20/03/2022
Modern vehicles are equipped with a myriad of sophisticated systems designed to enhance safety, efficiency, and driving comfort. Among these, the Tyre Pressure Monitoring System (TPMS) stands out as a vital component. Its primary role is to monitor the pressure in your vehicle's tyres, providing an early warning if pressure drops below safe levels. This not only contributes significantly to road safety by preventing blowouts and improving handling but also helps optimise fuel consumption and prolong tyre life. However, like any electronic system, TPMS can experience malfunctions, leading to that often perplexing warning light on your dashboard. Understanding what this light signifies, and crucially, how it can affect your vehicle's annual MOT test, is paramount for every responsible driver in the UK.
What Exactly is TPMS and How Does It Work?
The TPMS became mandatory for all new passenger vehicles sold in the European Union (including the UK prior to Brexit) from November 2014. Its purpose is straightforward: to alert the driver when one or more tyres are significantly under-inflated. There are two primary types of TPMS:
- Direct TPMS: This system uses individual pressure sensors mounted inside each tyre, often integrated with the valve stem. These sensors directly measure the tyre pressure and temperature and transmit this data wirelessly to the vehicle's onboard computer. If a significant pressure drop is detected, the system illuminates the TPMS warning light on the dashboard, and some advanced systems may even display the exact pressure of each tyre. The main advantage of direct TPMS is its accuracy and ability to detect pressure changes quickly, even when the vehicle is stationary. However, it can be more expensive to maintain due to battery-powered sensors that eventually need replacement.
- Indirect TPMS: This system is less complex and typically relies on the Anti-lock Braking System (ABS) wheel speed sensors. It doesn't directly measure tyre pressure but instead monitors the rotational speed of each wheel. When a tyre loses pressure, its rolling radius decreases, causing it to rotate at a slightly different speed compared to the other tyres. The indirect TPMS detects this discrepancy and triggers the warning light. While less expensive and requiring less maintenance than direct systems, indirect TPMS is not as precise and can only detect a pressure drop after the vehicle has been driven for a period. It also requires recalibration after tyre rotations or replacements.
Regardless of the type, the goal is the same: to keep you informed about the critical condition of your tyres. Ignoring the TPMS light can have serious consequences, from compromised handling and increased stopping distances to premature tyre wear and reduced fuel efficiency.
Why Is Your TPMS Light On? Common Causes
When the TPMS warning light illuminates, it's a clear signal that something needs attention. Here are the most common reasons:
- Low Tyre Pressure: This is by far the most frequent cause. A tyre that has lost pressure due to a slow puncture, temperature changes, or simply natural air loss will trigger the light. Always check your tyre pressures first with a reliable gauge.
- Faulty TPMS Sensor: In direct TPMS systems, each sensor has a battery with a finite lifespan, typically 5-10 years. Once the battery dies, the sensor stops transmitting data, and the TPMS light will come on. Sensors can also be damaged during tyre fitting, or their valve stems can corrode, leading to leaks or signal loss.
- TPMS System Malfunction: Less common but possible, there could be an issue with the TPMS control module itself, the wiring, or the receiver that collects data from the sensors. This indicates a more systemic problem rather than just a sensor or pressure issue.
- Tyre Rotation or Replacement Without Recalibration: Especially with indirect TPMS, if tyres are rotated or replaced without performing the necessary system recalibration, the light may illuminate as the system gets confused by new wheel speeds or sensor positions.
- Extreme Temperature Fluctuations: Air pressure inside tyres expands and contracts with temperature. A significant drop in ambient temperature can cause tyre pressure to fall enough to trigger the TPMS light, even if there isn't a leak. Conversely, pressure can rise with extreme heat.
It's crucial to understand that a solid TPMS light usually indicates low pressure or a system fault, while a flashing light (on some vehicles) often signifies a sensor malfunction or a system communication error immediately upon starting the vehicle.
TPMS and the MOT Test: What You Need to Know
This is where the information provided by Dan Powell becomes critical. For drivers in the UK, the annual MOT test is a non-negotiable hurdle. A malfunctioning TPMS can indeed lead to a MOT failure, but it's important to understand the specific criteria.
As Dan explains, the MoT tester will only fail the vehicle if it’s clear that the lamp indicates a TPMS system malfunction and not simply indicating that one or more of the tyre pressures is low.
Let's break this down:
- TPMS Light On Due to Low Pressure: If the TPMS light is on solely because one or more tyres are genuinely under-inflated, and the system itself is otherwise functioning correctly (i.e., it's accurately reporting low pressure), this should not be an automatic MOT failure. The solution here is simple: inflate the tyres to their correct pressure. Once the pressure is corrected, the light should extinguish, indicating the system is working as intended. However, it's worth noting that driving with significantly under-inflated tyres can lead to a 'dangerous' or 'major' defect in itself, regardless of the TPMS light.
- TPMS Light On Due to System Malfunction: This is where the MOT failure comes in. If the TPMS light is illuminated because there's a fault within the system itself – for example, a dead sensor battery, a damaged sensor, a faulty control unit, or a communication error – then it constitutes a 'major' defect and will result in an MOT failure. The tester's judgment relies on whether the lamp indicates a failure of the monitoring system rather than just its intended warning function. A permanently illuminated or flashing TPMS warning lamp indicating a system malfunction is a reason for failure.
Essentially, the MOT test focuses on the functionality of the safety system. If the TPMS is unable to perform its monitoring duties due to a fault, then the vehicle fails. If it's simply doing its job by warning you of low pressure, and the underlying pressure issue is rectified, then the system is deemed functional.
Troubleshooting and Resolving TPMS Issues
When your TPMS light comes on, here's a structured approach to troubleshooting:
- Check Tyre Pressures Immediately: Use a reliable tyre pressure gauge. Inflate all tyres to the manufacturer's recommended pressures (found in your owner's manual, on a sticker inside the driver's door jamb, or inside the fuel filler flap). Don't forget the spare tyre if your system monitors it.
- Drive Your Vehicle: After adjusting pressures, drive your car for a few miles (sometimes up to 10-20 miles) at a consistent speed. The system often needs time and movement to recalibrate and confirm the correct pressures. The light should turn off if low pressure was the only issue.
- Recalibrate the System (if applicable): Some indirect TPMS systems, and even some direct ones after tyre changes, require manual recalibration. Consult your owner's manual for instructions. This usually involves pressing a button or navigating through a menu on your dashboard.
- Inspect for Sensor Damage/Faults: If the light persists after checking pressures and recalibrating, it's likely a sensor issue. This is best left to a professional. A mechanic can use a diagnostic tool to read fault codes from the TPMS module, pinpointing which sensor or component is malfunctioning.
- Consider Sensor Battery Life: If your vehicle is older than 5-7 years and has direct TPMS, dead sensor batteries are a very common culprit. They cannot be replaced individually; the entire sensor typically needs replacing.
Preventative Maintenance for Your TPMS
While TPMS is largely maintenance-free, a few proactive steps can help ensure its longevity and accuracy:
- Regular Tyre Pressure Checks: Even with TPMS, manually checking your tyre pressures monthly is good practice. It catches slow leaks before the TPMS light comes on and ensures optimal performance.
- Professional Tyre Fitting: When replacing tyres, ensure the tyre shop is experienced with TPMS. Sensors can be fragile and easily damaged during removal and installation. It's often recommended to replace the valve stem service kit (valve core, cap, grommet, and nut) when new tyres are fitted, especially on direct TPMS systems, as these components can corrode.
- Consider Sensor Replacement with New Tyres: If your direct TPMS sensors are approaching the end of their battery life (e.g., 7-8 years old) and you're getting new tyres, it can be cost-effective to replace the sensors at the same time. This avoids paying for labour to remove and refit tyres specifically for sensor replacement later on.
Direct vs. Indirect TPMS: A Quick Comparison
| Feature | Direct TPMS | Indirect TPMS |
|---|---|---|
| Sensor Location | Inside each tyre valve | Uses ABS wheel speed sensors |
| Measurement Method | Direct pressure/temperature | Wheel rotation speed differences |
| Accuracy | High, real-time | Lower, reacts to changes in rolling radius |
| Detection Speed | Immediate, even when stationary | Requires driving for detection |
| Maintenance | Sensors with batteries need replacement | Less maintenance, no sensor batteries |
| Cost | Higher (sensors, fitting) | Lower |
| Recalibration | Less frequent, sometimes automatic | Often required after tyre changes/rotations |
Frequently Asked Questions About TPMS
Q: Can I drive with the TPMS light on?
A: While you technically can, it's highly advised against. The light is on for a reason – either low pressure (which is dangerous) or a system fault. Driving with low tyre pressure can lead to poor handling, increased stopping distances, excessive heat build-up (which can cause a blowout), and premature tyre wear. A system fault means you're driving without the safety net of pressure monitoring.
Q: How much does a TPMS sensor replacement cost?
A: The cost varies significantly depending on the vehicle make and model, and whether you use OEM (Original Equipment Manufacturer) or aftermarket sensors. Typically, a single sensor replacement can range from £50 to £150, including parts and labour. If all four need replacing, the cost will multiply.
Q: Do all cars have TPMS?
A: In the UK, all new passenger vehicles sold from November 2014 onwards are legally required to have TPMS fitted as standard. Older vehicles may not have it.
Q: Can I reset the TPMS light myself?
A: For low pressure issues, the light often resets itself after you inflate the tyres to the correct pressure and drive for a period. For system faults, some vehicles have a manual reset button or procedure outlined in the owner's manual. However, if the underlying issue is a faulty sensor or module, simply resetting the light won't fix the problem, and it will likely reappear.
Q: What's the difference between a flashing and solid TPMS light?
A: On many vehicles, a solid TPMS light typically indicates that one or more tyres are significantly under-inflated. A flashing TPMS light (which then may turn solid after a few seconds) usually signals a malfunction within the TPMS itself, such as a dead sensor battery or a communication error between a sensor and the vehicle's computer. Always refer to your vehicle's owner's manual for specific interpretations.
Conclusion
The TPMS is an unsung hero of modern vehicle safety, providing crucial warnings that can prevent accidents and save you money in the long run. While a TPMS warning light can be a nuisance, it's a call to action. Whether it's a simple case of low tyre pressure or a more complex system malfunction, addressing the issue promptly is essential for your safety, your vehicle's performance, and ensuring a smooth passage through your annual MOT test. Don't ignore that dashboard light; get your tyres and TPMS checked by a qualified professional if you can't resolve it with a simple pressure top-up. A well-maintained TPMS is a cornerstone of responsible driving.
If you want to read more articles similar to TPMS Failures: Your MOT & Safety Guide, you can visit the Maintenance category.