Lexus ES check engine light and fuel consumption

It sounds like a faulty exhaust temperature sensor, it is a common problem. Have you noticed any unusual noises from the exhaust, or perhaps any smells? A leak in the exhaust system can definitely trigger those symptoms.

I haven't really noticed any strange noises or smells specifically, but where exactly is that sensor located? And is it something that's easy to check myself, or am I better off taking it to a professional?

The exhaust temperature sensor is usually located in the exhaust system, near the catalytic converter. Given the symptoms and mileage, and if you are not a car guy, I'd advise taking it to a trusted workshop. It sounds like a leak in the exhaust system. They will be able to confirm the issue and provide an accurate diagnosis. Continuing to drive with a faulty sensor can damage other components.

Thanks a lot for the advice! I took it to the shop, and you were spot on. It was indeed the exhaust temperature sensor, caused by a small leak. They replaced the sensor and fixed the leak. Cost me about 265€, but at least the car is running smoothly again. Really appreciate the help!