Alcohol Interlocks In Cars
What alcohol interlocks are, when they are required, how they work and the consequences of misuse.
Alcohol interlocks stop the vehicle from starting when alcohol is detected in the driver’s breath sample. They are used as a safety measure in commercial transport and as a condition after sanctions for drink-driving, supporting the legal duty to drive sober in Norway.
How alcohol interlocks work
An interlock combines a breath handset and a control unit wired to the starter circuit. If the measured value exceeds the set threshold, the engine remains locked and the event is stored for follow-up.
| Component | Function | Notes |
|---|---|---|
| Handset | Guides the driver through the breath test | Displays status and fault codes |
| Measuring unit | Analyses alcohol level | Needs regular calibration |
| Control unit | Allows or blocks start | Records every attempt |
| Relay harness | Interrupts the starter circuit | Must be installed professionally |
Thresholds and monitoring
| Parameter | Typical value | Action when exceeded |
|---|---|---|
| Start limit | 0.10 mg/l (≈0.2‰) | Engine blocked |
| Retest | 5–30 minutes | Requires a rolling retest |
| Data retention | 6–24 months | Authorities may request the log |
For the legal basis, see Vehicle regulations .
When interlocks may be mandatory
| Situation | Example | Legal reference |
|---|---|---|
| Commercial transport | School transport contracts | Requirement in tender specifications |
| Sanctions | Alternative to a longer disqualification | Decided by authorities |
| Corporate policy | Internal HSE programme | Agreed within the company |
Many municipalities demand interlocks in Traffic control and flow to protect vulnerable passengers. Drivers with a drink-driving history can receive interlock conditions to regain their licence, see Disqualification, probation and penalty points .
Daily use
- Perform the breath test before start; the display confirms ready to drive
- Complete rolling retests safely when prompted while driving
- An interrupted or manipulated test blocks the next start and creates a log entry
- Keep the handset clean and warm in winter to secure accurate readings
For required routines, see Duty to remain sober .
Installation and approval
| Action | Responsible | Key requirement |
|---|---|---|
| Installation | Approved workshop | Follow the manufacturer’s instructions |
| Calibration | Certified technician | Normally every 6–12 months |
| Log review | Employer or Norwegian Public Roads Administration | Detects misuse or manipulation |
| Documentation | Driver/owner | Must be available during inspections |
Tampering with an interlock can trigger further penalties, as outlined in Sanctions: fines, confiscation, pledges and points .
Consequences of misuse
| Breach | Possible reaction | Outcome |
|---|---|---|
| Attempt to start with alcohol present | Log warning, start denied | Follow-up by employer or authorities |
| Tampering | Contract sanctions, licence issues | May extend disqualification |
| Missed calibration | Warning, usage ban | Service required before the next trip |
Regular follow-up lowers the risk of serious incidents; see Alcohol, BAC limit and drugs .
Safety benefits
- Prevents impaired driving within high-risk groups
- Strengthens corporate HSE work and documentation
- Builds trust among guardians and clients in passenger transport
- Provides clear sanction options for repeated breaches