At first, you may think that Tire Pressure Inoperative warning is caused by low tire pressure, but that is not always the issue. In this article, you will learn how the TPMS system works and common problems that may cause trigger message.
What does TPMS warning message mean on a Mercedes-Benz?
Every car manufacturer determines a certain pressure to which front and rear tires on their vehicles should be inflated.
These values vary depending on tire size and current vehicle loads, and drivers can find the recommended tire pressure by looking at the sticker on the frame of the driver's door.
Tire pressure outside the given limits will affect fuel economy, increase wear, and make the vehicle less stable. Therefore, drivers should regularly check if their tires are properly inflated.
Things like punctures and temperature changes may affect the tire pressure and cause it to drop while driving. Many drivers will not notice this and continue their trip, damaging the tire or even leading to a crash.
To prevent this, all 2000 and newer Mercedes-Benz cars have a series of sensors to monitor the pressure in all four tires. The TPMS system will warn the driver if any of them are under or overinflated.
However, if any of the components malfunctions, it will not monitor tire pressure anymore. When that happens in a Mercedes-Benz, it will warn the driver with a ‘Tire Pressure Monitor Inoperative’ message on the instrument cluster.
The tire pressure monitor is a relatively simple assembly with several components, which makes diagnosing eventual failures easier. These are the most likely causes, although this may differ depending on the Mercedes-Benz year and model.
Missing wheel sensors
Swapping between tire sets is one of the most common situations after which the Tire Pressure Monitor becomes inoperative. Many owners have separate sets of summer and winter tires, each with its own sensor. After each wheel change, it is necessary to reset the tire pressure monitor, during which the car will recognize new sensors.
There is a sensor within each tire that measures pressure and transmits that data to a corresponding control module. These sensors use a lithium-ion battery as a source of electrical power, which goes flat over time. In most cases, original batteries can last up to 10 years. Another possibility is mechanical damage, which can happen during tire fitment. The easiest way of solving this issue is by using a wireless reader, which verifies if the sensor is emitting a signal.
Blown Fuse / Bad Module
Simple as it may sound, a blown TPMS fuse is one of the likely causes here. The fuse is either located in the rear SAM control module or below the passenger seat, depending on the model. In some cases, the SAM control module can malfunction, resulting in TPMS warnings and electrical shorts.
With everything else ruled out, it is pretty likely that the tire pressure sensor control module is faulty. Most often, this happens because of moisture, which causes corrosion and leads to electrical shorts.
How do you reset TPMS in a Mercedes-Benz?
Mercedes-Benz tire pressure warning reset automatically as soon as you inflate the tires to the recommended pressure. The recommended pressure can be found in the sticker on the driver's door or behind the fuel door next to the fuel cap.
By resetting the Tire Pressure Monitor system, you will set the current tire pressure in your Mercedes-Benz as a reference value. This is why you should check the actual pressure in each wheel to make sure it matches the factory-recommended one.
- With tire pressures checked and adjusted, if needed, go inside the vehicle and turn on the ignition.
- Press the ‘Home’ button on the steering wheel to get into the menu on the central display. Inside it, use the arrow buttons to navigate towards the ‘Service’ menu.
- Click OK to enter the 'Service' menu, select 'Tire Pressure', and then click OK again to initiate the restart process.
- Before restarting, the system will ask you if the tire pressure is correct. You will do this by navigating to the ‘Yes’ option and selecting it with the OK button.
- A ‘Tire pressure monitor restarted’ message on the central display will tell you the procedure was carried out successfully.
Mercedes-Benz TPMS not working
After diagnosing a faulty sensor, which could be caused by a flat battery or mechanical damage, you will need to replace it.
However, this involves removing the tire from the rim, as the sensor is inside the wheel itself. This requires some specialized tools and equipment, so you may consider leaving this job to the tire shop.
If, however, you wish to do it yourself, there is a method that only requires essential tools.
- Start by removing the wheel in the question of the vehicle and placing it on the ground with the outer side facing upwards.
- Release all air from the wheel.
- Use a suitable wooden lath as a lever to separate the tire from the rim.
- Move the tire inward as much as possible to gain access to the pressure sensor.
- Using a ratchet with a suitable socket, undo the nut that holds the valve/sensor assembly.
- Pull the valve/sensor assembly from the rim and replace it with a new one.
- Refit everything back in reverse order of removal.
How to Code TPMS on a Mercedes-Benz
The TPMS is coded in two modules. Instrument clusters and central gateway (CGW). To disable TPMS, you will need to carry out variant coding on both these modules. Please note that disabling or coding out the TPMS may be illegal in certain countries.
The Tire pressure monitor will show in the variant coding as item 44, unique equipment code 475 known as Tire Pressure Monitor or Tire Pressure Loss Warner with Position Related Pressure Values.