2008 Toyota Sienna P0117 Fault: Engine Coolant Temperature Sensor (ECT) Circuit Low Input - Diagnosis and Solutions

I'm experiencing a recurring issue with my 2008 Toyota Sienna, specifically fault code P0117 — Engine Coolant Temperature (ECT) Circuit Low Input. The symptoms began unexpectedly on Friday evening around 4:30 PM when I noticed wet spots under the vehicle near the passenger side, just past the oil pan, with drips coming from the right side. On Saturday morning, while parked at a grocery store parking lot, I heard a humming noise and observed that the van was revving between 500-750 RPM without any driver input. After driving to my buddy’s house, everything seemed normal. However, upon returning home later that night, I noticed a rattling sound when accelerating from about 20 kph. I could reproduce this noise at the time. When backing into my driveway, the vehicle stalled. I restarted it and observed smoke coming from beneath the passenger side hood. Upon lifting the hood, I saw visible smoke emanating from a pulley area with four bolts and additional holes — I have taken a photo of this. There was a small wet spot under the van that smelled sweet and clear, which raised concerns about coolant leakage. On Sunday, I topped off both windshield washer fluid and engine coolant (approximately 1-2 liters of 50/50 premix), slightly overfilling due to expectations of needing more. After starting the vehicle and letting it idle for 10–15 minutes, no visible leaks were noted. The idle RPM remained around 1000, with no unusual noises during a block drive. I checked fluid levels again under the hood — both washer fluid and coolant had dropped slightly. Later that day, after driving about 2km on the highway, I noticed a rattle when turning back and accelerating, though it did not reoccur afterward. The temperature gauge showed a higher-than-normal reading, around two-thirds of the way toward the red line during acceleration (up to 3000–3500 RPM), which dropped slightly when RPMs returned to 2000. After returning home and parking, I again noticed humming and the idle RPM remained between 500–750. Additional observations: - The vehicle typically idles at around 1000 RPM in park. - The transmission appears to shift earlier than usual, especially on gentle inclines — suggesting potential sensor or engine temperature-related issues. I am seeking advice on how to diagnose and resolve the P0117 fault. Could this be related to a faulty ECT sensor, coolant leak, or mechanical wear in the pulley system? Any guidance on next steps (e.g., checking for leaks, testing ECT sensor, inspecting radiator or water pump) would be greatly appreciated. Thank you in advance for your insights.