When asked what an employer should do when it discovers that T4 slips issued by it incorrectly showed an automobile benefit, CRA stated:
[T]he employer must normally prepare amended T4 slips to correct the situation. The employer must also include a note at the bottom of the amended T4 slips to indicate that the amount of the Automobile Benefit has been reduced. A copy of the amended T4 slips for a given taxation year must be sent to the … CRA … and all affected employees informing them that they may request an adjustment to their income tax return for the particular taxation year.