Tire Pressure Monitoring System

What can go wrong

Mark Lemay

Tire pressure monitoring systems are now standard equipment on most vehicles sold in Canada. Problems with the TPMS system can cause not only illumination of the low tire pressure warning light but may also result in reduced performance on some vehicles.

There are 2 different types of tire pressure monitoring systems currently in use today: Direct and Indirect.

Indirect TPMS

On this type of system, the ABS wheel speed sensors are used to detect the rotational speed of the tire. As a tire looses air, the circumference of the tire will decrease causing the tire to speed up. This increase in speed is interpreted as a low tire and the customer is warned via the warning light. This system is an add on to the vehicles ABS and is subject to normal ABS types of failures such as bad wheel speed sensors and so forth. Usually there is a manual reset button if the system requires a reset. (Fig 1)

Direct TPMS

It consists of a sensor mounted in the tire plus the monitoring hardware. This system measures the pressure of the tire and on newer models the temperature of the air inside the tire and then relays this information via radio waves to a receiver located inside the vehicle. These systems require special tools in order to diagnose and reset the warning lights in the event of a failure.

Because the sensor can be located on the valve stem or banded to the center of the rim, care must be taken when breaking down tire beads not to damage the sensor. Care must also be taken when inflating the tire to get the correct tire pressure. The tire inflation information from the vehicle tire sticker must be used as opposed to the tire manufacturers recommended pressure.

System failures

They fall into two general categories. First the sensor is inoperative or inaccurate and secondly the receiver in the vehicle can not receive or interpret the sensor properly. To properly diagnose these systems you will need a TPMS tool that can read and display the sensor information.

Start testing by scanning the sensor. This is done by holding the TPMS tool on the sensor until the tool scans the sensor. If the sensor will not scan then the sensor is bad. If the sensor scans OK, check the sensor data to see if the data is correct. A typical scan is shown in fig 2.  Check that both tire pressure and temperature correspond with the actual tire pressure and temperature and check that the sensor battery is in good condition.

Second step in diagnosis

Once you have verified that the sensor is good, read the vehicle tire pressure data with a scan tool or use the vehicle display and compare the reading to the sensor readings. (fig 3) If the readings are different then the receiver is at fault.

Finally, when you replace a component, you will have to perform a relearn procedure on the vehicle. Check the vehicle service information for the correct procedure for the vehicle you are working on.