How To Know If Tpms Sensor Is Bad

The Tire Pressure Monitoring System (TPMS) is a critical safety feature in modern vehicles, designed to alert the driver to under-inflated tires. A properly functioning TPMS can improve fuel efficiency, extend tire life, and most importantly, prevent accidents. However, like any electronic system, TPMS sensors can fail over time. Knowing how to diagnose a faulty TPMS sensor is a valuable skill for any experienced DIYer, saving you time and money on unnecessary trips to the mechanic.

TPMS Background: Ensuring Tire Safety

Before diving into diagnostics, it's essential to understand the purpose and evolution of TPMS. Tire pressure monitoring systems were mandated in the United States following the Firestone tire recall in the late 1990s and early 2000s, which were linked to numerous accidents involving Ford Explorers. The goal was to provide drivers with a real-time warning of dangerously low tire pressure, allowing them to take corrective action before a blowout or loss of control.

There are two main types of TPMS:

• Direct TPMS: This system uses pressure sensors inside each tire, directly measuring the air pressure and transmitting that data wirelessly to the vehicle's computer (ECU - Engine Control Unit).
• Indirect TPMS: This system relies on the vehicle's ABS (Anti-lock Braking System) wheel speed sensors to detect changes in tire diameter. An under-inflated tire has a smaller diameter and therefore rotates at a slightly different speed than the other tires.

This article focuses on direct TPMS, as it's more common in newer vehicles and offers more accurate pressure readings. Direct TPMS also requires more involved troubleshooting when issues arise.

Technical Breakdown: How Direct TPMS Works

A direct TPMS consists of several key components working in concert:

• Tire Pressure Sensors: These sensors are typically mounted inside the tire, either strapped to the wheel or integrated into the valve stem. They contain a pressure sensor, a temperature sensor (in some cases), a transmitter, and a battery. The sensor continuously monitors tire pressure and transmits this data wirelessly.
• Receiver: The vehicle is equipped with a receiver, typically part of the ECU or a dedicated TPMS control module, that picks up the radio frequency (RF) signals from the sensors.
• ECU/TPMS Control Module: This module processes the data received from the sensors, compares it to pre-set thresholds, and triggers a warning light on the dashboard if the pressure in one or more tires falls below the specified level.
• Dashboard Indicator: A dedicated TPMS warning light (often a horseshoe-shaped symbol with an exclamation point) illuminates on the dashboard to alert the driver to low tire pressure. Some vehicles also display individual tire pressures on the instrument cluster.

The communication between the sensor and the receiver uses radio frequency (RF) signals. Each sensor has a unique ID, allowing the system to identify which tire is reporting the pressure. The sensors transmit data periodically, typically every few minutes, and more frequently when a significant pressure change is detected. The battery life of the sensors is typically 5-10 years, depending on usage and environmental conditions.

Diagnosing a Bad TPMS Sensor: Step-by-Step

Here's a breakdown of how to determine if a TPMS sensor is faulty:

1. Visual Inspection: Begin with a visual inspection of the tires and valve stems. Look for any signs of damage, such as cuts, bulges, or leaks. Check the valve stem for corrosion or damage, as this can interfere with the sensor's ability to seal properly. Pay close attention to the TPMS valve stem; these can be more fragile than standard valve stems.
2. Check Tire Pressure Manually: Use a reliable tire pressure gauge to check the pressure in each tire. Compare these readings to the pressures displayed (if available) on the vehicle's instrument cluster, and the recommended pressures on the sticker inside the driver's side doorjamb. Significant discrepancies between the manual readings and the TPMS readings can indicate a faulty sensor.
3. Scan for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner to check for TPMS-related DTCs. Common codes include:
   • C0700-C0799: TPMS Sensor Malfunction
   • C0705: Tire Pressure Sensor 1 Malfunction
   • C0710: Tire Pressure Sensor 2 Malfunction
   • C0715: Tire Pressure Sensor 3 Malfunction
   • C0720: Tire Pressure Sensor 4 Malfunction
   • C0750: Tire Pressure Sensor Location Undetermined

   The DTCs will provide valuable information about which sensor is malfunctioning, or if there are communication issues within the TPMS system. *Note that some generic OBD-II scanners may not read TPMS specific codes. You might need a scanner with enhanced diagnostic capabilities.*

4. TPMS Scan Tool: A dedicated TPMS scan tool is the most effective way to diagnose sensor issues. These tools can:
   • Read sensor data: Pressure, temperature, battery voltage, and sensor ID.
   • Activate sensors: Trigger the sensors to transmit data, allowing you to verify their functionality.
   • Program sensors: Program new sensors with the correct ID and relearn procedure.

   A low battery voltage reading from a sensor is a strong indicator that the sensor needs to be replaced. If a sensor fails to respond to activation, it's also likely faulty. Additionally, a *good* TPMS tool can often perform a "relearn" procedure, where the car relearns the sensor ID locations, which sometimes fixes intermittent issues after tire rotations.

5. Sensor Relearn Procedure: After replacing a TPMS sensor, it's usually necessary to perform a sensor relearn procedure. This procedure allows the vehicle's ECU to recognize the new sensor's ID. There are several types of relearn procedures:
   • Auto Relearn: This procedure requires driving the vehicle for a certain distance or time at a specific speed. The ECU automatically learns the sensor IDs during this process.
   • Stationary Relearn: This procedure requires using a TPMS scan tool to activate each sensor in a specific sequence while the vehicle is stationary.
   • OBD Relearn: This procedure requires using a TPMS scan tool to write the sensor IDs directly to the vehicle's ECU via the OBD-II port.

   The correct relearn procedure will vary depending on the vehicle make and model. Consult the vehicle's service manual or a reliable online resource for the correct procedure.

Common TPMS Issues and Maintenance Concerns

Here are some common issues that can affect TPMS sensor performance:

• Dead Battery: The most common cause of TPMS sensor failure is a dead battery. The battery life is limited and will eventually expire.
• Sensor Damage: Sensors can be damaged by impacts, corrosion, or improper installation.
• Corrosion: Corrosion can occur inside the tire or on the valve stem, affecting the sensor's ability to function properly. Using the correct valve stem caps (typically plastic or aluminum, not brass) can help prevent corrosion.
• Communication Issues: Interference from other electronic devices or damage to the antenna can disrupt communication between the sensors and the receiver.
• Incorrect Installation: Improper installation of the sensors can damage them or prevent them from sealing properly. *Always use the correct torque specifications when tightening the valve stem nut.*
• Relearn Procedure Failure: Sometimes the relearn procedure might fail, causing the TPMS light to remain on even after the sensors have been replaced. This could be due to incorrect procedure, a faulty scan tool, or underlying issues with the TPMS system.

Do's and Don'ts / Best Practices

Here are some best practices for maintaining your TPMS and preventing sensor failure:

• Do: Check your tire pressure regularly (at least once a month) using a reliable tire pressure gauge.
• Do: Rotate your tires regularly to promote even wear and extend the life of your tires and sensors.
• Do: Use the correct valve stem caps to prevent corrosion.
• Do: Have your tires and TPMS sensors inspected by a qualified technician during routine maintenance.
• Do: Use a TPMS-safe tire sealant if you experience a puncture. Some tire sealants can damage TPMS sensors.
• Don't: Ignore the TPMS warning light. Address low tire pressure promptly to prevent accidents and extend tire life.
• Don't: Over-inflate your tires. This can damage the tires and sensors.
• Don't: Use excessive force when installing or removing TPMS sensors.
• Don't: Mix and match different brands or types of TPMS sensors. Use sensors that are specifically designed for your vehicle.
• Don't: Assume a flashing TPMS light indicates low tire pressure. A flashing light often means a system malfunction or sensor failure, requiring professional diagnosis.

Conclusion

Diagnosing a faulty TPMS sensor requires a systematic approach, combining visual inspection, manual pressure checks, and the use of diagnostic tools. While a basic OBD-II scanner can provide some information, a dedicated TPMS scan tool is essential for accurate diagnosis and programming. If you're comfortable with basic automotive repairs and have access to the necessary tools, you can often diagnose and replace TPMS sensors yourself. However, if you're unsure about any aspect of the process, it's best to consult a qualified mechanic. Investing in a good quality TPMS scan tool can pay for itself over time, saving you money on diagnostic fees and allowing you to maintain your vehicle's TPMS system effectively. Remember to always follow proper safety procedures and consult your vehicle's service manual for specific instructions.