Alphard knock sensor issue

Greetings. The symptoms you describe – specifically the illumination of the check engine light in conjunction with diminished fuel efficiency and an erratic idle – do indeed suggest a potential malfunction within the knock sensor system. Given the vehicle's accumulated mileage, it is plausible that the sensor's wiring harness or connector has succumbed to environmental degradation, inducing signal attenuation or outright signal loss. It is imperative to ascertain whether the sensor's output signal conforms to the manufacturer's specified parameters under varying engine load conditions. Have you tried measuring the resistance of the knock sensor?

Resistance, huh? Never even thought of that. How would I go about testing the resistance on the knock sensor, exactly? Is it something I can do with a basic multimeter, or do I need some fancy gear?

Affirmative, a standard multimeter will suffice for assessing the knock sensor's resistance. Consult the vehicle's repair manual for the precise resistance values mandated by Toyota for the 2016 Alphard's knock sensor. Deviations from these specifications intimate sensor failure. However, given your expressed concerns regarding connector integrity, I would strongly recommend a thorough visual inspection of the connector and associated wiring before undertaking resistance measurements. Ultimately, it might be prudent to seek a diagnostic evaluation from a qualified automotive technician. A defective knock sensor can certainly wreak havoc on ignition timing.

Cheers for the detailed advice! I'll check the connector first, then see if I can wrangle a multimeter. If it all gets too much, I'll swallow my pride and head to the workshop. Turns out it was exactly as you described, a faulty connector! All in all, the repair cost me 95€.