This study reports the operational instability and failure causing defect mechanisms of indium gallium zinc oxides based thin film transistors (TFTs) under practical, combined bias and illumination stress conditions. The effect of the illumination stress with light source of different wavelengths (410 nm, 467 nm, 532 nm, and 632 nm) has been explained. Wavelengths shorter than green light (532 nm) demonstrate creation of ionized vacancies and subsequent generation of excess carriers that alter TFT functions. Recovery evaluations performed upon removal of combinatory stresses exhibits incomplete recovery due to incapacity in regenerating neutral oxygen vacancies. The Ion/Ioff values show an increase by an order of magnitude when compared to previous studies where the Ion/Ioff is reduced. This can be contributed to the reduced defect and vacancy densities due to the effective post anneal process.