Ignition interlock — Missouri Permit Test
Ignition interlock on the Missouri permit test — 5 rules, each with the reason behind it and a question on it while it is fresh.
The rules
An ignition interlock device connects to the vehicle's ignition, horn, and headlights, and measures the concentration of alcohol in a person's breath.
It is wired to more than the starter, so a driver cannot simply ignore it once the engine is running.
An ignition interlock device (IID) is a mechanical unit that is connected to the vehicle’s ignition, horn, and headlights, and measures the concentration of alcohol in a person’s breath.
The interlock requires a breath sample before the vehicle is started and periodically while it is being driven.
The second sample is what stops somebody else blowing into it at the start of the trip.
The IID requires vehicle operators to provide a breath sample before starting the vehicle and periodically during the operation of the vehicle.
The interlock must be maintained for at least six months from the reinstatement date, with no violations in the last three months.
The last three months are a clean stretch. A violation inside them restarts that part of the clock.
The IID must be maintained for a minimum of six months from your reinstatement date without any violations in the last three months.
You must pay for having the interlock installed and serviced every month, and failing to maintain it re-suspends your driving privilege.
The cost is yours and the obligation is continuous. Letting the service lapse is treated as a failure.
You must pay for having the device installed and for having it serviced every month.
If you are convicted of failing to have an ignition interlock installed as required, your driving privilege is revoked for one year, and a second offense takes it for five years.
Driving without the device is treated as seriously as the offense that required it.
A second-offense requires your driving privilege to be taken away for five years.
On the test
What does an ignition interlock device connect to?
The ignition, horn, and headlights. It is wired to more than the starter, so a driver cannot simply ignore it once the engine is running. Missouri Driver Guide, p.70: “An ignition interlock device (IID) is a mechanical unit that is connected to the vehicle’s ignition, horn, and headlights, and measures the concentration of alcohol in a person’s breath.”
How this works
Most practice tests hand you a bank of questions and leave the manual somewhere else. This one puts them together.
You read one law and the reason behind it, then answer a question on that law while it is still fresh. Get it right and you move on; get it wrong and you have found the gap while it is cheap to fix, not on the morning of the exam.
Learning one rule at a time, and proving it immediately, is what turns a manual you skimmed into something you actually know — and it makes studying feel like getting somewhere rather than like waiting to find out.
What it costs
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Where these come from
Every rule here is checked against Missouri Driver Guide, published by the Missouri Department of Revenue, and against the state code itself. Last checked 2026-09-16.
- src/assets/drivers-manuals/MO.pdf (Missouri Driver Guide, Revised August 2026, retrieved 2026-09-14)
- src/assets/drivers-manuals/MO.pages.txt (plain pdftotext over that PDF; sha256 pinned in SOURCES.json)
- src/assets/drivers-manuals/SOURCES.json (sha256, folio offset 2, folioMarker, nonLawPages and the extraction flag, each with its measured basis)