Radio Frequency Interference (RFI)

RFI can be caused by:

• Video equipment has been found to cause RFI especially when the video and power supply lines are near the TPMS.
• Car alarms (even those installed by the dealership) have been found to create enough RFI to cause the TPMS to malfunction or lose considerable range. These car alarms can be difficult to locate, as they are usually hidden somewhere out of the way for reduced accessibility.
• Many in-vehicle cell phone chargers have been found to cause considerable RFI. The vehicles with the power point closest to the BCM 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 battery 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.

Using Hit Rate Parameter Identifiers (PIDs) to Determine the Presence of RFI 

If an intermittent RFI issue is suspected, the information contained in the last 5 TPMS warning event Parameter Identifiers (PIDs) can be combined with specific Parameter Identifiers (PIDs) from the BCM to determine which TPMS sensors are being affected and if a RFI source is currently present in the vehicle.

The BCM module contains Parameter Identifiers (PIDs) tracking the number of TPMS messages received from the 4 trained TPMS sensors. These Parameter Identifiers (PIDs) can provide insight on the performance of the TPMS and can help establish the presence of a RFI source.

• TPM_HITS_LF (Tire Pressure Monitor Hit Rate Left Front) - The number of TPMS messages received by the BCM module from the LH front sensor
• TPM_HITS_RF (Tire Pressure Monitor Hit Rate Right Front) - The number of TPMS messages received by the BCM module from the RH front sensor.
• TPM_HITS_LRO (Tire Pressure Monitor Hit Rate Left Rear Outer) - The number of TPMS messages received by the BCM module from the LH rear sensor
• TPM_HITS_RRO (Tire Pressure Monitor Hit Rate Right Rear Outer) - The number of TPMS messages received by the BCM module from the RH rear sensor.

Method for determining if a RFI issue has been affecting the TPMS:

1. Collect the last 5 TPMS events and determine if they were due to system faults or low tire air pressure.
2. Collect the TPMS Hit Rate PID counters and compare them to the last 5 TPMS events.
3. If the TPMS Hit Rate PID counters are significantly different from each other or if the locations with lower hit rate values show up as fault locations in last 5 TPMS events and BCM DTC B124D:02 (Tire Pressure Sensor: General Signal Failure) is present, an intermittent RFI source is most likely preventing the signals from these TPMS sensors from reaching the BCM.
4. An intermittent RFI source can also be the case when all 4 TPMS sensors show up in the last 5 fault events and BCM DTC B1182:00 (Tire Pressure Monitoring System (TPMS): No Sub Type Information) is present. The possible cause would a strong source of RFI noise.

Original Equipment Manufacturer (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 concern, a dirty ground, improperly built ground shield or module can disable the system. Modules that have microcontrollers using clock circuits to create timing pulses for the microprocessor may radiate RFI.

Using Customer Electronics to Pinpoint Radio Frequency Interference 

This method can be a way to determine the cause of a concern well before the sensors and module are replaced with little or no effect on system performance. Since this takes more up front work, it relies on working with the customer to determine what equipment was being used at the time of the event. Question the customer about what kind of devices they are using. Determine which power points are being used and if necessary ask that the devices be activated to determine their affect on the TPMS.

Options for Eliminating Intermittent TPMS Operation Caused by RFI 

• If an Original Equipment Manufacturer (OEM) component or customer device is causing an RFI concern, replace the device.
• If a phone charger is causing an RFI concern, the customer should consult with their cell phone provider to acquire a different battery charger.
• If a device such as a dealer installed alarm is causing an RFI concern, move the device to another location in the vehicle. In the case of a portable device move the power cord to another power point location.

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