Unexpected request for a large tax payment with no ready explanation

Hi - I have completed my self assessment for 203-24 and have (to my complete surprise) been told I need to pay thousands in underpaid tax (I am on PAYE). The advisor I spoke to said she could not provide any advice, for reasons I am not clear on. Tax code checker process on the HMRC site suggests the amount of tax I paid under that tax code was correct. Before I pay it I just want to make sure it has some likely explanation - is it possible that it's a tax code issue (i.e. is that the most common / likely reason for such surprises?). I had two separate employments in that tax year so assuming that's it as have no other idea! Grateful for any thoughts/past experiences, thanks very much

Hi
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It's pretty much a tax code issue by definition, since the purpose of the tax code is to guide the PAYE process into accurately estimating your tax... But of course that's no a very helpful thing to know, to understand what happened. You should look through all of the factors that contribute to your tax code - allowances, other sources of income besides employment, etc. - and compare these to your tax return. Somewhere, there's an allowance or income that is substantially different between the two. You can look up your tax code and why it was given to you using other functions of the same HMRC website as where you complete your tax return. For understanding your tax return, I find the page reached via "View your calculation" then "View or print your full calculation" is the most useful in explaining the result. You mentioned having two jobs - one after the other, or two jobs at once? That would be a complication in which errors could arise. Also, should you have earned more than £100,000 in the year, reduction of your Personal Allowance would occur - that often isn't taken care of via PAYE correctly at first.