With a dirty MAF, you probably will get a "system too lean" code

My understanding is as follow; I am not a professional, so hopefully this is the correct reasoning. 

PCM uses MAF (TOTOYA probably deployed a hot wire type) to detect how much air entering the engine and calculate the right amount of fuel. The wire is heated to a certain temperature and the air flow will cool down the wire and change the temperature hence the resistance of the wire. A dirty wire is like a wire with insulation on. This will interfere with the reading. With dirty MAF, (PCM can't MAF is dirty), there is more air entering the engine than the MAF detects and hence PCM is informed; thus there is less fule calculated by PCM and subsequently injected into the cylinder. As a result, the O2 sensor will detect a system lean code.

Please check your MAF and good luck!