Quick answer

A TPMS sensor contains a pressure sensor (and often a temperature sensor), a small battery, and a wireless transmitter. It reads the tyre’s internal pressure, converts it into a digital signal, and transmits that data to a receiver. The receiver compares the readings to your set thresholds and alerts you if a tyre becomes unsafe.

What a TPMS sensor measures

Most modern TPMS sensors measure at least:

• Tyre pressure (typically displayed in PSI)

Many systems also measure:

• Tyre temperature (useful for detecting heat build-up from low pressure, heavy loads, or long towing runs)
• Rate of pressure change (to detect rapid leaks)

Temperature matters because heat is often the early warning sign. A tyre can look fine while the internal conditions are trending toward failure, especially when towing or travelling long distances.

What’s inside a TPMS sensor?

Whether the sensor is mounted externally on the valve stem or internally inside the tyre, most TPMS sensors include:

• Pressure transducer (measures air pressure inside the tyre)
• Temperature sensor (common on many systems)
• Microcontroller (processes and formats the data)
• Wireless transmitter (sends the readings to the receiver)
• Battery (powers the sensor for months or years depending on design)
• Seal and housing (protects the electronics from dust, water, and vibration)

The sensor takes a reading at set intervals (or when it detects significant change), then transmits the data packet to the receiver.

Direct TPMS vs indirect TPMS: which one uses sensors?

The term “TPMS sensor” usually refers to direct TPMS, because direct systems have a physical sensor at each tyre. Indirect systems use the vehicle’s ABS/wheel-speed sensors to estimate pressure changes and do not measure PSI directly.

If you want the full breakdown: Direct vs Indirect TPMS Explained.

External vs internal TPMS sensors

External sensors (valve cap style)

External TPMS sensors screw onto the valve stem like a cap. They measure the pressure through the valve and send readings wirelessly. External sensors are popular because they are:

• Easy to install (DIY)
• Easy to move between vehicles
• Simple to replace if a sensor fails
• Practical for caravans and trailers

Internal sensors (inside the tyre)

Internal sensors mount inside the tyre, usually at the valve stem. They are protected from damage and theft risk is lower, but installation and replacement require tyre removal and balancing.

Full comparison: Wireless vs Internal TPMS: Which Is Better?

How TPMS sensor data gets to you

A TPMS is a simple loop: measure → transmit → display → alert. Here’s how it works in practice:

1. The sensor measures pressure inside the tyre.
2. The sensor transmits the data via wireless signal to a receiver.
3. The receiver displays the reading as live PSI (and temperature if supported).
4. The receiver checks thresholds (your low/high pressure and temperature settings).
5. If unsafe, the system alerts you with an audible/visual warning.

If you monitor a caravan, the key factor is signal reliability between vehicle and caravan. That’s why towing systems often focus on range and stable transmission. For towing setups: Best TPMS for Caravans and Towing

What triggers a TPMS alert?

Alerts vary by system, but the most important ones include:

• Low pressure (often set 10–15% below your baseline)
• High pressure (pressure rises due to heat, over-inflation, or load changes)
• Rapid pressure loss (fast leak or puncture)
• High temperature (heat build-up that can indicate risk)
• Signal loss / sensor fault (sensor not communicating properly)

If you’re getting warnings you don’t understand, see: TPMS Fault Warning Meaning

How accurate are TPMS sensors?

TPMS sensors are generally accurate enough for safety monitoring, but readings can vary slightly from a manual gauge. What matters most is not a perfect number, but the trend: pressure dropping steadily, pressure dropping fast, or temperature rising abnormally.

A good habit is to confirm your baseline pressures with a quality manual gauge, then let the TPMS handle ongoing monitoring.

How long do TPMS sensor batteries last?

Battery life depends on the sensor type and how frequently it transmits data. In practical terms:

• External sensors often use replaceable batteries.
• Internal sensors may be sealed and require sensor replacement when the battery is depleted.

If your sensor stops transmitting, troubleshooting starts here: TPMS Sensor Not Reading? Here’s Why

Next step: installing a TPMS

If you’re ready to set one up, use this step-by-step guide: How to Install a Tyre Pressure Monitoring System

Or view the Safety Sam system here: Tyre Pressure Monitoring System

FAQs

Do TPMS sensors work on caravans and trailers?

Yes, as long as your system supports enough sensors and has reliable transmission range for towing setups. Start here: Best TPMS for Caravans and Towing.

Is a TPMS sensor the same as a valve cap?

Some TPMS sensors look like valve caps, but they contain electronics that measure and transmit pressure data. Standard valve caps only protect the valve from dust and moisture.

Why does tyre pressure increase while driving?

As tyres warm up, air inside expands and pressure increases. This is normal. Your TPMS helps you see how pressure changes with temperature and load.

Can a TPMS detect a puncture early?

Often yes. Many punctures leak slowly at first. A TPMS can alert you to a pressure drop trend before the tyre becomes dangerous or damaged.