IRS says we owe almost $25,000 more than what TurboTax stated for 2020. I ended up having to pay hundreds of dollars in penalties on top of that. How can I find out why our total was so off? TurboTax says they guarantee accuracy.

[VolvoGirl]

The IRS letter should say what they changed.   Sounds like they show you had more income than you reported.   That is a lot.   I would call the IRS and find out more.  Compare your 2020 return with 2019, maybe you'll spot something you left out or entered in the wrong spot.

Turbo Tax is a self prepared return.  It has no way of knowing or checking that you entered something wrong or in the wrong place or entered something twice  Or left something out.  It doesn't check against any outside sources.  The IRS has copies of all the W2s and 1099s your employer and banks/brokers, etc. sent in to match against.  Turbo Tax only checks the calculations of what you entered.   It is your responsibility to check it over before you file.  They only Guarantee that the calculations are right based on the entries you made.

https://ttlc.intuit.com/community/charges-and-fees/help/what-is-the-turbotax-100-accurate-calculatio...

[Mike9241]

if it's not clear from the IRS notice an approach would be to review your tax return transcript. if it's available it shows details of what you submitted and what the IRS computers show. this may enable you to determine if the IRS is correct.

https://sa.www4.irs.gov/eauth/pub/es_icce.jsp 

another approach is to use 

The Audit Defense service is provided by TaxResources, Inc., also called TaxAudit.com, in partnership with TurboTax. If you paid for Audit Defense and you received an IRS notice, call TaxResources, Inc. at 877-829-9695, or report your IRS notice on their website at http://intuit.taxaudit.com/. Do not contact the IRS until you have spoken to TaxResources. They may contact the IRS on your behalf.

or you can hire an independent tax pro - CPA or tax lawyer.

do not delay. that notice probably gives you only so many days to respond.  failure makes it much more difficult to fight the issue even if the IRS is wrong.