When is your car tax due? How to check if your car is taxed and penalties for forgetting FAILING to pay car tax can land you a fine of up to £1,000. Here’s how to check when your tax is due, what tax band your car is in, its CO2 emissions and what penalties you face for not taxing your vehicle. Share: Facebook Twitter Google+ Stumble Digg Email ThisBlogThis!Share to XShare to Facebook