|Code||Fault Location||Probable Cause|
|P0721|| Output shaft speed (OSS) sensor -range/performance problem |
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What Does Code P0721 Mean?
SPECIAL NOTES: Take note that when diagnosing code P0721 – “Output Speed Sensor Circuit Range/Performance”, non-professional mechanics are generally limited to a few basic troubleshooting and repair steps. On almost all modern automatic transmissions, this particular code can be caused by failures/malfunctions in control modules and/or circuits and systems that may at first glance not be directly related to the transmission output shaft speed sensor or its control system.
Therefore, the information in this guide is intended for general informational purposes only, and should NOT be used in ANY diagnostic/repair procedure for this code on ANY application without making proper reference to the manual for the affected application. Thus, if the information provided here does not resolve the problem, the better option is to refer the vehicle to the dealer or a specialist transmission repair shop for professional diagnosis and repair. END OF SPECIAL NOTES.
OBD II fault code P0721 is a generic code that is defined as “Output Speed Sensor Circuit Range/Performance”, and is set when the PCM Powertrain Control Module) or TCM (Transmission Control Module) detects an abnormal or unexpected voltage or signal from the transmission output shaft speed sensor or its control circuit. Note that this code only applies to applications with automatic transmissions. Also note that on some applications, the transmission output shaft speed sensor provides input data for the vehicle speedometer.
The primary purpose of the transmission output shaft sped sensor is to provide the PCM/TCM with input data on the rotational speed of the output shaft. The PCM/TCM uses this data (which is compared with input data from other transmission speed sensors) to calculate shift strategies, as well as the desired hydraulic pressure required for each gear ratio. Based on the total body of input data from all implicated speed sensors, but especially on input data from the output shaft speed sensor, the PCM/TCM controls the shift solenoids to accomplish appropriate gear shift points and patterns.
From the above it should be clear that if the PCM/TCM is deprived of accurate input data from the transmission output shaft speed sensor, the control module is unable to control gear shifts effectively since it is unable to direct hydraulic pressure to the correct shift solenoid at the right time.
The image below shows a typical transmission output shaft speed sensor. Note that depending on the application, these sensors may have two, three, or four wires in their connectors.
What are the common causes of code P0721 ?
Some common causes of code P0792 could include the following-
- Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
- Defective transmission output shaft speed sensor
- In some cases, other defective engine and/or transmission speed sensors can cause this code to set. However, in these cases the defective sensor(s) will almost certainly be indicated by codes that are dedicated to those sensors
- Low transmission fluid levels
- Dirty, contaminated, or degraded transmission fluid
- Failed or failing PCM/TCM. Note that control module failure is rare, and the fault must be sought elsewhere before any control module is replaced
What are the symptoms of code P0721 ?
Some common symptoms of code P0721 could include the following-
- Stored trouble code and an illuminated warning light
- Several other transmission related codes may be present as well. Common additional codes are P0720, P0721, P0722 and P0723
- Gear shifts may be harsh, erratic, or unpredictable
- Transmission may fail to engage any gear, or may be stuck in one gear
- On may applications, the transmission/engine may enter a fail-safe or “limp” mode
- Fuel consumption may increase considerably
- Engine may exhibit misfire-like symptoms
- Speedometer may display wildly inaccurate/erratic readings, or may not work at all
How do you troubleshoot code P0721 ?
NOTE: If the transmission is fitted with a dipstick, check the fluid level (and the condition of the transmission fluid) as a first step in the diagnostic/repair procedure. Adjust the fluid level as required, or replace the fluid and the internal filter as per the instructions in the manual if the fluid is dark in color, has a “burnt” odor, or has a thick, tarry consistency.
Assuming that the transmission fluid is serviceable, record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.
NOTE: If any other codes are present, refer to the manual to determine their relationship to P0721. Note however that codes that were stored after P0721 have likely set as a result of P0721, but in all cases where other codes precede P0721 these codes must be resolved before an attempt is made to diagnose P0721. Failure to do will almost certainly result in a misdiagnosis, wasted time, and the unnecessary replacement of parts and components.
Refer to the manual to locate the transmission output shaft speed sensor. On most applications, this sensor is located close to the output shaft. Also determine the color-coding and function of each wire in the sensor connector/harness.
NOTE: Check that the connector is fastened securely, since it is common for this connector to be left undone after routine maintenance and servicing.
Perform a thorough visual inspection of all associated wiring. Look for damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors. Make repairs or replace wiring as required.
If no visible damage to wiring is found, prepare to perform reference voltage (where applicable), resistance, ground integrity, and continuity checks on all associated wiring, but be sure to disconnect the sensor from all relevant control modules to prevent damaging the controllers.
Compare all obtained readings with the values stated in the manual, and make repairs or replace wiring as required to ensure that all electrical values fall within the ranges specified by the manufacturer.
TIP: Since the transmission output shaft speed sensor is sometimes very difficult to access, perform the above-mentioned tests at the connector of the relevant control module by back-probing the connector pins. Note however that while this method tests the circuit and the sensor at the same time, this way of testing the sensor and its circuit will not necessarily distinguish between a fault in the sensor and a fault in the circuit, and it may still be necessary to remove the sensor from the transmission for further testing or cleaning.
If Step 4 reveals an electrical issue, disconnect the sensor from its harness, and repeat Step 4 at the control module connector. If all electrical values check out, the sensor is defective. Conversely, if the sensor is disconnected and the fault persists, look for the problem in the wiring between the sensor connector and the control module. Make repairs as required, and repeat all tests to verify that the repair is successful.
If the output shaft speed sensor is shown to be defective, remove it from the transmission and inspect it for signs of mechanical damage, or the presence of excessive amounts of metal wear particles. Remove all wear particles, and check the sensors’ internal resistance to verify its overall condition. Replace the sensor with an OEM replacement if its resistance does not agree with the value specified in the manual.
Note however that if the sensor bears signs of mechanical damage on the part of the sensor that fits inside the transmission, there is nothing the average non-professional mechanic can do to fix the problem. Diagnoses of this type require removal and disassembly of the transmission, so in these cases, the better option is to refer the vehicle to the dealer or specialist transmission repair shop for professional assistance.
Clear all codes after all repairs are complete, and operate the vehicle normally to see if any codes return. In the event that any codes do return, suspect an intermittent fault that may require the use of advanced diagnostic equipment to diagnose and repair.