Understanding the OBD-II P0720 Code: Output Speed Sensor Circuit Malfunction
The OBD-II error code P0720, a common diagnostic trouble code in modern vehicles, signals a “Output Speed Sensor Circuit Malfunction”. This generic powertrain code applies to vehicles equipped with OBD-II systems, indicating an issue within the output speed sensor (OSS) circuit. While the code is broadly applicable, specific repair procedures can differ based on the vehicle’s make and model. As an automotive repair expert at autelfrance.com, I’m here to break down what this code means, its symptoms, potential causes, and diagnostic steps to resolve it effectively.
Decoding the P0720 Error
In vehicles with electronically controlled transmissions, the Output Shaft Speed Sensor (OSS) plays a crucial role. Positioned at the transmission’s output end, before the drive axles or driveshaft, the OSS monitors the output shaft’s rotational speed. This data is vital for the Powertrain Control Module (PCM) or Engine Control Module (ECM) to manage various transmission functions. These include:
- Shift Scheduling: Determining the optimal times for gear changes.
- Torque Converter Engagement: Controlling when the torque converter locks up for improved efficiency.
- Electronic Pressure Control (EPC): Regulating hydraulic pressure within the transmission for smooth operation.
When the PCM/ECM detects an anomaly in the OSS sensor’s circuit, it triggers the Obdii Error P0720. This malfunction means the control module is not receiving a consistent or plausible signal from the OSS, indicating a potential issue within the sensor itself or its associated circuitry. In the absence of reliable OSS data, the PCM/ECM might use estimations based on other sensor inputs to maintain basic transmission functionality. However, this can lead to compromised performance and potential drivability issues. Notably, the OSS is also frequently used to drive the speedometer, meaning a P0720 code can also impact your vehicle’s speed indication.
Alt: Location of the Output Speed Sensor (OSS) on a vehicle’s transmission, highlighting its role in monitoring output shaft speed for the engine control module.
Important Note: On certain vehicle models, the OSS may be integrated with the Transmission Fluid Temperature (TFT) sensor. Therefore, if a TFT sensor failure or wiring problem exists, it can indirectly cause an OSS malfunction and trigger the P0720 code. If you encounter additional codes related to the TFT sensor alongside P0720, investigate potential issues with both sensors and their shared wiring. This code is related to other OSS circuit malfunction codes such as P0721, P0722, and P0723, which may indicate similar but distinct issues within the OSS circuit range or performance.
Symptoms of a P0720 OBDII Error
When your vehicle logs a P0720 obdii error code, you may observe several noticeable symptoms, including:
- Delayed or Erratic Shifts: The transmission may hesitate or shift roughly between gears due to incorrect speed input.
- Non-Functioning Speedometer: The speedometer might stop working or display inaccurate readings as the OSS signal is often used for speed measurement.
- Check Engine Light (Malfunction Indicator Lamp) Illumination: The most immediate sign is the activation of the check engine light on your dashboard, indicating a detected problem within the vehicle’s system.
Alt: Dashboard view showing the Check Engine Light illuminated, a common indicator of OBD-II error codes like P0720, signaling a potential issue with the vehicle’s output speed sensor circuit.
These symptoms can impact driving comfort and potentially vehicle safety. Addressing the P0720 code promptly is crucial to prevent further complications and ensure optimal vehicle performance.
Potential Causes of the P0720 Code
Several factors can contribute to the triggering of the obdii error p0720. Pinpointing the exact cause is essential for effective repair. Common culprits include:
- Faulty Output Shaft Speed Sensor (OSS): The sensor itself may be defective due to wear, damage, or internal failure.
- Electrical Wiring or Connection Issues: Problems within the OSS circuit wiring, such as damaged wires, corroded connectors, loose connections, or shorts, can disrupt the signal and cause the error.
- Transmission Fluid Temperature (TFT) Sensor Malfunction: In vehicles where the OSS and TFT sensors are linked, a faulty TFT sensor or its wiring can indirectly affect the OSS reading and trigger the P0720 code.
Diagnosing and Repairing the P0720 Error
Diagnosing a P0720 obdii error requires a systematic approach to identify the root cause. Here are step-by-step diagnostic and repair procedures:
1. Output Shaft Speed Sensor Inspection:
- Sensor Operation: The OSS typically functions by using a Hall effect sensor or a magnetic pickup that generates a signal as a reluctor ring or gear on the transmission output shaft rotates. This signal, usually a frequency or voltage, corresponds to the shaft speed.
- Gear Check: Some OSS units utilize a plastic gear. Remove the sensor and inspect this gear for missing or worn teeth. Replace the gear if necessary.
- Wiring Harness Examination: Inspect the wiring harness connected to the OSS for any signs of damage, fraying, or loose connections.
- Resistance Test: Disconnect the OSS harness connector and use a Digital Volt Ohm Meter (DVOM) set to ohms. Measure the resistance across the sensor terminals. Refer to the vehicle’s factory shop manual for the specified resistance range. An out-of-range or open circuit (OL) reading indicates a need to replace the sensor.
- Power and Signal Check: Use a factory wiring diagram to check for power supply to the sensor and signal integrity at both the sensor and PCM/ECM harness connectors.
- Duty Cycle Monitoring (Advanced): With a graphing voltmeter or oscilloscope set to duty cycle, monitor the signal wire while the wheels are rotating. Look for voltage dropouts or deviations from manufacturer specifications, which could indicate wiring issues, TFT sensor problems, or a failing OSS. Advanced scan tools can also monitor the OSS signal while driving.
2. Electrical Wiring Troubleshooting:
- Visual Inspection: Carefully examine the entire wiring harness for loose connections, damaged wires, and corroded terminals.
- Continuity Test: Disconnect harness connectors at the OSS, TFT sensor (if applicable), and PCM/ECM. Using a DVOM in ohms mode and a wiring diagram, check for continuity and excessive resistance in each wire of the OSS circuit. High resistance suggests wire damage. An OL reading points to a broken wire.
- Short to Ground Test: With the DVOM in ohms mode, check for shorts to ground by testing each wire in the OSS circuit against a known good ground. An OL reading is expected. Resistance indicates a short to ground, possibly due to pinched or chafed wiring touching the vehicle chassis.
- Short to Power Test: Switch the DVOM to volts mode. With the key in the “Run” position, check for voltage on each wire in the OSS circuit with respect to a good ground. No voltage should be present. Voltage indicates a short to power, potentially from damaged wires contacting other circuits.
3. Transmission Fluid Temperature (TFT) Sensor Evaluation (If Applicable):
- Resistance Check: Disconnect the transmission wiring harness. Using a DVOM in ohms mode and a wiring diagram, measure the resistance across the TFT sensor terminals on the transmission side of the connector. Compare the reading to factory specifications. Replace the TFT sensor if the resistance is out of range or OL.
- Short to Ground Test: With the DVOM in ohms mode, check for a short to ground from either TFT sensor terminal to a known good ground. An OL reading is expected. Low resistance suggests a short to ground, possibly in the internal transmission wiring harness.
- Power Supply Check: On the vehicle side of the harness connector, check for power supply from the PCM/ECM to the TFT sensor using a DVOM in volts mode. The voltage should be battery voltage or a 5-volt reference voltage. Incorrect voltage could indicate a PCM/ECM issue or wiring problems.
By systematically following these diagnostic steps, you can effectively pinpoint the cause of the P0720 obdii error code and implement the necessary repairs, whether it’s replacing the OSS sensor, addressing wiring issues, or resolving TFT sensor problems. Always consult your vehicle’s service manual for specific procedures and specifications.
Need Further Assistance with P0720?
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Disclaimer: This information is for informational purposes only and should not be considered professional repair advice. autelfrance.com is not liable for any actions taken based on this information. Always consult a qualified technician for vehicle repairs.
