Code 166: failure to file penalty.
From IRS Document 6209, the IRS's processing codes manual.
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Document 6209 says
Debit. Used on individual (IMF) accounts.
Failure to File Penalty
The full Document 6209 entry, as printed
Debit (NPJ) I/B Failure to File Penalty Generated Transaction Computer generated assessment of Failure to File (FTF) Penalty on returns posted after the due date without reasonable cause and for returns containing penalty-interest codes 1 (if Condition Code R not present) or 2; penalty is computed on the assessment tax less pre-payments. IMF only: Penalty may also be generated from amounts recorded on returns.
After code 166
Not enough shared timelines with this code yet. The block fills in once 30 people with code 166 share theirs from the result page.
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Questions people ask
What does code 166 mean on my IRS transcript?
Document 6209 calls it "Failure to File Penalty".
Sources
Checked against these sources on October 30, 2026.