How does my car monitor tire pressure?
Your car tracks tire pressure with a tire pressure monitoring system (TPMS). It can rely on sensors inside the tires or on the vehicle’s wheel-speed data to estimate pressure. When a tire drops below the recommended level, the system alerts the driver with a warning light or a readout on the dashboard. Regularly checking and maintaining proper tire inflation helps safety, fuel efficiency, and tire life.
Two main ways cars monitor tire pressure
The following overview explains the two primary TPMS approaches found in modern vehicles and how they differ in what they measure and report.
Direct TPMS — Sensors are mounted in each tire, typically on or inside the valve stem. Each sensor records the actual tire pressure (and often temperature) and transmits the data to a central TPMS control module. The system can display the pressure for individual tires in the dashboard or infotainment system, and it requires a battery inside each sensor. When tires are replaced or rotated, a relearn or reset procedure is usually needed so the car recognizes the sensors’ IDs and aligns them with the correct wheels.
Indirect TPMS — There are no tire-mounted pressure sensors. Instead, the system uses ABS wheel-speed sensors and other data to detect changes in tire circumference that come with under-inflation. If the software detects a discrepancy, it triggers the warning light. Indirect TPMS does not provide exact PSI readings, and there is typically no battery to replace because there are no onboard sensors in the tire. Some cars that rely on indirect TPMS can still display a rough pressure indicator or rely on the warning light alone.
Both approaches aim to prevent under-inflation, which can affect handling, braking, and tire wear. The choice between direct and indirect TPMS depends on vehicle design, cost, and manufacturer preferences. In most new cars, you’ll encounter one of these systems as a standard safety feature.
What happens when the TPMS warning light is on
When the warning indicator appears, drivers should take immediate but safe steps to verify tire condition and address potential issues. The steps below outline a practical course of action for most passenger cars.
- Safely pull over and inspect the tires for obvious damage or leaks. Do not drive long distances on a visibly damaged tire.
- Check tire pressures when the tires are cold, using a reliable gauge. Inflate each tire to the vehicle manufacturer’s recommended PSI, which is listed on the door jamb placard or in the owner’s manual.
- After inflating, recheck the pressures to confirm they’re within the recommended range. Remember that driving heats tires and can raise pressure, so use cold readings for accuracy.
- If the light stays on after inflation, check for slow leaks, punctures, or faulty sensors. For a direct TPMS, a damaged sensor or dead battery could trigger a persistent alert; for an indirect TPMS, the system may need recalibration or service if the indicators don’t resolve.
- If you’re unsure or you can’t locate a cause, visit a tire or automotive service shop. A technician can perform a TPMS relearn/reset procedure (if required), inspect sensors, and confirm proper inflation.
Responsive action to a TPMS warning is a key safety step. Under-inflated tires can compromise steering, braking performance, and increase the risk of a blowout, especially in hot weather or on long trips.
Maintaining your TPMS and tires
Keeping your TPMS functioning properly involves routine tire maintenance and occasional system checks. Below are best-practice guidelines to help you stay ahead of potential issues.
- Check tire pressures monthly and before long trips, using the recommended PSI as your target.
- Ensure you inflate tires when they’re cold to avoid over- or under-inflating due to heat buildup.
- If your vehicle uses direct TPMS, be aware that each sensor has a battery life (typically several years). When a sensor fails or is replaced, a relearn procedure may be required so the system properly recognizes the new or reinstalled sensor.
- During tire rotations or replacement, confirm that the TPMS relearn or reset is performed as required by your vehicle’s manufacturer. Some cars automatically relearn after driving for a short period, while others need manual steps.
- Keep valve caps on all tires and inspect sensors for damage or corrosion if you notice a TPMS fault message.
- If you replace a tire, wheel, or sensor, verify the correct sensor type and frequency for your car and have the service provider handle installation and programming if needed.
Proper maintenance helps ensure the TPMS provides accurate alerts and helps you avoid unnecessary trips to the shop for sensor-related issues.
Summary
Tire pressure monitoring systems are a smart safety feature that comes in two main flavors: direct TPMS, which uses individual tire sensors to report exact pressures, and indirect TPMS, which estimates pressure using wheel-speed data. When the TPMS light illuminates, check tire pressures with a gauge, inflate to the recommended level, and seek professional help if the light remains on or if you suspect a sensor problem. Regular tire maintenance—checking pressures, rotating tires, and replacing sensors as needed—helps maintain performance, fuel efficiency, and safety on the road.
How does my car know the tire pressure?
Your car knows tire pressure through a Tire Pressure Monitoring System (TPMS), which uses either a direct sensor inside each wheel to measure pressure or an indirect sensor that uses the ABS wheel-speed sensors. Both systems send data to the car's computer, which triggers a dashboard light when the pressure is too low.
This video explains how TPMS works: 56sPep Boys AutoYouTube · Jun 1, 2012
Direct TPMS
- How it works: A sensor is mounted inside each tire and directly measures the air pressure. The sensor then wirelessly transmits the information to the car's computer.
- What it detects: It can provide a precise, real-time pressure reading for each tire.
- Battery life: The sensors contain a battery that is not typically user-replaceable and is designed to last for about 5-10 years.
Indirect TPMS
- How it works: This system uses the car's existing Anti-lock Braking System (ABS) sensors to monitor the speed at which each wheel rotates.
- What it detects: An underinflated tire has a slightly larger diameter and will spin at a different speed than the other tires. The computer detects this speed difference to determine if a tire is low.
- Advantages: This system doesn't require separate sensors in each wheel, so it has no batteries to replace.
This video demonstrates how to identify a tire pressure monitoring system: 1mDillAirControlsYouTube · Mar 7, 2013
What to do when you see the light
- 1. Check your tires: Use a tire pressure gauge to manually check the air pressure in all four tires as soon as possible.
- 2. Find the recommended pressure: The correct pressure is usually on a sticker located on the driver's side door jamb.
- 3. Inflate or deflate: Adjust the tire pressure to the recommended PSI.
- 4. Reset the system (if needed): After adjusting the pressure, you may need to press a reset button in your car. Driving for a short time may also be necessary for the system to recognize the new pressure.
- 5. Seek professional help: If the light comes back on shortly after, there may be a slow leak or a faulty sensor that needs to be checked by a mechanic.
How accurate is a tire pressure monitoring system?
Tire Pressure Monitoring Systems (TPMS) are generally accurate, with studies showing differences of only a few PSI from a master gauge, and systems are often accurate to within 1 psi. However, TPMS is best used as a warning system rather than a replacement for manual checks, as sensors can malfunction or lose accuracy over time, and the system may not alert you to gradual pressure loss or small changes.
Accuracy and performance
- Close to actual pressure: A study by AAA Newsroom found the average percentage difference between a TPMS reading and actual pressure was between 1.2%1.2 %1.2% and -1.5%negative 1.5 %−1.5%.
- Within 1-3 PSI: Direct TPMS sensors are generally accurate to within 1 psi, although some difference of up to +/- 3 psi can be acceptable.
- System limitations: The system's primary function is to warn you when pressure is critically low, not to provide a precise reading at all times.
When to check manually
- Regularly: It is still important to manually check your tire pressure with a gauge at least once a month.
- Consider gradual changes: A TPMS will not always detect gradual pressure loss or issues like a slow leak from the valve stem.
- To ensure optimal performance: Manual checks ensure your tires have the optimal pressure for fuel efficiency and tire lifespan, as the TPMS is not a substitute for this.
What to do if you suspect an issue
- Check with a trusted gauge: Use a quality manual gauge to double-check the TPMS reading, especially if it seems off.
- Calibrate the system: Make sure your TPMS is calibrated, which is often done during regular service.
- Replace sensors if needed: TPMS sensors have a battery that typically lasts 5 to 10 years and may need to be replaced as they age.
Why does my car say low tire pressure but tires are fine?
A car may show a low tire pressure warning even if the tires seem fine due to temperature changes, a need for a TPMS reset, or a system malfunction. To address this, manually check and adjust the tire pressure using a gauge, as cold weather can lower pressure and the TPMS may need time or a manual reset to recalibrate. If the light remains on, a failing sensor is a likely cause, and a professional diagnosis may be necessary.
Potential causes and solutions
This video explains common reasons for the TPMS light to stay on: 54sFixITYouTube · Jan 18, 2025
- Temperature changes: Cold weather can cause tire pressure to drop, triggering the warning light. As you drive, the tires will warm up, increasing pressure, and the light should turn off.
- TPMS needs a reset: After adjusting tire pressure, the system may need a manual reset. Check your owner's manual for the specific procedure, which usually involves a button or menu option.
- Delayed recalibration: The system may need to drive for a few miles at a certain speed to recognize the new pressure and turn off the light.
- Slow puncture: A slow leak might not be visually obvious, but it can still cause the warning light to come on over time.
- Malfunctioning TPMS sensor: The sensors in the wheels can fail over time and send a false warning.
- Incorrect inflation pressure: It's best to check pressure when tires are cold using a reliable gauge and inflate them to the PSI recommended on the driver's side door jamb, not the maximum pressure listed on the tire itself.
What to do
- 1. Manually check all four tires: Use a reliable pressure gauge to check all tires, including the spare, while they are cold.
- 2. Inflate to the correct pressure: Add air to reach the recommended PSI listed on the sticker in your driver's side door jamb. Be careful not to overinflate.
- 3. Drive the vehicle: Drive for a few miles to allow the TPMS to recalibrate. Some vehicles may require a manual reset.
- 4. Reset the system: If the light is still on, consult your owner's manual to reset the TPMS light.
- 5. Get professional help: If the light remains on after these steps, a sensor may be faulty, and you should have the system professionally diagnosed.
This video shows how to reset the tire pressure indicator: 59sHondaYouTube · May 15, 2019
Does the car automatically detect the tire pressure or in some time?
As the acronym suggests, TPMS monitors tire pressure, and it does so automatically. Depending on the vehicle, you may even be able to see individual tire pressure readings. If tire pressure drops 25% from the set pressure, depending on the vehicle, a warning light will come on to alert you.
