How does an ignition interlock work, in practice

How does an ignition interlock work, and where does it sit?

Two parts. A handset on a coiled cable, usually clipped near the centre console, which is what you blow into. And a control module wired into the starter circuit out of sight behind the dash. The handset takes the sample; the module decides whether the engine is allowed to crank.

Only Ontario’s approved providers — Smart Start Canada and ALCOLOCK Canada — may lease and install one. No driving school installs devices, ours included.

Starting the car: the initial breath sample

Ontario’s published rule is exact: “If the device detects a blood alcohol concentration (BAC) over the preset limit of 0.02 the vehicle won’t start. If you are under the preset limit, the vehicle will start and you will be asked to provide breath samples at random times while the vehicle is running.”

Getting under 0.02 is rarely the difficulty. The technique is. The unit is not just measuring alcohol, it is checking that the sample came from deep lung air, and it verifies that by requiring a specific pattern: a long, even exhale with a hum or note behind it. Blow too softly, too briefly, or in bursts, and it rejects the sample without ever reaching a reading.

Most people fail twice before the first sample registers, purely on delivery. That is normal, it is not recorded as a fail, and it is the single most useful thing to have practised before an examiner is watching you do it.

Rolling retests: breath samples while you are driving

MTO defines a rolling retest as when “the device will prompt drivers to pull over and provide a breath sample while the vehicle is running to verify the driver has not consumed any alcohol after initially starting the vehicle”. It arrives without warning, at random, and the first one is genuinely alarming.

Here is what happens if you miss it or fail it, in Ontario’s own terms. The device will issue a warning, record the event, and activate specific alarm systems (for example, lights flashing, horn honking) until you turn off the ignition.

The practical response: signal, pull over safely, blow, carry on. Treat it as a phone call you have to take.

Lockouts and what triggers them

A lockout is the device refusing to allow a start. The usual causes are a reading over 0.02, a run of rejected samples, a missed service appointment, or a tamper condition detected by the unit. Some lockouts clear on their own after a wait; others need an unlock code from your provider, which is chargeable — Smart Start listed $84.75 as at 27 July 2026.

Cold weather is its own category. At −20 °C the handset wants warming before it will read reliably, and the sensible routine is to keep it inside the car overnight rather than in a cold glovebox. This is Ottawa, so plan for it.

The 60-day service appointment

Verbatim from the Reduced Suspension programme page: “All activity with the ignition interlock device is recorded when it is used. Every 60 days, you must return to the ignition interlock service provider to download data and calibrate the device.”

Two reasons not to miss it. The calibration keeps the sensor honest, and a missed appointment is a programme violation — the same category as tampering, carrying a six-month extension for a first offence, eighteen months for a second and three years after that. It is the cheapest mistake on this page to avoid and one of the most expensive to make.

Everything the device records, and who reads it

Every start attempt, every reading, every rejected sample, every rolling retest and every missed one. The log goes to your provider at each 60-day download, and from there into the monitoring record MTO relies on.

Two things get extended off the back of that log, and they are not the same thing.

• Performance failures — blowing over 0.02, or missing a rolling retest. Only counted during your monitoring window: the last three months for a first-time convicted offender, the last six for subsequent convictions.
• Programme violations — tampering, a tampering conviction, a conviction for driving without the device, or a missed appointment. Counted for the entire duration.

The full extension table sits with the rest of the ignition interlock program in Ontario.

False positives: mouthwash, medication and yeast

Approved devices use alcohol-specific fuel-cell sensors. That is better than the cheap semiconductor sensors in novelty breathalysers, which react to all sorts of things — but it means the device detects ethanol from any source, and it cannot tell where the ethanol came from.

Mouthwash, some medications and yeast-containing foods can leave mouth alcohol that reads on the unit. The remedy is unglamorous and it works: rinse with water and wait ten to fifteen minutes before blowing. Mouth alcohol dissipates; alcohol in your blood does not.

Can someone else blow for you?

Physically, on some units, at some moments. Practically, it is one of the worst decisions available to you.

It is tampering or circumvention depending on how it is characterised, and either way it is a programme violation with a six-month extension minimum, rising to eighteen months and then three years. A conviction for tampering is separate again. And the rolling retest exists precisely to defeat it: the prompt comes at random while you are driving, when the person who blew for you at the kerb is no longer in the car.

Living with it: borrowed cars and work vehicles

An “I” condition means you may drive only a vehicle with an approved device fitted. There is no exemption for a friend’s car, a courtesy car, a rental, or a work van. Fines run $200 to $1,000 for a passenger vehicle and up to $20,000 for a commercial one, with impoundment of 45, 90 or 180 days depending on how many times it has happened.

The clause that catches families: a vehicle owner who knowingly lets a person with an “I” condition drive their vehicle can also be charged. If your employer runs a fleet, that conversation is better had early than after a roadside stop.

Learning to use one before it is installed

You can. The device in our training vehicle is ours, on our own service schedule, and nothing you do during a lesson touches the monitoring record of the unit leased to you. That is the whole point of practising on someone else’s: the mistakes cost nothing.

Ten hours covers the blow until it is reliable, several deliberate rolling retests until the alarm is boring, cold starts, and the road-test manoeuvres alongside. The ignition interlock device training in Ontario is set out hour by hour.