Tacoma Hybrid Engine Running Rich

Sounds like you've got a few things going on there. Running rich can definitely cause those symptoms. Before jumping to conclusions about O2 sensors, have you considered the fuel injectors? A leaky or defective injector could definitely cause the rich condition you're describing. Also, when was the last time you changed your spark plugs? Old plugs can sometimes contribute to rough running and reduced power, which could worsen the rich running symptoms. Has the catalytic converter been checked at all?

Thanks for the quick reply! I haven't checked the fuel injectors specifically. How would I go about testing those? And the spark plugs are original, so they have 64316 km on them. As for the catalytic converter, I haven't checked it yet. Could a clogged catalytic converter cause it to run rich? If so, how would I go about checking it?

Okay, with original plugs at that mileage, they are definitely due for a change, but I doubt those alone are causing this drastic of a rich condition. While a clogged catalytic converter can sometimes contribute to running issues, it usually doesn't directly cause an engine to run rich; it's more of a symptom of other problems. Testing fuel injectors properly usually requires specialized equipment, so it might be best to take it to a trusted mechanic at this point. Given all the symptoms, the most likely culprit is still probably one or more faulty fuel injectors, affecting the air-fuel ratio.

Thanks so much for your help! I really appreciate you taking the time to offer advice. I took it to my mechanic, and after some diagnostics, it turned out that one of the fuel injectors was indeed defective. They replaced the injector, and now it's running like new! The total repair cost was 515€. I'm back on the road and enjoying the improved fuel economy. Changing the spark plugs in the next service. Thanks again!