How To Tell Which Tpms Is Bad

Tire Pressure Monitoring Systems (TPMS) have become a standard feature in modern vehicles, offering increased safety and fuel efficiency. But like any sensor-based system, TPMS components can fail. Identifying which sensor is faulty can save you time, money, and frustration. This article will guide you through the process of diagnosing a bad TPMS sensor, enabling you to tackle this common issue effectively.

Understanding TPMS Technology

Before diving into diagnostics, it's crucial to understand the basics of how TPMS works. There are two main types of TPMS: direct and indirect.

Direct TPMS

Direct TPMS uses pressure sensors mounted inside each wheel. These sensors directly measure the tire pressure and transmit this data wirelessly to the vehicle's Electronic Control Unit (ECU). The ECU then displays the pressure readings, usually on the instrument cluster. Each sensor has a unique ID, allowing the system to identify which tire is experiencing a pressure issue. Direct TPMS provides accurate, real-time pressure information and can even report tire temperature in some cases.

Indirect TPMS

Indirect TPMS relies on the vehicle's Anti-lock Braking System (ABS) wheel speed sensors. It monitors the rotational speed of each wheel. A deflating tire has a smaller rolling radius, causing it to rotate faster than properly inflated tires. The system detects this difference in wheel speed and infers that a tire is low on pressure. Indirect TPMS is less accurate than direct TPMS because it doesn't directly measure pressure. It's also susceptible to false alerts caused by factors other than low tire pressure, such as uneven tire wear or driving on irregular surfaces.

This article focuses on diagnosing issues with direct TPMS, as it's more common and offers more specific diagnostic opportunities.

Diagnosing a Faulty TPMS Sensor: A Step-by-Step Guide

Identifying a bad TPMS sensor requires a systematic approach. Here's a breakdown of the diagnostic process:

1. Visual Inspection

Start with a simple visual inspection of all tires. Look for obvious signs of damage, such as punctures, cuts, or embedded objects. Check the tire pressure using a reliable handheld pressure gauge. Compare the reading from the gauge with the reading displayed by the TPMS system (if available). Significant discrepancies may indicate a sensor issue. Also, examine the valve stems for corrosion or damage. Corroded valve stems can cause air leaks and contribute to sensor failure.

2. Using a TPMS Scan Tool

A TPMS scan tool is an indispensable tool for diagnosing TPMS problems. These tools can communicate with the TPMS sensors, read sensor data (pressure, temperature, battery status, sensor ID), and trigger the sensors to transmit data. Here's how to use a TPMS scan tool effectively:

1. Wake Up the Sensors: Many TPMS sensors enter a "sleep" mode to conserve battery life. The scan tool can send a signal to "wake up" the sensors and initiate communication.
2. Read Sensor Data: Once the sensors are active, the scan tool can read the sensor's pressure reading, temperature, battery voltage, and unique sensor ID. Compare the pressure readings displayed by the scan tool to the actual tire pressure measured with a handheld gauge. A significant difference points to a faulty sensor. A low battery voltage reading (typically below 2.5 volts) indicates that the sensor's battery is nearing the end of its lifespan and may need replacement.
3. Check for Error Codes: The scan tool can read Diagnostic Trouble Codes (DTCs) related to the TPMS system. Common TPMS-related DTCs include:
   • C0700-C0799: Tire Pressure Monitoring System Malfunction
   • C0701-C0705: Tire Pressure Sensor Circuit Malfunction (specific to each tire position)
   • C0750: TPMS Sensor Battery Low
4. Sensor Location Confirmation: The scan tool can also help you determine if the sensors are correctly registered to the vehicle. If the scan tool displays a sensor ID that doesn't match the tire's actual location, the system may need to be relearned (see Step 4).

3. Troubleshooting Without a Scan Tool

If you don't have access to a TPMS scan tool, you can still perform some basic troubleshooting, although it's less precise:

1. Tire Rotation Test: If you suspect a specific sensor is faulty but are unsure, rotate the tires. After rotation, drive the vehicle for a short distance (typically 10-20 miles) to allow the TPMS system to register the new sensor positions. If the low-pressure warning follows the rotated tire, it strongly suggests that the sensor in that tire is the problem.
2. Slow Leaks: Submerge the tire and wheel in water (or use a spray bottle with soapy water) to check for slow leaks around the valve stem and the tire bead. Small leaks can cause gradual pressure loss and trigger the TPMS warning. If you find a leak, repair it before assuming the sensor is faulty.

4. Relearning (Sensor Programming)

After replacing a TPMS sensor, or after rotating tires, you typically need to perform a relearn procedure to teach the vehicle's ECU the new sensor IDs or positions. There are three main types of relearn procedures:

1. Auto-Relearn: Some vehicles automatically learn the new sensor IDs after driving for a certain distance (typically 10-20 miles). The vehicle's owner's manual will specify the requirements for auto-relearn.
2. Stationary Relearn: This procedure requires a TPMS scan tool. The scan tool is used to manually enter the sensor IDs into the vehicle's ECU.
3. OBD Relearn: This procedure also requires a TPMS scan tool that can connect to the vehicle's OBD (On-Board Diagnostics) port. The scan tool communicates with the ECU to learn the new sensor IDs.

Common TPMS Issues and Maintenance Concerns

• Sensor Battery Life: TPMS sensors are battery-powered, and their batteries typically last 5-10 years. Once the battery dies, the entire sensor needs to be replaced.
• Corrosion: Corrosion can occur around the valve stem, especially in areas with harsh winters or salty road conditions. This corrosion can damage the sensor and cause air leaks.
• Physical Damage: TPMS sensors can be damaged by impacts with potholes, curbs, or other road hazards.
• Improper Installation: Incorrect installation of TPMS sensors can lead to premature failure. Always follow the manufacturer's instructions carefully when installing new sensors.
• Compatibility Issues: Ensure that the replacement TPMS sensors are compatible with your vehicle's make, model, and year. Using incompatible sensors can cause communication problems and system malfunctions.

Do's and Don'ts / Best Practices

• Do use a TPMS scan tool for accurate diagnostics.
• Do check the tire pressure regularly using a handheld gauge, even if your vehicle has TPMS.
• Do replace all TPMS sensors at the same time when one sensor fails, especially if they are all of similar age. This helps prevent future issues and ensures consistent performance.
• Do use high-quality replacement TPMS sensors from reputable manufacturers.
• Don't ignore TPMS warnings. Low tire pressure can affect handling, braking, and fuel efficiency.
• Don't overtighten the valve stem nut when installing TPMS sensors. This can damage the sensor and cause air leaks.
• Don't use tire sealants in tires equipped with TPMS sensors. Sealants can damage the sensors and clog the valve stems.

Conclusion

Diagnosing a faulty TPMS sensor requires a systematic approach, combining visual inspection, the use of a TPMS scan tool (if available), and basic troubleshooting techniques. While some issues, like low tire pressure or a simple slow leak, can be easily resolved, others may require sensor replacement and relearning. If you're comfortable with basic automotive repairs, you can often diagnose and replace TPMS sensors yourself. However, if you're unsure or lack the necessary tools, it's best to consult a qualified mechanic. By understanding how TPMS works and following the steps outlined in this article, you can confidently identify and address TPMS problems, ensuring the safety and efficiency of your vehicle.