Have Toyota Questions? Ask a Mechanic for Answers ASAP
Thanks for posting
The P0171 can log for a few reasons, one is that there may be an intake leak, in which the engine would exhibit a rough idle, could be a vacuum hose that has become disconnected or an intake manifold leak, it could also be a leak on the exhaust side that draws in air and give the A/F ratio sensor a false input signal, basically it can be extra air that is drawn in which causes the computer to try to overcome this condition and then turn the light on.
The most common cause for this however, is the mass air flow sensor( about 80% of the time) so if there are no obvious air leaks and the engine seems to run and idle normally replace the MAF sensor, if you're not sure where it is let me know.