P0721 Output Speed Sensor Range/Performance
OBD-II Trouble Code Technical Description
Former Service Manager & Air Force Mechanic
Output Speed Sensor Range/Performance
What does that mean?
This is a generic powertrain diagnostic trouble code (DTC) and applies to OBD-II vehicles. That may include but is not limited to vehicles from VW, BMW, Mercedes, Chevrolet, GMC, Allison, Duramax, Dodge, Ram, Ford, Honda, Hyundai, Audi, etc. Although generic, the exact repair steps may vary depending on year, make, model and powertrain configuration.
The P0721 OBD-II trouble code is associated with the Transmission Output Speed Sensor.
When the Powertrain Control Module (PCM) detects a malfunction within the Output Speed Sensor Circuit a wide variety of codes can be triggered depending on the specific vehicle and the specific automatic transmission.
Some of the more common code responses associated with Transmission Output Speed Sensor issues are codes P0720, P0721, P0722 and P0723 based on the specific malfunction that alerts the PCM to set the code and activate the Check Engine Light.
The purpose of the Transmission Output Speed Sensor is to provide the PCM with signal that indicates the speed of the transmission output shaft rotation. The PCM uses these readings to control the shift solenoids. The solenoids direct fluid between various hydraulic circuits and change the transmission gear ratio at the appropriate time. Based on the vehicle and the transmission configuration the Output Speed Sensor may also control the speedometer reading. An automatic transmission is controlled by bands and clutches that change gears by having fluid pressure in the right place at the right time. This process starts with the Transmission Output Speed Sensor.
Code P0721 is set by the PCM when the Output Speed Sensor is out of range or not functioning properly.
What is the severity of this DTC?
The severity of this code normally starts as moderate, but it can progress to a more severe level rapidly when not corrected in a timely manner.
Photo of a transmission speed sensor:
What are some of the symptoms of the code?
Symptoms of a P0721 trouble code may include:
- Poor fuel economy
- Check Engine Light illuminated
- Transmission does not shift
- Transmission Shifts roughly
- Possibly misfire-like symptoms
- PCM puts the engine into limp mode
- Speedometer shows incorrect or erratic reading
What are some of the common causes of the code?
Causes for this P0721 transmission code may include:
- Defective Output Speed Sensor
- Dirty or contaminated fluid
- Dirty or clogged transmission filter
- Defective Coolant Temperature Sensor
- Defective Transmission Valve Body
- Restricted hydraulic passages
- Faulty Shift Solenoid
- Corroded or damaged connector
- Faulty or damaged wiring
- Faulty PCM
What are some P0721 troubleshooting steps?
Prior to beginning the troubleshooting process for any malfunction you should research the Technical Service Bulletins (TSB's) for the specific vehicle by year, model and transmission. In some situations this can save a lot of time in the long run by pointing you in the right direction.
The first step is to make sure the fluid level is correct and inspect the condition of the fluid for contamination. You should also check the vehicle records to verify the last time that the filter and fluid was changed if possible. A thorough visual inspection to check the associated wiring for obvious defects such as scraping, rubbing, bare wires, or burn spots is next. Check the connectors and connections for security, corrosion and damaged pins. This should include all wiring and connectors to the output speed sensor, transmission solenoids, transmission pump and the PCM. Depending on the configuration, the transmission linkage should be inspected for security and freedom of movement.
The advanced steps become very vehicle specific and require the appropriate advanced equipment to perform correctly. These procedures require a digital multi meter and the specific technical references for the vehicle. You should follow the specific troubleshooting guidelines and sequence for your vehicle. Voltage requirements may very based on the specific model of the vehicle and the transmission configuration.
Continuity checks are always performed with the power removed from the circuit to avoid shorting the circuit and creating more damage. Unless otherwise specified by the technical data, the normal readings for wiring and connections should be 0 ohms of resistance. Resistance or no continuity is an indication of faulty wiring that is open or shorted and must be repaired or replaced.
- Changing the fluid and filter
- Replacing a faulty Output Speed Sensor
- Repairing or replacing a faulty shift solenoid
- Repairing or replacing a defective transmission valve body
- Flushing the transmission to clean the passages
- Cleaning corrosion from connectors
- Repairing or replacing wiring
- Flashing or replacing PCM
- Engine misfire problem
- Internal transmission problem
- Driveline problem
Hopefully, the information in this article has been helpful to point you in the right direction to correct the problem with your Transmission Output Speed Sensor trouble code. This article is strictly informational and the specific technical data and service bulletins for your vehicle should always take precedence.
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