Quick answer
A light that comes on and stays on means at least one tire is low. Check all four with a gauge while they're cold, and set them to the pressure on the vehicle's tire information label, not the number on the sidewall. A light that flashes for about a minute at start-up and then stays on means the monitoring system itself isn't working and needs an inspection.
On this page
The tire pressure light is the horseshoe-shaped symbol, a tire in cross-section, with an exclamation mark inside. Michelin Canada says it comes on when one or more tires is a quarter or more below the recommended pressure, so by the time you see it, the tire is already well down. Start with what the light is doing, because each pattern points somewhere different.
What each light pattern means
| What the light does | What it usually means | What to do |
|---|---|---|
| Comes on and stays on | At least one tire is low (Bridgestone Canada) | Check all four with a gauge, set them to the label pressure, and find out why the tire lost air |
| Comes on in the cold, goes out after you've driven | A tire sitting just above the warning point. Overnight cold lowers its pressure; daytime warmth and driving raise it again | Check the tires cold and top up any that are low |
| Flashes for about 60 to 90 seconds at start-up, then stays on | The monitoring system isn't working. Bridgestone says it can't warn you about low pressure until it's repaired | Check the pressures with a gauge, then have the system inspected |
| Comes on after new tires, a rotation or a seasonal swap | Transport Canada says a malfunction warning comes on if tires are changed or rotated and the system isn't reset | Look up the reset in the owner's manual. If the light stays on, have the sensors and system checked |
Unless your dash shows each tire's pressure, the light won't say which tire is low or by how much. Put a gauge on all four and you'll know both, even when every tire looks normal.
Can I keep driving with the light on?
You can't tell from the light how far a tire has dropped. Transport Canada says some systems warn only once a tire is significantly under-inflated, and that when the system warns, you should measure the pressure as soon as possible. Too little air makes the sidewalls bend and flex too much, it adds, which can lead to sudden tire failure, and it advises against driving on a seriously under-inflated tire.
Why it comes on when the weather turns cold
Tires lose pressure as the air cools: about 7 kPa (1 psi) for every 5°C drop, according to Transport Canada. A tire set on a 15°C afternoon reads about 28 kPa (4 psi) lower at −5°C the next morning. That's enough to push a tire that was already a little low past the warning point, and Transport Canada's TPMS page says large swings in outdoor temperature can bring on the warning.
Michelin Canada describes what drivers then see: the light comes on on a cold morning and goes out after a drive, because the tires warm up and their pressure rises. The tire is still low when the light goes out, and the next frost can bring the light back. Bridgestone's advice covers both cases: check every tire with a gauge and add air to any that are low.
Tires also leak a little on their own. Michelin says they've been known to lose up to 1 psi (about 7 kPa) a month, so a set nobody has checked since summer may start the cold season already low.
How to check and set your tire pressure
Transport Canada recommends measuring at least once a month, spare included, and says a monitoring system doesn't replace that. Bridgestone adds a check before a long trip or when you're carrying extra load.
- Find the right number. It's on the vehicle's tire information label, usually on the driver's door edge or door post, sometimes in the glove box or on the fuel door, and in the owner's manual. The pressure moulded into the tire's sidewall is the most the tire can hold, not the pressure your vehicle needs (Transport Canada).
- Measure cold. Transport Canada counts tires as cold once the vehicle has been parked for at least three hours or driven less than 2 km. Michelin says never to let air out of a hot tire to match the label: the higher reading is normal and drops as the tire cools.
- Use your own gauge. Michelin calls gas station gauges often unreliable.
- Check all four, not just the one that looks low. Bridgestone notes a monitoring system may not catch tires that lose pressure at the same rate, and may be set to warn below the pressure the vehicle needs to carry its load.
- Reset it if your vehicle needs that. Transport Canada says newer systems let the driver reset them, and the owner's manual explains how for your vehicle. If the light stays on after the tires are set correctly, Michelin says to have the system inspected, since sensors can fail or need recalibration after a tire change.
Adding extra air so the light stays off creates a different problem. Transport Canada says an over-inflated tire rides on the centre of its tread, with a smaller contact patch and less grip.
If the light keeps coming back
A tire that needs air again within days or weeks is losing it somewhere. Besides the tire itself, Michelin lists the seal between the tire and the rim, the valve, the joint between the valve and the rim, and the wheel. A puncture is the other common cause. Michelin says minor damage in the centre of the tread is often repairable by a trained professional, while damage in the sidewall or shoulder, or to a tire that has been driven flat, can't be repaired. The Wheels & Tires counter handles flat and valve stem repairs.
Running low also shows in the tread. Bridgestone says an under-inflated tire wears on both shoulders, and an over-inflated one in the centre. Wear on only the inside or outside edge is a different story: Transport Canada lists misalignment among the causes of uneven wear, and suggests an alignment check when you see it. A low tire on one side can also make the car pull to one side.
Winter wheels and the TPMS light
If your winter tires are on their own wheels, the sensors matter as much as the tires. Transport Canada says the system may not work properly with winter tires unless the correct pressure sensors for your vehicle are installed, and that anyone buying new rims should make sure the sensors fit them. A swap or rotation without a reset also brings on the malfunction warning.
While that warning is on, Bridgestone points out, the system can't warn you about low pressure, so lean on your gauge until it's repaired.
If you're still deciding whether a second set of wheels is worth it, sensors belong in that decision. For the set coming off, see storing the off-season set.
When to call the tire counter
West Yale Wheels & Tires, at 44360 Yale Rd, Chilliwack, handles TPMS diagnosis and sensor replacement: it diagnoses the warning and replaces sensors when needed. Call the tire line at 604-391-1766 with:
- What the light does: stays on, flashes and then stays on, or comes and goes with the temperature
- When it started: after a cold night, after new tires, a swap or a rotation, or with no obvious trigger
- Your cold pressures in kPa, next to the figures on the tire information label
- Whether one tire keeps needing air
