What does it mean?
On electronically controlled transmissions, the output shaft speed (OSS) sensor is located at the transmission end before the drive axles or driveshaft to read the output shaft speed, which can be used to determine shift programming, torque converter clutch engagement timing, and Electronic Pressure Control (EPC) within the transmission. The Powertrain Control Module / Engine Control Module (PCM / ECM) has determined there is a malfunction with the OSS sensor. The PCM / ECM can substitute values based on other sensors to determine shift timing, torque converter operation, and EPC pressure if the OSS sensor information is unavailable. The OSS sensor is also commonly used for the speedometer.
Note: On some vehicles, the OSS sensor is aligned with the Transmission Fluid Temperature (TFT) sensor; if the TFT sensor or its associated wiring fails, the OSS sensor may also malfunction. If there is another code associated with the TFT sensor along with the OSS sensor, suspect a possible issue 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:
Delayed shifts
Speedometer not working
Check Engine Light on (Malfunction Indicator Lamp)
Potential causes
Causes
of this DTC may include:
Output shaft speed sensor
Electrical wiring or connections
Transmission fluid temperature sensor
Diagnostic steps
/ repair
Output shaft speed sensor – The OSS sensor operates by passing an electric current through the sensor; when the reluctor ring or gear at the transmission output shaft end rotates, the voltage changes according to speed. It is typically displayed as a frequency or voltage corresponding to the shaft’s rotational speed. Some vehicles use a plastic gear at the OSS sensor end. Remove the OSS sensor from the transmission end and check the gear’s condition to see if teeth are missing or worn, replacing the gear if necessary. Check the wiring harness for any signs of damaged wiring or loose connections. Check the OSS resistance by disconnecting the harness connector and using a digital volt-ohm meter (DVOM) set to the ohms scale, with one lead on each of the terminal connectors. The resistance should meet the specifications determined by the factory service manual.
If the resistance is out of range or above the limit (OL), replace the sensor. Verify that power is reaching the sensor and at the PCM harness connector using a factory wiring diagram as a reference. If the OSS sensor is in line with the TFT sensor, the voltage will vary based on transmission fluid temperature. Use a graphing voltmeter 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 to a known good ground. The duty cycle only displays when the vehicle is moving or the tires are rotating. If the duty cycle shows voltage drops or the voltage is outside manufacturer 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 that the wiring harness has no loose connections, loose wiring, or dirty terminals. Disconnect each of the harness connections from the vehicle’s wiring harness, the output shaft speed sensor, the TFT sensor if applicable, and the PCM/ECM. Using the DVOM set to the ohms scale, with one lead at each end of the harness connectors using the factory wiring diagram as a reference, check for excessive resistance. If the resistance is very high, trace the wiring through the circuit between each connector to determine the cause. If the resistance indicates an upper limit reading (OL), look for any broken or damaged wiring. Check for a short to ground in the wiring harness sections by moving the positive or negative lead to a known good ground.
The voltmeter should read OL; if there is resistance, there may be a short to ground, and the wiring harness should be checked for pinched or damaged wiring or areas that might be rubbing against 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 on or run position. No voltage should be present; if voltage is present, there may be a short to power – check the wiring harness for any pinched or damaged wires that might have come into 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 using the DVOM set to the ohms scale. With the transmission wiring harness connector, using the factory wiring diagram as a reference to locate the appropriate wires for the TFT sensor on the transmission side of the connector and the positive and negative leads on one of the two wires, the DVOM should read resistance according to factory specifications. If the reading is out of specifications or above the limit (OL), replace the TFT sensor inside the transmission.
Check for a short to ground by moving one of the leads to a known good ground; the voltmeter should read very high resistance or an upper limit (OL); if you find very little resistance, suspect a short to ground or a short inside the transmission’s internal wiring harness. Check the vehicle side of the transmission harness connector for PCM/ECM power using the factory wiring diagram as a reference. Set the DVOM to a volts scale with the positive lead on the PCM/ECM power wire and the negative lead on a known good ground; the voltage reading should be battery voltage or 5-volt reference voltage. If the voltage is not within specifications, suspect a possible short to ground, a faulty PCM/ECM, or a wiring issue between the transmission and the PCM/ECM.