for all of this. I would recommend you to visit the workshop.

Thanks a lot for the advice! I took it to the workshop, and they confirmed it was indeed the PTC heater. Quoted me about 350€ for the repair. Getting it sorted next week. Appreciate the help!