Kilopascals to PSI

The Sensor Sends Kilopascals, the Dash Shows PSI

A direct tyre-pressure monitoring sensor is a small transducer sitting on the valve stem, and almost every one of them encodes its reading in kilopascals before broadcasting it at 315 or 433 MHz. The body module then converts that to whatever the cluster is configured to display. So the value on a scan tool's live-data list, the value on the door placard and the value on the dash can all describe the same tyre in two different unit systems at once.

Conversion factor: 1 kPa = 0.145037738 psi. A placard reading 240 kPa cold is 34.81 psi, which is why the cluster shows 35 psi and why an inflator set to "35" leaves the sensor a couple of kilopascals above the placard figure.

What Governs the Warning Lamp

Direct systems measure, indirect ones infer

A direct sensor reports an actual pressure and usually a tyre temperature. An indirect system watches wheel speeds through the ABS rings and never produces a pressure number at all.

The telltale follows a percentage rule

Regulation puts the low-pressure lamp around 25 % below the placard cold pressure, with a floor value written into the rule, so the trigger travels with the vehicle instead of sitting at one fixed psi.

Displays round, sensors quantise

Many sensors report pressure in steps of a few kilopascals, so a psi readout can only move in jumps of roughly a third of a psi even before the cluster rounds it to a whole number.

Wheel work needs a relearn

The module maps a sensor ID to a corner. Rotate the wheels or fit new sensors and the pressures stay correct while the positions do not, until the relearn procedure is completed.

Checking a Threshold Before You Reach for the Inflator

Diagnosing a monitoring complaint usually means lining up three figures — placard, live sensor value and shop gauge — that rarely arrive in the same unit.

Put the placard value in as kilopascals

Door-jamb placards in most markets print kPa first: 220, 240, 250. Enter it and the psi equivalent appears while you type, so you know what the cluster ought to be showing.

Enter three quarters of it to find the trip point

Take the placard × 0.75 and convert that instead — 240 kPa becomes 180 kPa, which is 26.11 psi. Anything a customer reports below that explains the lamp with no fault anywhere in the system.

Flip the direction for your own gauge reading

The swap button (↔) turns the page into psi → kPa, so a stick gauge or a digital inflator can be laid straight against the sensor's live value. The hand version is a multiply by 6.894757.

Copy the plain figure onto the work order

Copying puts the number alone on the clipboard — no unit, no spaces — which is what a job-card field or a before-and-after pressure log expects. Ctrl + C inside a field behaves the same way.

Compare like with like: placard values are cold pressures. A sensor that has been rolling for twenty minutes is describing a warm tyre, so a live-data figure sitting above the placard is not automatically over-inflation.

Placard, Trip Point and the Margin Between Them

Common placard cold pressures, the point 25 % below each one where the low-pressure telltale is expected to illuminate, and how much pressure a tyre can quietly shed before that happens.

Placard cold pressure (kPa)	Placard (psi)	Trip point, 25 % below (kPa)	Trip point (psi)	Margin lost first (psi)
200 kPa	29.01 psi	150 kPa	21.76 psi	7.25 psi
220 kPa	31.91 psi	165 kPa	23.93 psi	7.98 psi
230 kPa	33.36 psi	172.5 kPa	25.02 psi	8.34 psi
240 kPa	34.81 psi	180 kPa	26.11 psi	8.70 psi
250 kPa	36.26 psi	187.5 kPa	27.19 psi	9.06 psi
260 kPa	37.71 psi	195 kPa	28.28 psi	9.43 psi
280 kPa	40.61 psi	210 kPa	30.46 psi	10.15 psi
300 kPa	43.51 psi	225 kPa	32.63 psi	10.88 psi

The last column is the part drivers never expect. On a 240 kPa vehicle nearly nine psi can vanish before anything lights up, and every kilopascal of that is lost fuel economy and extra shoulder wear. The monitor is a backstop against a puncture, not a replacement for a gauge.

Useful Details While Chasing a Monitoring Fault

Either box accepts the reading

Put the placard on one side or the live sensor value on the other; the opposite field follows immediately, so all four corners can be stepped through without resetting the page.

Turn it round for the shop gauge

One press of the swap control puts psi on the left, the direction you want when a digital inflator reads psi and the scan tool insists on kilopascals.

Bar and kg/cm² sit in the same menu

Searchable dropdowns hold all 26 pressure units on both sides, so an imported placard printed in bar or kg/cm² is handled on this page rather than in another tab.

Figures clean enough to paste

Values carry up to eight decimals with a space between thousands, and copying strips everything except the digits so the repair record stays tidy.

TPMS Warning-Light Questions

At what pressure does the low-pressure telltale actually illuminate?

The rule most manufacturers build to is 25 % below the placard cold pressure, with a minimum floor written into the regulation so a very low placard value still gives a sensible trigger. On a 240 kPa placard that works out at 180 kPa, or 26.11 psi. Individual vehicles may warn a little earlier, and many add a separate fast-deflation alert that reacts to the rate of loss rather than the absolute value.

What is the practical difference between direct and indirect monitoring?

A direct system carries a battery-powered sensor inside each wheel reporting a real pressure — that is where the kilopascal value comes from, and it is what makes a per-corner display possible. An indirect system compares wheel rotation speeds, since a soft tyre has a smaller rolling radius and turns slightly faster. It needs no extra hardware, but it can only say that one wheel is out of step, it drifts when all four fall together, and it must be reset by the driver after every inflation.

Why does a rotation need a relearn even though nothing was deflated?

The module stores a sensor ID against each wheel position. Move the wheels and the pressures are still transmitted correctly, but the display attributes the front-left reading to the rear-right corner. Some vehicles relearn on their own after a period of driving, some need a magnet or a trigger tool at each wheel in a defined order, and some are done through a scan tool. Until it is finished, hunting the "low" corner sends you to the wrong tyre.

Why does the cluster show whole psi when the sensor works in kilopascals?

Two roundings stack up. The sensor transmits pressure in quantised counts — often a few kilopascals each, already about a third of a psi — and the cluster then rounds that to a whole or half psi for display. A tyre genuinely at 34.81 psi and one at 35.2 psi can therefore show the same figure, so a one-psi disagreement between the dash and your gauge is normal rather than proof of a failing sensor.

Why does the warning appear on a frosty morning and clear itself by midday?

Gauge pressure tracks absolute temperature, so a tyre sheds roughly 7 kPa — about 1 psi — for every 5.5 °C (10 °F) the air cools. A car parked at the placard on a mild evening can start a cold morning a good 15 kPa lower, and if it was already marginal that is enough to cross the trip point. Driving warms the tyre and the lamp goes out again. The fix is to reinflate to the placard while the tyres are still cold, not to wait for the light to clear on its own.