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Error Code P0721 is defined as Output Speed Sensor Range/Performance. This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to present. This may include, but not limited to, vehicles from, Allison, Audi, BMW, Chevrolet, Dodge, Ford, GMC, Honda, Hyundai, Mercedes Benz, Ram, Volkswagen, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model, and powertrain configuration.
Error Code P0721 is associated with Transmission Output Speed Sensor. When the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) determines a malfunction within the OSS (Output Speed Senor) circuit a wide variety of codes can be triggered, depending on the specific vehicle and specific automatic transmission.
The job of the Transmission Output Speed Sensor is to provide the PCM with the signal that tells the speed of the transmission output shaft rotation. The PCM uses these readings to control shift solenoids. The solenoids direct fluid between various hydraulic circuits and change the transmission gear ratio at the right time. Based on the vehicle and the transmission configuration, the Output Speed Sensor may also control the speedometer reading. Vehicles with automatic transmissions are 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.
Error Code P0721 is set when the PCM determines that the Output Speed Sensor is not working properly, or out of range.
Other OSS related error codes include
As with most codes, this code activates the Check Engine light. Other common symptoms include:
- Transmission unable to shift properly or shifts roughly
- Misfire-like symptoms
- Engine in “limp mode”
- Incorrect or erratic reading in speedometer
Multiple factors lead to this code, such as:
- Defective OSS
- Dirty or clogged transmission filter
- Dirty/contaminated transmission fluid
- Defective coolant temperature sensor
- Clogged/restricted hydraulic passages
- Defective transmission valve body
- Corroded or damaged connector
- Faulty wiring
- Defective PCM (rare)
How to Check
As with any malfunctions, it’s best to refer with the TSB (technical service bulletins) before starting your diagnosis for this error code. Make sure to check with the vehicle year, model, and transmission.
The first step is to check and make sure the fluid level and condition is correct, there should be no contamination. Also, make sure to check the vehicle’s records and check the last time the filter and fluid were changed. A thorough visual inspection on the associated wiring is also needed; make sure to check for obvious defects, such as bare wires, burn spots, rubbed and scraped wires.
Make sure to check the connectors and connections for security, corrosion, and damaged pins. Also, check the connectors and wirings to the output speed sensor, transmission pump, transmission solenoids, and PCM. Depending on the configuration, the transmission linkage must be inspected for security and freedom of movement.
Advanced steps may vary depending on the vehicle, and require the right advanced equipment to perform correctly. This requires a digital multimeter and specific technical references. Also, follow the right troubleshooting guidelines and sequence for the vehicle. Voltage requirement may also vary based on the specific model of the vehicle and transmission configuration.
Always check for continuity; avoid shorting the circuit and doing more damage by making sure the power is removed from the circuit. Unless specified in the technical data, normal readings for wirings and connections must be at 0 ohms of resistance. Resistance or no continuity means faulty wiring that is open or shorted and must be repaired or replaced.
How to Fix
Depending on the diagnosis, common repairs for this code include:
- Changing of fluid and fluid filter
- Replacement of defective OSS
- Repair or replacement of defective shift solenoid
- Repair or replacement of defective transmission valve body
- Flushing the transmission to clean the passages
- Cleaning of corroded connectors
- Repair or replacement of faulty wiring
- Flashing or replacement of PCM
Not being able to know exactly how fast the vehicle is moving due to a defective speed sensor can be dangerous, so it’s essential to have this problem addressed as soon as possible.
The most common misdiagnosis for this code are an internal transmission problem, engine misfire, and driveline problem. Also, some jump into blaming the fuel delivery system. In any case, these lead to unnecessary replacement of a vehicle speed sensor.