Tires - Tire Pressure Monitoring System

SOURCE: Subaru Tech Tips

TITLE: Tire Pressure Monitoring System

APPLIES TO: All models

SERVICE TIP:

If you receive customer concerns that their Tire Pressure Monitoring System (TPMS) warning light came on when the vehicle was first started after sitting overnight and the light went out after driving a few miles, there may NOT be anything wrong with the system. With the approach of colder weather in many parts of the country the air pressure in the tire may have been below the pressure threshold of the system. This threshold is 26.20 p.s.i.

For example purposes only, let's say that at 70°F the front tires are inflated to 32 p.s.i. and the rear tires are inflated to 30 p.s.i. Overnight, the ambient temperature dropped down to 45°F. This would decrease the pressure in the tires by 2.5 lbs. 70 - 45 = 25 degree change in temperature. Tire pressure will drop 1 p.s.i. for every 10 degree change in temperature. 25 / 10 = 2.5 p.s.i. The rear tire pressure of 30 p.s.i. minus a 2.5 p.s.i. drop in pressure due to the ambient temperature equals 27.5 p.s.i. Take into account any other factors such as the tire(s) being wet with rain, mud, snow, or other substance, that reading could drop more. Also figure in the differences in tire pressure measuring gauges when the tires were inflated and you have the possible reason for the TPMS light coming on when the customers started driving their cars and the light going out after driving a few miles. The tires have now warmed up and the warming of the tires will cause a pressure increase in the tire and now satisfy the pressure threshold of the TPMS system. Hence, the light goes out. So before diving into the system looking for a problem that may not be there, you may want to question the owner as to the weather conditions in their area.

NOTE: The owners may want to adjust their tire pressures to compensate for the colder temperatures in their area as the seasons change. At no time should the tires be over inflated to compensate for this concern. Always observe the information in the vehicles Owner's Manual when it comes to properly inflating the tires on customers' vehicles.