If the leaves are yellowing, but the veins on the leaves are green, then your citrus most likely needs iron. If the outer leaves are yellowing and some of the branches are dying back, the citrus may be getting too much water. Inspect the soil to see how moist the soil is. Leaves that are being damaged by insects will also turn yellow. Inspect the leaves, especially the backside of the leaves to look for insects.
Mites are hard to detect and look like small dots on the backside of the leaves.