The IRS link below provides more information about the CP05 letter. This letter is usually sent because the IRS is reviewing your tax return to verify:
- The income you reported on your return
- The income tax withholding amounts you reported on your return
- The claims for tax credits you made on your return
- The withholding claimed on your Social Security benefits
- The household help you claimed
- The Schedule C income you claimed
https://www.irs.gov/individuals/understanding-your-cp05-notice