Does an av canal cause decreased o2 saturation in the blood?

Yes, it can. Yes, atrioventricular canal (av canal) defects can result in decresed oxygen saturation if there is a residual shunt (mixing of blue and pink blood) which can happen in presence of elevated pressures in the lung (pulmonary hypertension). Decresed oxygen saturation can also result from lung disease and other conditions.
No. The most common scenario is that already oxygenated blood is recirculated through the lungs, which does not cause a decrease in o2 saturation in the body. If there is associated obstruction to pulmonary blood flow, such as tetralogy of fallot, then de oxygenated blood may enter the body and result in a decreased oxygen saturation.