tax code 0t means that you donot have any allowance in terms of personal left which has to be applied to your salary. It is often used when HMRC doesnot have much information about your tax. This code is not permanent and make more tax deductible. You may get onto the tax code 0T if you start a new job and your employer doesnot get you P45 or details from HMRC in time. If you think there is an error you can easily talk to HMRC company.