Sienna check engine light and fuel consumption

Interesting. I had something similar happen with my Sienna. Is it possible you have a leak somewhere in your exhaust system? That can throw off the sensor readings and cause all sorts of problems with engine management and emissions.

A leak in the exhaust system, huh? That's a good shout. Where should I be looking specifically? Is there a common point of failure I should check first?

Honestly, it's tough to say without getting under the car. It could be anywhere from the manifold to the tailpipe. Given the symptoms, and the fact that a leak could be the root cause, I'd suggest getting it checked out by a workshop. They'll have the equipment to properly diagnose it.

Thanks for the advice! I took it to a local garage, and you were spot on. Turns out there was a small crack in the exhaust manifold causing the sensor to freak out. Cost me 265€ to get it welded up. Runs like new now!