Radio Frequency Interference (RFI)

WARNING: This page is about a different car, the 2016 Ford Expedition. However, it is still accessible from the selected car via links, so may be relevant.

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.
Anti-theft 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 anti-theft 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. Vehicles with the power point closest to the TPM module are the most affected. Most cell phone chargers do not produce high levels of RFI all the time, it depends on the state of charge of the cell phone battery. The phone battery must be almost completely discharged in some cases for the charger to produce enough RFI to interfere with TPMS operation.
Power supplies and DC -to-AC inverters typically create RFI. Most consumer grade equipment has very little filtering or shielding.

Using Hit Rate PIDs to Determine the Presence of RFI

If an intermittent RFI issue is suspected, the information contained in the last 5 TPMS warning event PIDs can be combined with specific 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 contains PIDs which keep track of the number of TPMS messages received from the 4 trained TPMS sensors. These 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 LF sensor.
TPM_HITS_RF (Tire Pressure Monitor Hit Rate Right Front) - The number of TPMS messages received by the BCM module from the RF sensor.
TPM_HITS_LRO (Tire Pressure Monitor Hit Rate Left Rear Outer) - The number of TPMS messages received by the BCM module from the LR sensor.
TPM_HITS_RRO (Tire Pressure Monitor Hit Rate Right Rear Outer) - The number of TPMS messages received by the BCM module from the RR sensor.

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

Collect the last 5 TPMS events and determine if they were due to system faults or low tire air pressure.
Collect the TPMS Hit Rate PID counters and compare them to the last 5 TPMS events.
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 TPM module.
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 is most likely a strong source of RFI noise.

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 which have microcontrollers using clock circuits to create timing pulses for the microprocessor may radiate RFI.

Using Customer Electronics to Pinpoint RFI

This method can be a way to determine the cause of a concern before the sensors and module are replaced with little or no effect on system performance. Discuss with the customer what types of devices they were using when the event occurred. Determine which power points are being used and, if necessary, ask for the devices be activated to determine their affect on the TPMS.

Options for Eliminating Intermittent TPMS Operation Caused by RFI

If an OEM component or customer device is causing a RFI concern, replace the device.
If a cell phone battery charger is causing a 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 anti-theft alarm is causing a 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 caused by the RFI.