Introduction:
The P0720 diagnostic trouble code (DTC) indicates a malfunction within the output speed sensor circuit of your vehicle's transmission. This sensor plays a crucial role in monitoring the speed of the transmission's output shaft, providing vital data to the engine control unit (ECU) or transmission control module (TCM) for proper gear shifting and overall vehicle performance. A faulty output speed sensor or circuit can lead to various issues, including erratic shifting, limp mode, and even complete transmission failure. Understanding the causes, symptoms, and troubleshooting steps associated with P0720 is essential for diagnosing and resolving this problem effectively.
Comprehensive Table: P0720 Output Speed Sensor Circuit Malfunction
| Category | Description | Possible Solutions |
|---|---|---|
| Definition | P0720 indicates that the vehicle's ECU/TCM has detected an irregular or out-of-range signal from the output speed sensor circuit. This signal is used to determine the rotational speed of the transmission's output shaft, which is then compared to other speed sensors (like the input speed sensor and wheel speed sensors) to manage gear shifts. | N/A |
| Common Symptoms | Erratic or harsh shifting, failure to shift, limp mode (reduced engine power), speedometer malfunction, Check Engine Light illuminated, transmission slipping, increased fuel consumption, difficulty starting the vehicle, ABS light may illuminate, Traction Control light may illuminate. | Address the root cause of the P0720 code. |
| Possible Causes | Faulty output speed sensor, damaged or corroded wiring/connectors in the output speed sensor circuit, shorted or open circuits, issues with the tone ring (reluctor ring) on the output shaft, low transmission fluid level, contaminated transmission fluid, faulty ECU/TCM (rare), mechanical transmission problems, debris interfering with sensor operation, incorrect sensor installation. | Diagnose and repair the specific cause. |
| Diagnostic Steps | 1. Scan for DTCs and freeze frame data. 2. Inspect wiring and connectors for damage. 3. Check transmission fluid level and condition. 4. Test the output speed sensor using a multimeter. 5. Inspect the tone ring for damage. 6. Monitor sensor data with a scan tool while driving. 7. Perform a wiggle test on the wiring harness. 8. Check for TSBs (Technical Service Bulletins). 9. Check the sensor's resistance against the manufacturer's specifications. 10. Verify proper sensor mounting. | Follow a systematic diagnostic approach to identify the root cause. |
| Sensor Types | Variable Reluctance Sensor (VRS): Generates an AC voltage signal proportional to the speed of the rotating tone ring. Hall Effect Sensor: Uses a magnetic field and a semiconductor to generate a digital signal indicating the speed of the tone ring. | Identify the sensor type used in your vehicle for accurate testing. |
| Tone Ring/Reluctor Ring | A toothed ring attached to the output shaft that interacts with the output speed sensor. Damage to the tone ring (bent, cracked, or missing teeth) can cause inaccurate speed readings. It can be integrated into the output shaft or be a separate component. | Inspect the tone ring carefully for any signs of damage. |
| Wiring & Connectors | The wiring harness and connectors connecting the output speed sensor to the ECU/TCM are crucial for transmitting the sensor's signal. Corrosion, damage, or loose connections can disrupt the signal and trigger the P0720 code. This includes checking for shorts to ground or shorts to power within the circuit. | Thoroughly inspect wiring and connectors, repair or replace as needed. |
| Transmission Fluid | Low or contaminated transmission fluid can affect the internal components of the transmission, including the output speed sensor. Proper fluid level and condition are essential for optimal transmission performance. Check for burnt smell or unusual color. | Check and correct the transmission fluid level. Perform a transmission fluid flush if necessary. |
| ECU/TCM | In rare cases, a faulty ECU/TCM can misinterpret the signal from the output speed sensor, leading to the P0720 code. This is usually diagnosed after ruling out all other possible causes. Internal failures within the module can cause incorrect processing of the sensor signal. | Consult with a qualified technician for ECU/TCM diagnosis and potential replacement or reprogramming. |
| Potential Consequences of Ignoring | Continued driving with a P0720 code can lead to further transmission damage, potentially requiring costly repairs or complete transmission replacement. Limp mode can severely restrict vehicle performance and create unsafe driving conditions. Poor fuel economy is also a potential consequence. | Address the problem promptly to prevent further damage. |
| Testing the Sensor | Resistance Test: Use a multimeter to measure the resistance across the sensor terminals. Compare the reading to the manufacturer's specifications. Voltage Test (VRS): With the wheels turning (safely supported), measure the AC voltage output from the sensor. Signal Test (Hall Effect): With the wheels turning (safely supported), check for a square wave signal on a scope or scan tool. | Refer to the vehicle's service manual for specific testing procedures and values. |
| Tools Required | OBD-II scanner, multimeter, wiring diagram, socket set, wrench set, jack and jack stands (for safe wheel turning), transmission fluid, rags, safety glasses, gloves, potentially a oscilloscope for detailed signal analysis. | Gather the necessary tools before starting the diagnosis. |
| Related DTCs | P0721 (Output Shaft Speed Sensor Range/Performance), P0722 (Output Shaft Speed Sensor No Signal), P0723 (Output Shaft Speed Sensor Intermittent), P0500 (Vehicle Speed Sensor Malfunction), transmission-related shift solenoid codes, ABS codes related to wheel speed sensors. | Consider the possibility of related issues affecting the output speed sensor circuit. |
Detailed Explanations:
Definition:
The P0720 code signifies that the vehicle's computer has detected an abnormal signal from the output speed sensor circuit. This sensor's primary function is to monitor the speed at which the transmission's output shaft is rotating. The ECU/TCM uses this information to manage gear shifts and other transmission-related functions, comparing it to the input speed sensor and wheel speed sensors. When the signal falls outside the expected range or is erratic, the P0720 code is triggered.
Common Symptoms:
Several symptoms can indicate a P0720 code. These include erratic or harsh shifting, where the transmission shifts roughly or at inappropriate times. The transmission might fail to shift altogether, leaving the vehicle stuck in one gear. Limp mode, a safety feature that reduces engine power, might be activated. The speedometer may malfunction, displaying inaccurate readings. The Check Engine Light will illuminate, alerting you to the problem. Other potential symptoms include transmission slipping, increased fuel consumption, difficulty starting the vehicle, and potentially illuminated ABS and Traction Control lights.
Possible Causes:
The causes of a P0720 code can range from simple to complex. A faulty output speed sensor is a common culprit. Damaged or corroded wiring and connectors in the sensor circuit can disrupt the signal. Shorted or open circuits can also cause the code. Issues with the tone ring (reluctor ring) on the output shaft, such as damage or debris, can interfere with the sensor's readings. Low or contaminated transmission fluid can affect internal transmission components. In rare cases, a faulty ECU/TCM or mechanical transmission problems might be the cause. Incorrect sensor installation can also trigger the code.
Diagnostic Steps:
Diagnosing a P0720 code requires a systematic approach. Start by scanning for DTCs and freeze frame data using an OBD-II scanner. Inspect the wiring and connectors for any signs of damage or corrosion. Check the transmission fluid level and condition. Test the output speed sensor using a multimeter to check its resistance and signal output. Inspect the tone ring for damage or debris. Monitor sensor data with a scan tool while driving to see how the sensor is performing in real-time. Performing a wiggle test on the wiring harness can help identify intermittent connection problems. Check for TSBs (Technical Service Bulletins) related to the P0720 code for your vehicle. Verify proper sensor mounting to ensure correct operation.
Sensor Types:
There are two primary types of output speed sensors: Variable Reluctance Sensors (VRS) and Hall Effect Sensors. VRS sensors generate an AC voltage signal that is proportional to the speed of the rotating tone ring. Hall Effect sensors use a magnetic field and a semiconductor to produce a digital signal indicating the speed of the tone ring. Identifying the type of sensor used in your vehicle is crucial for accurate testing and diagnosis.
Tone Ring/Reluctor Ring:
The tone ring, also known as a reluctor ring, is a toothed ring attached to the output shaft. It interacts with the output speed sensor to generate the speed signal. Damage to the tone ring, such as bent, cracked, or missing teeth, can cause inaccurate speed readings and trigger the P0720 code. It's important to carefully inspect the tone ring for any signs of damage.
Wiring & Connectors:
The wiring harness and connectors that connect the output speed sensor to the ECU/TCM are essential for transmitting the sensor's signal. Corrosion, damage, or loose connections can disrupt the signal and trigger the P0720 code. This includes checking for shorts to ground (where the wire is making an unintended electrical connection to the vehicle's chassis) or shorts to power (where the wire is making an unintended electrical connection to a power source) within the circuit. Thoroughly inspect the wiring and connectors, repairing or replacing them as needed.
Transmission Fluid:
Low or contaminated transmission fluid can negatively affect the internal components of the transmission, including the output speed sensor. Proper fluid level and condition are vital for optimal transmission performance. Check for a burnt smell or unusual color in the transmission fluid, as these can indicate problems within the transmission.
ECU/TCM:
In rare cases, a faulty ECU/TCM can misinterpret the signal from the output speed sensor, leading to the P0720 code. This is usually diagnosed after ruling out all other possible causes. Internal failures within the module can cause incorrect processing of the sensor signal. Consulting with a qualified technician for ECU/TCM diagnosis and potential replacement or reprogramming is recommended.
Potential Consequences of Ignoring:
Ignoring a P0720 code can have serious consequences. Continued driving with the problem can lead to further transmission damage, potentially requiring costly repairs or even complete transmission replacement. Limp mode can severely restrict vehicle performance and create unsafe driving conditions. Poor fuel economy is also a potential consequence of a malfunctioning output speed sensor.
Testing the Sensor:
Testing the output speed sensor involves several methods. A resistance test uses a multimeter to measure the resistance across the sensor terminals, which should be compared to the manufacturer's specifications. A voltage test (VRS), performed with the wheels turning (safely supported), measures the AC voltage output from the sensor. A signal test (Hall Effect), also performed with the wheels turning (safely supported), checks for a square wave signal using an oscilloscope or scan tool.
Tools Required:
Diagnosing and repairing a P0720 code typically requires several tools. These include an OBD-II scanner to read the DTCs, a multimeter for electrical testing, a wiring diagram to understand the sensor circuit, a socket set and wrench set for removing and installing components, jack and jack stands for safely lifting the vehicle and turning the wheels, transmission fluid for topping off or replacing the fluid, rags for cleaning, and safety glasses and gloves for personal protection. An oscilloscope might be needed for detailed signal analysis.
Related DTCs:
Several other DTCs can be related to the P0720 code. These include P0721 (Output Shaft Speed Sensor Range/Performance), P0722 (Output Shaft Speed Sensor No Signal), P0723 (Output Shaft Speed Sensor Intermittent), P0500 (Vehicle Speed Sensor Malfunction), transmission-related shift solenoid codes, and ABS codes related to wheel speed sensors. Considering the possibility of related issues affecting the output speed sensor circuit is essential for accurate diagnosis.
Frequently Asked Questions:
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What does the P0720 code mean? It indicates a malfunction in the output speed sensor circuit, affecting transmission performance.
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Can I drive with a P0720 code? Driving is not recommended as it can lead to further transmission damage and unsafe driving conditions.
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How do I fix a P0720 code? Diagnosis and repair involve inspecting wiring, testing the sensor, and addressing any underlying transmission issues.
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How much does it cost to fix a P0720 code? The cost varies depending on the cause, ranging from a sensor replacement to more extensive transmission repairs.
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Is the output speed sensor the same as the vehicle speed sensor? While related, they are not always the same. The output speed sensor measures the transmission's output shaft speed, while the vehicle speed sensor often uses a wheel speed sensor.
Conclusion:
The P0720 code indicates a problem with the output speed sensor circuit, which can lead to various transmission-related issues. A thorough diagnosis and prompt repair are crucial to prevent further damage and ensure safe vehicle operation.