TPMS Failure: What Happens at the MOT?

The Tyre Pressure Monitoring System, or TPMS, has become an increasingly common feature on modern vehicles, designed to alert drivers to underinflated tyres. While crucial for safety and fuel efficiency, a malfunctioning TPMS can have implications beyond just a dashboard warning light. Specifically, many drivers wonder what happens if their TPMS sensor fails the MOT test. This article aims to demystify the process, explain the consequences, and provide guidance on ensuring your vehicle remains compliant.

Understanding the MOT Test and TPMS

The MOT test is an annual inspection designed to ensure vehicles are safe and environmentally sound. While its primary focus is on critical safety components like brakes, lights, and steering, it also encompasses a range of other checks. The inclusion of TPMS in the MOT has evolved over time, reflecting its growing importance in vehicle safety. As of recent regulations, a faulty or missing TPMS, particularly if the warning light remains illuminated, can indeed lead to an MOT failure.

Reasons for TPMS Failure in the MOT

Several issues can cause a TPMS to fail an MOT inspection. These generally fall into a few key categories:

• Illuminated Warning Light: The most common reason for a TPMS-related MOT failure is the tyre pressure warning light remaining illuminated on the dashboard for a prolonged period. This indicates a fault within the system that the tester is required to note.
• Missing Sensors: If a TPMS sensor has been deliberately removed or is otherwise missing from a wheel, this can also result in a failure. The system is designed to have sensors on all wheels that are equipped with it.
• System Malfunction: Beyond a simple low-pressure alert, the TPMS can malfunction due to various internal faults. This could be a faulty sensor, a problem with the control unit, or issues with the wireless communication between the sensors and the vehicle's computer. Any of these will typically trigger the warning light.
• Incorrect Sensor Type: While less common, using non-compatible or incorrect type of TPMS sensors after a replacement can also lead to system errors and a subsequent MOT failure. It's vital to use sensors that are specifically designed for your vehicle make and model.

Consequences of a TPMS MOT Failure

Failing an MOT due to a TPMS issue means your vehicle is not deemed roadworthy in its current state. The immediate consequence is that you will not receive a new MOT certificate. You will be issued with a refusal of an MOT certificate, detailing the reasons for failure. You will then need to have the identified faults rectified and present the vehicle for a partial re-test. Driving a vehicle without a valid MOT certificate is illegal and can lead to fines and other penalties.

What Constitutes a TPMS Fault for MOT Purposes?

It's important to distinguish between a temporary TPMS alert and a persistent fault. A TPMS alert that illuminates briefly when you first start the engine, or if you have recently adjusted tyre pressures and the system hasn't recalibrated, is usually not a failure point. However, if the warning light stays on continuously while driving, or if it comes on and stays on after starting the engine, this signifies a fault that the MOT tester must address.

Testers will check the following:

• Absence of the warning light when the ignition is turned on (it should illuminate and then go out).
• The warning light does not remain illuminated after the engine has started.
• No sensors are visibly missing from the wheels.

Common Causes of TPMS Sensor Failure

Understanding why your TPMS might be failing can help in preventing issues. The most frequent causes include:

• Battery Depletion: TPMS sensors have small internal batteries that power them. Over time, these batteries can deplete, leading to sensor failure. This is a very common issue, especially on older vehicles.
• Sensor Damage: The sensors are located within the wheel assembly and can be susceptible to damage. This can occur during tyre changes, from impact with kerbs, or from road debris.
• Corrosion: The metal components of the sensor, particularly the valve stem, can corrode over time, especially in areas with harsh weather conditions or where de-icing salts are used. This corrosion can affect the sensor's functionality.
• Re-learning Issues: After new tyres or sensors are fitted, the TPMS often needs to be 're-learned' or recalibrated to the vehicle. If this process isn't done correctly, the system may not communicate properly, leading to a warning light.
• Faulty Control Module: Less commonly, the central control module that receives signals from the TPMS sensors can fail, causing the entire system to malfunction.

Can You Fix a TPMS Fault?

Yes, TPMS faults are generally fixable. The approach to fixing the issue will depend on the specific cause:

• Battery Replacement: For sensors with depleted batteries, the battery can sometimes be replaced, although in many cases, the entire sensor unit is replaced as the battery is integrated.
• Sensor Replacement: If a sensor is damaged or corroded, it will need to be replaced. It's often recommended to replace all TPMS sensors at once, especially if they are of a similar age, to avoid repeated issues.
• Recalibration: After replacing sensors or tyres, a recalibration process is necessary. This can often be done using diagnostic tools. Some vehicles have a manual recalibration procedure that the driver can perform.
• Software Updates: In some instances, a software update for the vehicle's control module might resolve communication issues.

Pre-MOT Checks for TPMS

To avoid an unpleasant surprise at your MOT appointment, it's wise to perform some basic checks beforehand:

• Observe the Dashboard: Pay attention to your TPMS warning light. Does it illuminate when you start the engine and then go off? If it stays on, you have a problem.
• Check Tyre Pressures Regularly: Manually check your tyre pressures against the manufacturer's recommendations (usually found on a sticker in the driver's doorjamb or in the owner's manual). While this doesn't directly test the TPMS, it ensures your tyres are correctly inflated, which is the primary purpose of the system.
• Listen for Unusual Noises: While not directly related to the warning light, any unusual noises from a wheel could indicate damage to a sensor.

Frequently Asked Questions (FAQs)

Q1: My TPMS light came on briefly, then went off. Will this fail my MOT?
Generally, no. A temporary illumination of the TPMS warning light when you start the engine is normal. It's only when the light remains illuminated continuously that it indicates a fault that could lead to an MOT failure.

Q2: I just had new tyres fitted, and now my TPMS light is on. What should I do?
It's likely that the TPMS sensors need to be recalibrated or 're-learned' to your vehicle. Contact the tyre fitting centre to have this done. If they confirm it's done correctly and the light persists, then there might be a fault with one of the sensors.

Q3: Can I disable my TPMS if it's faulty?
While it might be technically possible to disable the TPMS on some vehicles, this is not recommended and is unlikely to be permitted for an MOT test. The system is a safety feature, and its intended functionality, or lack thereof indicated by a warning light, is what the MOT tester assesses.

Q4: How much does it cost to fix a TPMS sensor?
The cost can vary significantly depending on the vehicle, the type of sensor, and whether it's a simple recalibration or a full sensor replacement. Expect to pay anywhere from £50 to £200 or more per sensor, plus fitting and recalibration costs.

Q5: Are all vehicles fitted with TPMS required to have it working for the MOT?
If a vehicle was originally manufactured with a TPMS, then the system must be in working order and the warning light must extinguish after starting the engine for it to pass the MOT. If a vehicle was never fitted with TPMS, it is not a mandatory requirement.

Conclusion

The Tyre Pressure Monitoring System is a vital safety feature, and its functionality is taken seriously during the MOT test. A persistent TPMS warning light or missing sensors will result in a failure. By understanding the common causes of TPMS faults and performing regular checks, you can ensure your vehicle remains compliant, safe, and passes its MOT with flying colours. If you suspect a TPMS issue, it's always best to address it with a qualified mechanic before your next inspection.

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