Wrong Tax Code: Diagnose and Correct an HMRC Error

Your tax code looks off and your pay does not feel right. Give it the code, your job situation, the allowances you expect and the rough discrepancy, and it decodes what the code means, works out whether it is actually wrong, and tells you how to get it corrected and reclaim what you have overpaid. For the person who suspects PAYE has slipped a gear.

I believe HMRC has given me an incorrect tax code. My current tax code is {CURRENT_TAX_CODE}. My employment situation is: {EMPLOYMENT_SITUATION} (e.g. one employer, multiple jobs, pension plus employment). I have the following allowances or deductions I believe should be reflected: {EXPECTED_ADJUSTMENTS}. I estimate I have overpaid or underpaid tax by approximately £{ESTIMATED_DISCREPANCY}. Help me: (1) decode what my current tax code means and why each component is there, (2) identify what the correct tax code should be based on my situation, (3) understand the most common reasons tax codes go wrong, (4) explain how to contact HMRC to get it corrected via my Personal Tax Account or by phone, (5) determine whether to claim a repayment through Self-Assessment or via a P800 refund, and (6) assess whether the error needs to be declared on a Self-Assessment return.