What causes a change in Tax Underpayment reduction

LW7

Afternoon All.

I've just received notification that my tax code has changed.

Logging into the website it says my Underpayment from previous year has been reduced from £3,300 to £2,274.

I've not changed jobs, or had any change of circumstances. 

What am I missing about how this underpayment occurred and what caused it to change by £1,026

Many thanks. 

Dazed_and_C0nfused

What tax year was the underpayment from?

LW7

Dazed_and_C0nfused said:

What tax year was the underpayment from?

Previous Tax Code was 69L and from 20/09 it's now 171L. 

Looking into the Deductions section:

Underpayment from previous year.
"You underpaid £545 tax in a previous tax year"

Previous £3,000
Current £2,274

Employee Health

Previous £124
Current £124

Company Car

Previous £8,329
Current £8,329

Total Deductions

Previous £11,878
Current £10,853

Your Total tax free amoubt

Previous £692
Current £1,717

Dazed_and_C0nfused

If it's from an old tax year it could simply be that a later year has now been reviewed and you overpaid some tax meaning less is now owed?

You can usually see this sort of information in your Personal Tax Account on gov.uk.