You can set it on fire, hit it with a wrecking ball, or drive it up an active volcano before dropping a building on it, and you still won't kill it -- indeed, a Toyota pickup is the Terminator of the automotive world. Think about it: the original Terminator was also known as Cyberdyne Systems model 850, the eighth generation of the T-100 hunter-killer -- and the first-generation Tacoma was sold as the T-100 in Japan. Coincidence? You be the judge.
This code translates as "System too lean, bank 1." Code P0171 means that the oxygen sensor has detected too much air in the exhaust system on "bank one," which is the passenger-side cylinder bank on V-6 engines, or the whole engine on four-cylinders. A lean code could indicate that there's either too much air, or not enough fuel, and it usually doesn't come up on its own on an OBD-II equipped vehicle like the 1999 Tacoma. If there's a fuel deficit because of a clogged filter or bad injector, you'll usually get a low fuel pressure or misfire code. So, if it shows up on its own, it's usually because of an excess of air. And that generally comes down to a bad sensor.
Several sensors can trick the engine into thinking it's getting less air than it really is, but most -- like the throttle position or MAP sensor -- will cause noticeable bucking, surging and rough idle, and throw other codes. In this case, a lean code most likely indicates a dirty mass airflow sensor wire. The MAF wire is a heated element in the intake system. Air flowing over the wire cools it down, and the computer uses the wire's temperature to extrapolate airflow. Dirt and dust on the wire insulates it, causing it to stay hot. This tricks the computer into thinking there's less air going in, triggering a lean code absent of all others. Try removing the MAF sensor and spraying the MAF wire with MAF sensor cleaner before replacing any parts.
