Technical Description of OBD-II Trouble Code
Malfunction in the Circuit of the Output Speed Sensor
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to vehicles equipped with OBD-II. While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
What does that mean?
In electronically controlled transmissions, the Output Shaft Speed Sensor (OSS) is located at the end of the transmission before the drive axles or driveshaft. Its purpose is to measure the speed of the output shaft, which is used to determine shift scheduling, torque converter engagement schedule, and Electronic Pressure Control (EPC) within the transmission. The Powertrain Control Module (PCM) or Engine Control Module (ECM) has detected a malfunction with the OSS sensor. If the OSS sensor information is unavailable, the PCM/ECM may use values from other sensors to determine shift timing, torque converter operation, and EPC pressure. The OSS sensor is also commonly used for the speedometer.
Note: In some vehicles, the OSS sensor is connected to the Transmission Fluid Temperature (TFT) sensor. If there is a failure with the TFT sensor or its associated wiring, the OSS sensor may also malfunction. If there is another code related to both the TFT sensor and the OSS sensor, there may be a possible problem with either sensor or the wiring between them.
This code is similar to codes P0721, P0722, and P0723.
Symptoms
Symptoms of a P0720 trouble code may include:
Potential Causes
Possible causes of this DTC may include:
Diagnostic / Repair Steps
Output shaft speed sensor – The OSS sensor operates by passing an electrical current through the sensor. As the reluctor ring or gear on the end of the transmission output shaft rotates, the voltage changes in relation to the speed. This change in voltage is usually displayed as a frequency or a voltage that corresponds to the speed of the shaft rotation. Some vehicles use a plastic gear on the end of the OSS sensor. To determine if the teeth are missing or worn, remove the OSS sensor from the end of the transmission and inspect the condition of the gear. Replace the gear if necessary. Check the wiring harness for any signs of damage or loose connections. To check the resistance of the OSS, remove the harness connector and use a Digital Volt Ohm Meter (DVOM) set to the ohms scale, placing each lead on one of the terminal connectors. The resistance should be within the specification determined by the factory shop manual.
If the resistance is out of range or over the limit (OL), replace the sensor. Check for power going into the sensor and at the PCM harness connector using a factory wiring diagram as a reference. If the OSS sensor is connected to the TFT sensor, the voltage will vary based on the temperature of the transmission fluid. Use a graphing volt meter or oscilloscope set to the duty cycle scale with the positive lead on the signal side of the sensor (either after the sensor or at the PCM/ECM harness connector) and the negative lead on a known good ground. The duty cycle will only be displayed while the vehicle is in motion or the tires are spinning. If the duty cycle has any dropouts in voltage or the voltage is outside the manufacturer’s specifications, suspect excessive resistance in the TFT sensor, wiring harness, or the OSS sensor itself. If an advanced scan tool is available, the OSS sensor signal can also be monitored while driving the vehicle.
Electrical wiring – Check the wiring harness for any loose connections, loose wiring, or dirty terminals. Unplug each of the harness connections from the vehicle wiring harness, output shaft speed sensor, TFT sensor (if equipped), and the PCM/ECM. Use the DVOM set to the ohms scale, with one lead on each end of the harness connectors, using the factory wiring diagram as a reference, to check for excessive resistance. If the resistance is very high, trace the wiring back through the circuit between each connector to determine the cause. If the resistance indicates an over limit (OL) reading, look for any broken or damaged wiring. Check for a short to ground in the wiring harness sections by moving either the positive or negative lead to a known good ground.
The volt meter should indicate an OL reading. If there is resistance present, there may be a short to ground, or the wiring harness should be checked for pinched or damaged wiring, or areas that could be rubbed through and touching the vehicle chassis. Check for a short to power by switching the DVOM to the volts scale, with the positive lead on the wire being checked and the negative lead on a known good ground with the key in the run or on position. No voltage should be present. If there is voltage present, there may be a short to power. Check the wiring harness for any pinched or damaged wires that may have come in contact with each other.
Transmission fluid temperature sensor – On vehicles equipped with a TFT sensor in-line with the OSS sensor, the TFT sensor can be checked for resistance by using the DVOM set to the ohms scale. With the transmission wiring harness connector disconnected, use the factory wiring diagram as a reference to locate the appropriate wires to the TFT sensor at the transmission side of the connector. Place the positive and negative leads on either of the two wires, and the DVOM should read resistance within the factory specifications. If the reading is outside the specifications or over the limit (OL), replace the TFT sensor inside the transmission.
Check for a short to ground by moving either of the leads to a known good ground. The volt meter should read very high resistance or over the limit (OL). If there is very little resistance found, suspect a short to ground or a short inside the transmission internal wiring harness. Check the vehicle side of the transmission harness connector for power from the PCM/ECM using the factory wiring diagram as a reference. Set the DVOM to the volts scale with the positive lead on the power supply wire from the PCM/ECM and the negative lead on a good known ground. The voltage reading should be battery voltage or 5 volts reference voltage. If the voltage is outside the specifications, suspect a possible short to ground, faulty PCM/ECM, or a problem in the wiring between the transmission and the PCM/ECM.
