Direct tire pressure monitoring systems

There are two main types of direct tire pressure monitoring system currently in use. These are known as 'high line' and 'low line'.
High line system

If the vehicle is fitted with low frequency (LF) transmitters near each wheel, the vehicle may use these to force the sensors to transmit. In this case, the TPM may not transmit on its own, but the vehicle will periodically command the sensors to send their information.

In addition, the TPMs will be forced to transmit when the ignition is switched on. This will give an early indication of low pressure without having to have the vehicle's receiver switched on when the vehicle is not in use. The transmitters are usually activated one at a time in sequence so that the vehicle can inform the driver of the location of the wheel with low pressure. This information can then be used later for localisation by matching the TPM's unique ID with its position discovered by this sequential activation. This method is used on some high line systems where the TPM also transmits periodically.

On some vehicles only three LF transmitters are used in order to save money. The vehicle assumes that transmissions from a nearby TPM which has not been woken up by the LF belong to the TPM located where there is no LF transmitter.

High line systems are inherently more expensive than low line systems but they have the advantage of the vehicle knowing the pressure when started without draining the vehicle's main battery and providing localisation. These systems tend to be used on higher end models.
Low line system

In this system, the TPM units transmit on their own at fixed or random intervals. As the individual TPMs on the vehicle do not know if another TPM is transmitting at the same time, it is possible to have collisions between messages transmitted. Measures have to be taken to ensure that the message is received by the vehicle. On some systems the message is re-transmitted multiple times to reduce the effect of interference (communication). The transmission pattern can be random or pseudo random to reduce the chance of collisions between transmissions from the sensors on the vehicle.

Another method of attempting to avoid collisions is simply to transmit more frequently such as once per minute. In addition, if the TPM detects a rapid change in pressure or too high a temperature, it will start to transmit more frequently so that the vehicle has more chance of receiving the information. The low line system is used on the majority of vehicles due to its lower cost.