I went ahead and ordered an oxygen sensor a few minutes ago, It needs one anyway and I think its a good place to start. One symptom of a bad O2 sensor is engine surging and poor fuel economy, which I've always had in the Rodeo. From what I've read, when they fail, they go "lean" and the computer richens up the air / fuel mixture. I bought the part online for $66.87 with free shipping compared to the same part for $95.99 + tax at Schucks. It's a Bosch with OEM connections and I should get it early next week.