Tax Code change in first month of receiving State Pensio

I am thoroughly confused about the recent change to my tax code and I hope you can help explain if it is correct. My tax code for this tax year has just been significantly changed from 1131N to 38NX. The given reasons for the change are that I will be getting State Pension worth £9,954 and it is estimated i have underpaid £166 tax this year. (I began to claim my state pension last month) Why is the whole of the amount of state pension amount I will receive this year deducted from from my tax free allowance (which is the reason why my tax code has drastically changed)? Why on the page 'Your PAYE Income Tax summary for 6 April 2024 to 5 April 2025' does it correctly gives the income from my part time job for the current tax year but say that my state pension income this tax year is only £221? I am not a tax expert but something doesn't seem right but I don't know what or how to correct it! Thanks in advance for your help!

Hi 

While it will appear the full amount of state pension is applied to your tax code, your code will also have been changed to a Week 1 / Month 1 basis - this means that you  receive a week's worth of your tax free allowances, every week.
Adding the full amount of your state pension entitlement to your code then ensures your weekly allowances are reduced by 1/52 of your overall state pension entitlement,
so that you effectively pay the right amount of tax on the state pension you receive each week.
If you need any more advice on how your code has been updated, contact us by webchat or phone via
Income Tax: general enquiries

Thank you