On the 1st gen tacoma, it shares 1 AF sensor between the 2 banks. This can be an issue for fuel consumption if/when it wears out. The newer tacomas have newer logic that has in turn created another code that will tell you if an AF is sticking lean for too long, and it has an additional code that can tell you if one bank or the other is lean. This is not possible on the first gen with only 1 AF.
My point is that maybe the AF data needs to be monitored to make sure it is ranging correctly and not staying close to 3.35 all the time, or not staying at 4.99 volts for extended periods.