The device in an Ottawa winter

Why the cold matters at all

An approved device uses an alcohol-specific fuel-cell sensor, and a fuel cell is a chemical instrument. Chemistry slows down when it is cold. At −20 °C the handset can take noticeably longer to become ready, and a sample delivered before it is ready is more likely to be rejected.

That is not a fault and it is not a reading. It is the unit refusing to guess.

The one habit that solves most of it

Keep the handset inside the cabin overnight, not in the glovebox. A glovebox on the outside wall of the car gets colder than the cabin, and a handset left there is the single most common cause of a slow January start.

Some drivers bring the handset indoors overnight where the lease allows it. Ask your provider first — disconnecting it may register as a tamper event, and tampering is a programme violation with a six-month extension.

Budget the extra ten minutes

In summer the routine is thirty seconds. In February, allow for the unit to warm, one rejected sample, and a second clean one. Ten minutes covers it comfortably. The drivers who have problems are the ones who allowed no margin and then rushed the blow.

Rolling retests in winter

A rolling retest prompt does not care about the weather. You still pull over safely and blow. The one seasonal difference is that on a cold morning the cabin may still be warming, so give the sample a fraction more time and a fraction more air.

And the thing worth repeating: missing or failing one triggers an alarm — lights and horn until the ignition is switched off. The engine does not cut out. How the ignition interlock rolling retest behaves is covered in full in the device guide.

Mouthwash, coffee and a cold start

The winter version of the false-positive problem is the hot drink on the way to the car. If you have used mouthwash, taken certain medications, or eaten something yeast-containing, rinse with water and wait ten to fifteen minutes. Mouth alcohol dissipates. A reading over 0.02 is a performance failure regardless of cause, so the waiting is the whole strategy.

Practise it cold, deliberately

Most people learn the blow in a warm car in daylight and then meet their first −20 °C morning alone. Part of the ten-hour ignition interlock training course is doing it in the conditions you will actually face, because this is Ottawa and half the year is winter.