All four tires on your Accord look full in the driveway, and the tire pressure light is on anyway. On a 2008 to 2012 Accord, a battery-powered sensor sits inside each wheel, and that battery wears out with age. From 2013 on, the wheels hold no sensors, and the ABS wheel-speed sensors set the light instead. A low tire turns slightly faster than the other three, and that difference in speed is what the system flags.

The year decides whether the fix sits in a wheel sensor on 2008-2012 cars or in a calibration from 2013 on, with any leak found on either. At Driven Auto Care of San Ramon, every tire pressure visit starts with the model year, because the year separates a fix inside the wheel from a fix in the calibration. On the earlier generation, a sensor whose internal battery has worn out stops reporting, and we find that wheel and replace its sensor.

How Does Wheel Speed Show a Low Tire?

The newer design is called indirect TPMS, because it never measures air pressure directly. Each wheel already has a speed sensor that feeds the ABS brakes and counts how fast that wheel spins. The system is programmed to compare those four counts with each other as the car rolls. Once a tire loses air, it sags slightly and rolls on a smaller circle, so it turns faster to cover the same ground. The light comes on after a tire has lost a sizable share of its cold pressure, a drop the sidewall barely shows.

That softer tire turns more times over each mile, and the speed sensors register the difference. One new tire with a different overall diameter shifts the count in the same way, so we check sizes and tread at all four corners. That same comparison explains why topping up one tire leaves the light lit.

Why Adding Air Leaves the Light On

Filling the low tire at a gas station seems like it should put the light out on the way home. On the indirect system, the light stays on after air is added until the system is calibrated. Calibration is needed after air is added and after any rotation or tire replacement. Calibrating stores a fresh baseline of how fast each of your four tires turns at the right pressure, and every later count is measured against it.

Steady Light or Blinking Light

How the light behaves in the first moments after start-up separates two different problems. A steady light is a sign of low pressure in one or more tires. When the light blinks for a short while and then stays on, the cause is a fault inside the monitoring system, which has switched into a fail-safe mode. Adding air changes nothing in that case, so the work moves from the air hose to the scan tool. With the indirect system, a wheel-speed sensor fault can bring the ABS light on beside the tire pressure light. We read the fault codes and the live wheel-speed data to find the corner that stopped reporting.

Finding The Leak Before The Calibration

A slow leak in one tire keeps bringing the light back. Something as small as a nail in the tread or a worn valve stem lets pressure fall over days rather than hours. We find the leak on the tire that dropped and repair it, since refilling it leaves the leak in place.

Calibration records whatever pressures the four tires hold at that moment. We set all four tires cold to the pressure printed on the driver's door jamb before calibrating. Cold means the car has sat parked long enough that heat from the drive over has not raised the reading, so the number we set is the number printed on the jamb. If one tire is high and another low during calibration, the baseline is wrong from the start. On a 2013 or newer Accord, the light goes out once the leak is repaired and the system is calibrated on four tires set cold to the door-jamb pressure. Calibration completes over a stretch of steady driving, so we put the car through that stretch before it goes home.

Tell Us Whether The Light Blinked First

Bring your Accord to Driven Auto Care of San Ramon, and on the call mention whether the light blinked before it stayed on. When you pick up a 2013 or newer Accord, the leak has been found and repaired and the calibration drive is done, and on a 2008-2012 car you'll know which wheel sensor stopped reporting. Call our San Ramon shop at (925) 830-4701.