TPMS Failures Explained

Understanding TPMS Malfunctions and MOT Failures

In the UK, the Driver and Vehicle Standards Agency (DVSA) introduced significant changes to the MOT inspection manual for cars and passenger vehicles on the 17th June 2019. Among these updates, one particular change has caused a stir within the automotive industry: the reclassification of a malfunctioning Tyre Pressure Monitoring System (TPMS) from a ‘minor’ to a ‘major’ defect. This means that from the 20th June 2019 onwards, any vehicle presenting with a non-functioning TPMS will, unfortunately, fail its annual MOT test.

This shift in policy highlights the growing importance of tyre safety and the role technology plays in ensuring vehicles are roadworthy. For motorists, it underscores the need to maintain their TPMS diligently. For garages and MOT centres, it presents both a challenge and a significant business opportunity. Understanding the reasons behind TPMS failures is now more critical than ever to keep vehicles compliant and customers safe.

What is TPMS and Why is it Important?

TPMS, or Tyre Pressure Monitoring System, is an electronic system designed to monitor the air pressure inside the pneumatic tyres of various types of vehicles. It alerts the driver when one or more tyres are significantly under-inflated, presenting a serious decrease in performance, increased danger to oneself and others, fuel economy, and tyre wear. The system typically consists of sensors within each wheel, a central control unit, and a warning light on the dashboard. Since November 2014, TPMS has been a mandatory fitment on all newly registered vehicles in the UK. This legislation was enacted to improve road safety by reducing the risks associated with under-inflated tyres, such as blowouts and poor handling.

The Evolution of MOT Defect Categories

Before May 2018, MOT tests categorised defects as either a pass, a fail, or an advisory. However, the DVSA introduced a new system classifying defects as ‘minor’, ‘major’, or ‘dangerous’. This revised structure provides a clearer indication of the severity of a fault.

• Minor Defects: These are issues that do not pose an immediate risk to safety but could potentially lead to problems in the future. They are recorded as advisories, much like before, informing the owner of potential maintenance needs.
• Major Defects: These are more serious faults that could compromise the safety of the driver, passengers, or other road users. They could also pose a risk to the environment. A vehicle with a major defect will fail its MOT.
• Dangerous Defects: These are faults that present a direct and immediate risk to road safety or have a serious impact on the environment. A vehicle with a dangerous defect must not be driven until it is repaired.

The crucial change for TPMS is its reclassification. Previously, a TPMS malfunction might have been a ‘minor’ defect, meaning the car could still pass the MOT with an advisory. Now, it’s a ‘major’ defect, necessitating repair for the vehicle to pass. This means that simply ignoring a TPMS warning light is no longer an option if you want your vehicle to pass its MOT.

Common Reasons for TPMS Failure

Several factors can contribute to a TPMS malfunction. Understanding these common causes can help diagnose and rectify the issue:

1. Low Battery in Sensor: TPMS sensors are battery-powered. The average lifespan of these batteries is typically between 5-10 years. Once the battery depletes, the sensor can no longer transmit data, leading to a system failure. This is one of the most frequent reasons for TPMS faults as vehicles age.
2. Damaged Sensor: The sensors are located inside the wheels, often near the valve stem. They can be damaged during tyre fitting or removal, or by road debris, potholes, or even corrosion. A physically damaged sensor will cease to function.
3. Faulty Sensor Unit: Like any electronic component, TPMS sensors can develop internal faults and fail over time, regardless of battery life or physical damage.
4. Receiver/ECU Issues: The TPMS system relies on a receiver and an Electronic Control Unit (ECU) to process the data from the sensors. Malfunctions in these components can also cause the system to fail, even if the sensors are working correctly.
5. Interference: In rare cases, strong radio frequency interference from other electronic devices or aftermarket accessories could disrupt the communication between the TPMS sensors and the vehicle's ECU.
6. Incorrect Tyre Pressures: While not a system failure, significantly incorrect tyre pressures (either too high or too low) will trigger the TPMS warning light. It’s important to differentiate between a system fault and a genuine low-pressure warning. Always check your tyre pressures regularly with a reliable gauge.

The Impact on Garages and MOT Centres

With TPMS now a mandatory pass/fail criterion, garages and MOT centres are experiencing an increased demand for TPMS diagnostics and repairs. Vehicles manufactured after November 2014 will inevitably start showing TPMS faults as they age and their sensors’ batteries naturally deplete. Garages that are equipped to handle TPMS repairs – offering diagnostics, sensor replacement, and system re-learning/programming – are well-positioned to capitalize on this new business. Conversely, those unable to offer these services risk losing customers to competitors who can.

The complexity of TPMS systems means that specialised tools and knowledge are often required. Not all independent garages may have the necessary equipment or training. This is where brands like VDO, part of the Continental Corporation, play a crucial role. They provide diagnostic tools, programming devices, and replacement TPMS sensors, enabling workshops to service and repair these systems effectively and get vehicles back on the road safely and legally.

What to Do if Your TPMS Fails

If your TPMS warning light illuminates on your dashboard, it’s essential to take it seriously. Here’s a step-by-step approach:

1. Check Tyre Pressures: The first and simplest step is to manually check the pressure in all your tyres using a reliable tyre pressure gauge. Ensure they are inflated to the manufacturer’s recommended levels, which can usually be found on a sticker in the driver’s side doorjamb or in your vehicle’s handbook. Adjust pressures as needed.
2. Drive for a Short Period: After adjusting tyre pressures, drive the vehicle for a few miles at a moderate speed. Some TPMS systems need to recalibrate. If the light goes off, the issue was simply low tyre pressure.
3. Consult Your Vehicle Handbook: Your handbook will explain what the TPMS warning light means and may offer specific troubleshooting advice for your model.
4. Seek Professional Diagnosis: If the warning light persists or flashes (which often indicates a system fault rather than just low pressure), it’s time to visit a qualified mechanic or MOT centre. They have the diagnostic tools to identify the specific fault, whether it’s a sensor battery, a damaged sensor, or an issue with the control unit.
5. Sensor Replacement and Programming: If a sensor needs replacing, a new sensor will need to be fitted and then ‘paired’ or programmed to your vehicle’s ECU. This process varies depending on the vehicle manufacturer.

TPMS Sensor Lifespan and Replacement Considerations

As mentioned, TPMS sensor batteries have a finite lifespan. When they fail, the entire sensor unit typically needs to be replaced, as the batteries are not usually designed to be user-replaceable. When replacing TPMS sensors, it’s advisable to replace them in sets, especially if multiple sensors are nearing the end of their expected battery life. This proactive approach can prevent further failures shortly after the initial repair.

When purchasing replacement sensors, ensure they are compatible with your vehicle’s make, model, and year. Original Equipment (OE) sensors or reputable aftermarket equivalents (like those from VDO) are recommended for optimal performance and longevity.

Table: Common TPMS Faults and Solutions

Frequently Asked Questions (FAQs)

Q1: How long do TPMS sensors last?
TPMS sensor batteries typically last between 5 to 10 years, depending on the manufacturer and usage. The entire sensor unit usually needs replacement when the battery fails.

Q2: Can I ignore the TPMS warning light?
No. Since June 2019, a malfunctioning TPMS is a major MOT defect, meaning your vehicle will fail its MOT test if the system is not working correctly.

Q3: How much does it cost to fix a TPMS fault?
The cost can vary significantly depending on the nature of the fault. A simple tyre pressure adjustment is free. Replacing a single TPMS sensor and programming can range from £80 to £200 or more, depending on the vehicle and the sensor type.

Q4: Can I disable the TPMS system?
It is not recommended and may be illegal to disable a safety system like TPMS. Furthermore, attempting to do so will likely result in an MOT failure.

Q5: Do all my tyres need TPMS sensors?
Yes, under current regulations, all vehicles fitted with TPMS from the factory must have a functioning system for all wheels, including the spare if it is equipped with a sensor.

In conclusion, the mandatory nature of TPMS functionality for MOT tests means that drivers must pay closer attention to their tyre pressure and the health of their TPMS. By understanding the common causes of failure and seeking professional help when needed, motorists can ensure their vehicles remain safe, compliant, and pass their annual MOT with flying colours.

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