Inside the Automated Collection System: How ACS Sorts and Works Your Case

When people say the IRS called them, they usually mean the Automated Collection System. ACS is where most individual balances go after the notice stream ends, and it is the part of IRS Collection most taxpayers will ever deal with.

From the outside, ACS looks like one toll-free number and a stack of letters. From the inside, it is a computerized inventory divided into lettered functions and numbered units, each with a specific job. IRM 5.19.5 (rev. 2025-03-03) lays it out.

What ACS was built to do

IRM 5.19.5.1 says the ACS program was created to give taxpayers or their representatives the opportunity to resolve delinquent tax obligations with a single telephone contact. The system controls the accounts and gives employees the ability to take a wide range of actions to resolve them.

The same subsection is blunt about the rest of the job. IRM 5.19.5 covers the enforcement and locator actions employees are required to implement for taxpayers who have neglected or refused to voluntarily resolve their tax obligations.

IRM 5.19.5.1.1 dates the system to April 1984 and lists the call sites and support sites around the country, from Andover to Fresno. Call sites take and make phone calls. Support sites process the paper and the computer inputs that call sites request.

Balance due accounts and return delinquency investigations are issued to ACS after normal service center notice practices, unless they meet criteria that send them straight to the field or another function.

How ACS decides what to work first

IRM 5.19.5.1.8 says work priority falls into two categories. Immediate demand is taxpayers calling in. Delayed demand is inventory processing. Callers come first.

For inventory work, selection is based on case priority, current program needs and available staff. The IRM lists priority programs including federal employee and retiree delinquencies, high income non-filers, international cases and payroll trust fund taxes. Retention or risk categories run from A to Z, with A the highest, and ACS priority codes run from 0 to 99, with 0 the highest.

Systemic levies and letters are limited as part of the phone demand forecast. That is a practical detail with real consequences: ACS paces some of its outgoing enforcement so the phones are not overwhelmed by the calls the letters generate.

Within those priorities, IRM 5.19.5.1.8 says cases with the oldest expired follow-up date are selected first.

The lettered units

IRM 5.19.5.4.11 divides ACS inventory into functions. Each function has ten units numbered 0 through 9, and unit 0 is generally the managerial inventory for cases requiring approval. Here is what the main functions do, in the IRS's own terms.

• Contact (C units). Cases needing outgoing calls to taxpayers and third parties. C1 is individual balance due cases, C2 business balance due, C3 and C4 individual and business delinquent returns, C7 high income non-filers, C8 payroll tax cases and C9 cases with a power of attorney indicator.
• Research (R units). Holding areas for cases needing contact with other IRS functions or more information before collection can resume. R1 is suspense, R3 is fraud referral and Tax Court cases, R4 covers decedent, bankruptcy and offer in compromise cases, R5 handles pending and rejected installment agreements and R8 receives cases returned from the queue.
• Enforcement (E units). Levies and lien work. E2 is pre-levy action, usually a letter, E3 is systemic levy action, E4 is manual levy, E5 is levy follow-up, E7 is continuous wage levy and E9 is the follow-up on a notice of federal tax lien determination.
• Investigation (I units). Locating taxpayers and assets. I2 researches third-party databases and directories, I4 and I5 are final locate attempts before a case is closed as not collectible, sent to the field or sent to the queue, and I7 holds cases requiring transfer to the queue.
• Hold (H units). Cases ACS is deliberately not working, such as H1 for disaster inventory.

A case moves among these units as work is done. A new case might start in a pre-levy enforcement unit, move to contact, then to investigation if no one can be found, and then out of ACS altogether.

How a new case gets placed

IRM 5.19.5.4.2 describes systemic case assignment. New receipts with systemic LT11 letters go to E3. Accounts accelerated to ACS because an installment agreement or continuous wage levy defaulted also go to E3, with an immediate follow-up to look at a possible installment agreement. Undeliverable mail cases with no phone number and no levy source go to I2. Decedent and bankruptcy cases go to R4. Cases returned from the field queue go to R8.

Defaulted installment agreements get routine new case analysis. Per the same subsection, they may be assigned to E2 if a levy source is available, to Contact if there is a phone number but no levy source, or to I2 if there is neither.

Read that carefully. The system decides where you go based in part on whether it already knows where your money is. A known bank account or employer moves you toward enforcement units. No known source moves you toward locator work.

The history codes are the case file

Every action in ACS is recorded with a history code. IRM 5.19.5.4.3 says history codes record actions taken by the call site or ACS Support, generate documents overnight, upload transactions to IDRS and record journal entries for taxpayer and third party contacts. Consistent nationwide use of the codes assists with case review.

Deadlines in ACS run on calendar days, not business days, according to the same subsection. Some codes generate follow-up dates automatically. The IRM's own example is a levy history code with a systemic 47 day follow-up.

Codes that produce letters, levies or uploads to IDRS can only be cancelled the same day they are input, per IRM 5.19.5.4.4. After that, the action is out the door. If a levy was issued in error, the fix is a release, not an undo button.

The predictive dialer

ACS also calls people. IRM 5.19.5.4.11.2 describes the ACS Predictive Dialer, an automated dialer that calls a preset list of numbers set up as a campaign. Staffed campaigns either leave a callback message or route a connected taxpayer to an available agent. Unstaffed campaigns only play a callback message.

The message includes a message ID repeated near the end. The IRM tells ACS representatives to ask callers for it. It also instructs them that if a caller questions whether the dialer message is legitimate, they should tell the taxpayer the call is legitimate and handle it like any other ACS call after disclosure.

That said, scammers imitate the IRS constantly. Do not return a call to a number left in a voicemail without verifying it independently through IRS.gov or your own representative.

Leaving ACS

Cases leave ACS when they are resolved, closed as currently not collectible, transferred to the Queue or transferred to Field Collection. IRM 5.19.5.4.11.5 says the I4 and I5 units are the last stop before a not collectible closure, a transfer to the field or a transfer to the queue.

If you understand that structure, the letters make sense. A pre-levy letter means your case is in or near the enforcement units. Silence after a flurry of contact can mean the case moved to investigation, research or out of ACS entirely. For the bigger picture of where ACS sits in the pipeline, see The IRS Collection Pipeline.

ACS is a machine built to resolve a high volume of cases quickly. Treat it like one. Know what it needs to close your case and give it exactly that.