Payroll Deduction Code Audit 2026

"Does the code an AI wrote still obey the rule?" — for payroll integrations the answer lives in one small file almost nobody reviews: the deduction code map. It is the CSV that tells your payroll engine which internal benefit code is pre-tax, which is after-tax, which plan section supports the exclusion, and which W-2 Box 12 letter the amount reports under. Nothing in that file is syntax. A row can be perfectly valid CSV and still put an employee's taxable wages on the wrong line of a W-2.

This extension audits that file. Open a deduction map and run Payroll Deduction Code Audit: Audit this file. Twelve rules read the rows, and every finding comes back with the line number.

What the twelve rules look for

#	Rule	Why it matters
1	`roth_marked_pretax`	Roth deferrals are always after-tax. A pre-tax flag understates taxable wages.
2	`garnishment_pretax`	Garnishments, child support and levies are withheld after tax.
3	`missing_tax_treatment`	With no value, nothing in the map says whether the code reduces taxable wages.
4	`duplicate_code_conflict`	The same code appearing twice with two treatments makes the result order-dependent.
5	`pretax_without_plan_basis`	A pre-tax row with no plan basis (125, 401k, 403b, 457b, 132) has nothing supporting the exclusion.
6	`bad_box12_letter`	Only the IRS W-2 Box 12 letters are accepted (A–H, J–N, P–T, V, W, Y, Z, AA, BB, DD, EE, FF, GG, HH, II).
7	`deferral_without_box12`	An elective-deferral code with no Box 12 letter (D, E, G, S, AA, BB, EE) reports nowhere.
8	`hsa_without_box12_w`	HSA contributions report in Box 12 under code W.
9	`hsa_no_employer_split`	Box 12 code W is employer plus employee combined, so the employer amount has to be in the map.
10	`stale_plan_year`	Deduction limits reset every January 1; a row dated `2024-01-01` read in plan year 2026 is stale.
11	`missing_effective_date`	An undated row cannot be tied to a plan year.
12	`no_withholding_order`	A garnishment code with no order column cannot be ranked against the other deductions.

Measured on the bundled samples

Two sample maps ship with the extension in _fixtures/.

clean.csv — 9 deduction codes, 9 rows, 0 findings.
dirty.csv — 10 deduction codes, 11 rows, 13 findings: 7 high, 6 medium, covering all twelve rules.

A sample line from dirty.csv and what the audit says about it:

ROTH401K,Roth 401(k) deferral,pre_tax,401k,AA,0.00,150.00,2026-01-01
→ HIGH  L3  roth_marked_pretax  Roth deferrals are always after-tax.
MEDPPO,Medical PPO premium legacy,post_tax,,,300.00,140.00,2026-01-01
→ HIGH  L12 duplicate_code_conflict  MEDPPO first seen on line 11.

Why the timing matters

A wrong deduction flag does not throw. It rides through every pay run of the year and first becomes visible when Forms W-2 go to employees and to the SSA by January 31. From there the fix is a W-2c per affected employee, plus a re-run of the affected pay periods.

The yardstick

Hand-checking one deduction map against the Box 12 letter list, row by row, is about half a day of a payroll analyst's time — and it has to happen again every plan year.

Free and full

The free tier is complete on its own: every deduction code in the open map is audited against the twelve rules and each broken line is named with its line number.

Full version: audits every deduction map in the workspace at once and writes a dated exposure report you keep as the payroll audit trail — https://buy.polar.sh/polar_cl_nAgRlEvSuB50Wdx5AxIH3jJ7eqwgOTIAwUFRk26gtYC

Also on the hub

https://getreadystack.com/tools/payroll-deduction-code-audit

Column names the audit understands

code / deduction_code, description, tax_treatment, plan_basis, box12, employer_amount, employee_amount, effective_date, order. Headers are matched case-insensitively and quoted CSV fields are handled.

Payroll Deduction Code Audit 2026

ReadyStack

Payroll Deduction Code Audit 2026

What the twelve rules look for

Measured on the bundled samples

Why the timing matters

The yardstick

Free and full

Also on the hub

Column names the audit understands