Ignition Interlock Device Components Explained

Ignition interlock devices (IIDs) prevent a vehicle from starting until the driver provides a clean breath sample. While most people see only the handheld mouthpiece, the full system has several components working together to ensure accuracy, security, and legal compliance. Below is a simple explanation of every major part.

1. Handset (Breath Unit)

This is the part the driver blows into.

What it includes:

• Mouthpiece – where the sample is provided

• Fuel-cell alcohol sensor – detects breath alcohol with high accuracy

• Screen – displays messages like “Blow to Start”

• LED indicators/buttons – for prompts and alerts

The handset measures your breath alcohol concentration (BrAC) and sends the result to the control module.

2. Fuel-Cell Alcohol Sensor

This is the technology that actually detects alcohol.

How it works:

• Breath passes through the sensor

• Alcohol triggers an electrochemical reaction

• The device converts that reaction into a BrAC reading

Fuel-cell sensors are trusted because they are the same style used in many police breath testers.

3. Control Module (Head Unit)

This is the “brain” of the entire system, usually installed under the dashboard.

Its job:

• Stores all test results

• Records fails, missed retests, and tamper attempts

• Controls whether the vehicle can start

• Tracks date, time, and event logs

Everything the driver does is recorded automatically.

4. Camera Unit (Required in Many States)

Most states now require a camera to prevent someone else from blowing into the device.

What the camera records:

• A photo during every test

• Low-light images at night

• The driver’s mouth and device position

• Evidence of tampering or substitution

The camera works passively — the driver does nothing extra.

5. Wiring Harness & Ignition Connection

The wiring harness connects the IID to the vehicle.

What it does:

• Provides power to the device

• Communicates with the ignition system

• Blocks the starter during failed tests

• Maintains tamper-evident seals

Only a certified technician can install or remove these connections.

6. Data Storage & Reporting System

IIDs must report driver activity to monitoring agencies.

Data includes:

• Passed tests

• Failed tests

• Missed retests

• Photos

• Tamper warnings

• Lockout events

Depending on the model, data is sent via cellular network or downloaded at service appointments.

Why This Matters

Understanding the parts of an interlock device helps drivers:

• Avoid violations

• Know what the camera sees

• Understand how tests are logged

• Prepare for installation

• Comply with state requirements

The system is designed to be tamper-proof, accurate, and fair.