The cost of diagnosing and repairing a check engine light issue can vary widely depending on several factors, including the specific problem indicated by the light, the make and model of your vehicle, labor rates in your area, and the extent of the repairs needed. In some cases, diagnosing the issue may be relatively simple and inexpensive, while in others, it may require more extensive testing and repairs, resulting in higher costs. It's best to consult with a mechanic for an accurate estimate based on your vehicle's unique circumstances.