TPMS and keyless entry can be difficult to understand because these systems rely on transmitting, receiving and decoding radio waves, but these signals are invisible. You can see wires connecting modules and components and observe them with a meter or scope but seeing radio signals takes intelligent tools.
Radio waves are a type of electromagnetic radiation that have artificially organized patterns of waves that transmit information. Still confused? Think of a TPMS sensor; it transmits a very low power signal that goes in all directions. The wave is absorbed and reflected off the vehicle and environment. The antenna on the vehicle — which could be on the windshield, in the wheel well or other locations — receives part of the wave.
TPMS sensor signals are low power and low frequency with the majority of sensors transmitting at 315 MHz or 413 MHz. Keyless entry fobs also operate at this frequency. The transmitters in TPMS sensors are low signal devices subject to FCC Part 15 and are Class C devices. The signals are unencrypted, but the signal is so minimal that it cannot be read by anyone more than 100 feet away.
The other thing to remember is that similar frequencies in the same area can interfere with each other. In the 315-413 MHz range are a lot of consumer products, like home alarm systems and home automation products like smart light bulbs. In some cases, electronic devices such as phone chargers can emit unintended electromagnetic radiation.
If a TPMS sensor transmitted all the time, that sensor will not last very long. Most TPMS sensors will transmit when movement is detected through a simple accelerometer inside. If the wheel stops moving, the sensor will stop broadcasting after a programmed amount of time. But once it is triggered, the sensor transmits on a predetermined interval set by the manufacturer. A sensor will immediately send a signal if it detects a sudden loss in pressure. A sensor never receives radio signals during normal operation. The only time a sensor receives a signal is when a TPMS tool activates the sensor by emitting an electromagnetic pulse at a specific frequency.
There are some models that can send a signal to indicate reduced battery voltage if it goes beyond a specified limit, but not all sensors send such a signal, and this information is often inconsistent. For instance, cold temperatures can cause a temporary voltage reduction that only corrects itself once the tires warm up. The only proven way to know how much battery life remains is to fully use the sensor to depletion.
The TPMS system will not turn the light on if a single transmission is not received. It takes multiple missed signals. The system knows that a missed or garbled transmission might be an external issue like a sensor on another vehicle transmitting at the same time or interference from the sensor being behind a brake caliper. It is like a misfire monitor; it only sets a code if the problem reaches a specific threshold.
How do you measure, observe and confirm a sensor or vehicle component is transmitting? The first strategy is to look for a result in the scan tool data PIDs. If you are having issues with a key fob or TPMS sensor, look at the data. It might be a command to unlock the doors that is seen in the BCM or keyless entry module.
If you are looking to measure the strength of a signal it requires specialized tools. Many TPMS tools include a feature to measure signals from key fobs and smart keys. These tools can also ping TPMS sensors and force them to transmit. This is a great feature if you are dealing with a random sensor.
The other type of radio signals you might have to deal with are carrier waves used by the keyless entry system. These radios waves range between 125 to 140 kHz. The waves are modulated to send information between the keys and antennas around the vehicle. These are for smart keys that can remain in the driver’s pocket. The antennas around the vehicle will send out these carrier waves along with the keys. These frequencies are used for very short distances to share security information. The antennas are located in the doors, tailgate and center console.
To test these antennas and keys, you will need a special probe for your scope. This probe can detect the carrier wave using a scope that has a fast sample rate. You will not be able to decode the information, but this probe can be very helpful when diagnosing a no start or condition where the owner can’t open a door.
Radio Wave Diagnostic Tips
- Perform relearns away from other vehicles and sources of electromagnetic interference like alarms and home appliances.
- If a vehicle can’t receive signals from a sensor during a relearn process, try rolling the vehicle forward a few feet to unblock a sensor.
- To avoid radio frequency issues, try relearning sensor positions with a TPMS tool that can interface with the TPMS module through the OBDII port.
- If a sensor can’t be activated with a TPMS tool, try rapidly deflating the tire by pushing in the valve stem. Hold the tool near the tire to see if the tool received the sensor’s ID.
- Do not retrain or program sensors around tire balancers and mounting machines. These machines cause sensors to transmit by deflating the tire rapidly or activate the sensor due to rotation. A vehicle might pick up the signal from a sensor that is not on the vehicle and when the vehicle is pulled out of the bay, the TPMS light will come on.
- Look for dash cams and radar detectors mounted on the windshield. Often these devices can cause interference that can block the TPMS antenna that might be mounted on or near the windshield.
- On long vehicles like pickups and cargo vans, the distance from the sensor to the antenna is pushed to the limits. It can get worse if the truck is filled with cargo like steel pipes or sheet metal.