Component Tests and General Diagnostics

Tire Pressure Monitoring System

Component Test

Tire Pressure Monitoring System (TPMS) Intermittent Conditions

Radio Frequency Interference (RFI) can create intermittent Tire Pressure Monitoring System (TPMS) system issues that are difficult to pinpoint. RFI can be caused by the addition of aftermarket equipment that blocks the TPMS sensor(s) transmissions from getting to the TPMS/Vehicle Security Module (VSM) located behind the glove compartment. Devices causing RFI could block the module from communicating with any of the 4 sensors, and others may intermittently block just the rear sensors due to their distance from the module. The purpose of this Component Test is to provide information and techniques to help diagnose intermittent issues.

TPMS Last Warning Event PID Definitions

The TPMS/VSM uses the TPMS last warning event PIDs to store detailed information about the last 5 times the TPMS indicator was activated. These PIDs can be used to acquire more information about a particular TPMS event, but must be used carefully.

EVT1_IGN thru EVT5_IGN

The number of key cycles since the TPMS indicator was activated. This PID will cycle from zero to 255 and then start over from zero again. Default is FF, this can be used to determine how long ago a TPMS event occurred and the time (in key cycles) between events.

EVT1_TLOC thru EVT5_TLOC

This is the last programmed location for the sensor identifier causing each TPMS event. Due to tire rotation, the sensor may no longer be at the original location. It is suggested that all the PIDs be recorded, the system retrained, and then the sensor identifier PIDs be used to pinpoint the actual location of each sensor.

EVT1_PSI thru EVT5_PSI

This is the air pressure associated with each TPMS indicator event. This can be used along with the function code to clearly identify the TPMS events that were strictly due to low pressure. It can also be used to determine when a sensor is transmitting inaccurate air pressure.

EVT1_STAT thru EVT5_STAT

Describes the warning status of each TPMS event by using the information received from the TPMS status (TP_STAT) PID. If there is a communication issue, the status could be Normal.

- Unknown
- Normal (normal operation)
- Low (low pressure event)
- Fault (sensor fault or system fault)

EVT1_TxID thru EVT5_TxID

This is the identifier of the sensor involved in each TPMS event. EVT1 is the most recent event that triggered the TPMS indicator. Default is FF FF FF FF.

Wheel Rotation and Sensor Training Techniques

Moving a Problem Sensor/Wheel to a Different Position

If a sensor in a certain location has caused several events, yet the sensor will train and seems to operate normally, moving that particular wheel to a different location on the vehicle is a good way to isolate the issue to a certain sensor/wheel location. The wheels should be rotated followed by a vehicle road test. This can be done in an attempt to replicate the issue. This will determine if the issue followed the sensor or remained in the original sensor location.

Training Sensors in a Different Order

This is a technique to get past a left front sensor that may not be responding to determine if the remaining sensors will train to the module. This can help save time determining if other sensors are having issues or if the module is experiencing training difficulties with a certain location.

NOTE: Training known good sensors from another vehicle cannot differentiate between a faulted module and RFI, as some noise source could be preventing the module from hearing the original sensors as well as the known good sensors.

Training Known Good Sensors From Another Vehicle

This is a technique that can be used to differentiate between a sensor and module issue. If the module in the vehicle cannot train any of its own sensors, and likewise cannot train known good sensors from another vehicle, then the issue is with the module or the RFI, and not with the original sensors. The original sensors should not be replaced.

Items That Cause Radio Frequency Interference (RFI)

Non-OEM Equipment

The following equipment has been found to sometimes cause RFI:

- Video equipment has been found to cause RFI especially when the video and power supply lines are near the TPMS/VSM.
- Car alarms (even those installed by dealerships) have been found to create enough RFI to cause the TPMS systems to malfunction or lose considerable range. These car alarms can sometimes be tricky to locate, as they are usually hidden somewhere out of the way for reduced accessibility. This sometimes means they are inside or behind the instrument panel near the TPMS/VSM.
- Many different in-vehicle cell phone chargers have been found to cause considerable RFI. The vehicles with the power point closest to the TPMS/VSM are the most affected. It must be noted that most cell phone chargers do not produce high levels of RFI all the time. This depends on the state of charge of the cell phone battery. The phone must be almost completely discharged in some cases.
- Power supplies and DC/AC inverters typically create a lot of RFI. Most consumer grade equipment has very little filtering or shielding.

OEM Modules

In some cases, the RFI may actually be caused by a module or ground on the vehicle. Depending on the severity of the issue, a dirty ground, improperly built ground shield or module can disable the system. Modules that have microcontrollers use clock circuits to create the timing pulses for the microprocessor and these may also radiate RFI.

Using Customer's Electronics to Pinpoint the Radio Frequency Interference (RFI) Source

This can be a way to determine the cause of an issue well before the sensors and module are replaced with little or no affect on the system performance. Since this takes more upfront work, it relies on working with the customer to determine what equipment was being used at the time of the event.

Options for Eliminating Intermittent TPMS Conditions Caused by Radio Frequency Interference (RFI)

- If an OEM component or customer device is causing an RFI issue, the device can be replaced.
- If a phone charger is causing an RFI issue, the customer should consult with their cell phone provider to acquire a different phone charger.
- If a device such as an dealer-installed alarm is causing an RFI issue, the device could be moved to another location on the vehicle. In the case of a portable device, the power cord could be moved to another power point location.

In summary, if the RFI source is present and cannot be moved or replaced, the intermittent issue will remain. The TPMS system must accept RFI and the unwanted system operation it can cause.