Fault code P0721 – output shaft speed sensor range/performance

Fault code P0721 is called “Output Shaft Speed (OSS) Sensor Range/Performance” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.

Technical description and explained code P0721

Diagnostic Code P0721 indicates that the operating range of the output speed sensor (OSS) does not match. Without an accurate output speed signal, the Transmission Control Module (TCM) cannot shift gears properly, so this code is set.

In electronically controlled transmissions, the output shaft speed sensor (OSS) is located at the end of the transmission in front of the drive axles or driveshaft. The output shaft speed readout can be used to determine shift timing. As well as torque converter timing and electronic pressure monitoring inside the transmission.

When the transmission control module (TCM) detects a fault, the PCM/ECM can substitute values based on other sensors to determine shift timing. Thereby attempting to adjust the torque converter and pressure if OSS sensor information is not available.

The OSS sensor is also typically used to display speedometer readings, so the data on it may not be correct.

On some vehicles, the OSS sensor is in line with the transmission fluid temperature (TFT) sensor. If the TFT sensor or associated wiring is faulty, the OSS sensor may also malfunction.

Therefore, if there are other errors associated with the TFT sensor with code P0721, you should look for a possible problem with either of these sensors or the wiring between them.

Symptoms of vehicle malfunctions

The main signal that an error P0721 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.

It can also be warning signs such as:

1. The “Check engine” control lamp on the control panel will light up (the code will be memorized as a fault).
2. Other related trouble codes may also be present.
3. Inoperative speedometer or tachometer, or malfunction.
4. Hard shifting of gears.
5. Problems with gear shifting.
6. Engine not accelerating well.
7. Ignition jerking/missing at idle or under load.
8. Engine stalls or fails to start.
9. Reduced power output of the engine.
10. Increased fuel consumption.

Error P0721 is quite serious, as the conditions that led to its persistence may cause damage to the transmission.

Factors that can cause this error code

The error code P0721 can mean that one or more of the following problems have occurred:

• Faulty output shaft speed sensor (OSS).
• Transmission fluid temperature sensor (TFT) has failed.
• Opened, shorted, or shorted wiring or connectors on these sensors.
• Sometimes the cause is a faulty PCM module.

How to fix or reset OBD-2 code P0721

Some suggested steps for troubleshooting and fix the error code P0721:

1. Read all stored errors with an OBD-II scan tool.
2. Clear the errors and test-drive to find out if error P0721 appears again.
3. If the code appears again, inspect the wires and connectors for damage.
4. Test the output shaft speed and transmission fluid temperature sensor input circuits, and if there is a problem, replace the faulty sensor.
5. If necessary, flash or replace PCM.

Diagnose and repair of problems

It’s a good idea to first look up the technical service bulletins (TSB) for your specific make of car with error P0721. If the problem is described, following the instructions can save you time and money in diagnosing and correcting the problem.

Visually inspect all wiring and connectors leading to the output shaft speed and transmission fluid temperature sensor. Carefully inspect for abrasions, exposed wires, or broken wires. Also look for melted or otherwise damaged areas. Repair if necessary.

Electrical wiring

Inspect the wiring harness for loose connections, loose wires, or dirty terminals. Disconnect all connections from the vehicle wiring harness, output shaft speed sensor, TFT sensor and PCM / ECM unit.

Using a multimeter on an ohm scale, 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 over the limit, look for broken or damaged wiring. Check for a ground fault in the wiring harness sections by moving the positive or negative lead to a good ground.

If resistance is present, there may be a short to the ground. The wiring harness should be checked for damaged wiring or sections that may be frayed from touching the vehicle chassis.

Faulty wiring can quite often be the cause of error P0721.

Output shaft speed sensor

Some vehicles use a plastic gear on the end of the OSS sensor. Remove the OSS sensor and check the condition of the pinion. To determine if any teeth are missing or worn, replace the pinion if necessary.

Check the wiring harness for possible signs of damaged wiring or loose connections. Measure resistance by removing the connector and using a digital multimeter set on the ohm scale. If the resistance is out of range or above the limit, replace the sensor.

Check that the sensor and PCM wiring harness connector are energized using the factory wiring diagram. If the OSS sensor is on the same line as the TFT sensor, the voltage will vary based on transmission fluid temperature.

Transmission fluid temperature sensor

Also, a faulty transmission fluid temperature (TFT) sensor can be the cause of error P0721. This is usually seen on vehicles equipped with a TFT sensor installed in line with the OSS sensor.

We also check the TFT sensor for resistance with a multimeter on the ohm scale. If the reading is out of specification or exceeds the limit, replace the TFT sensor inside the transmission.

Check the wiring harness connector for power from the PCM / ECM, using the factory wiring diagram as a reference. Place a multimeter on the volt scale with the positive wire on the power wire from the PCM / ECM. And place the negative wire on the ground.

Depending on the vehicle model, the voltage reading should be 12 volts or a 5 volt reference voltage. If the voltage is not within specifications, you may suspect a short to ground.

Also do not rule out a faulty PCM / ECM or a wiring problem between the transmission and the PCM / ECM.

On which vehicles does this problem occur most frequently

Fault code P0721 can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:

• Audi
• BMW
• Chevrolet
• Citroen (C4)
• Dodge (Caravan, Nitro, RAM)
• Ford (F-150, Focus, Ranger)
• GMC
• Geely
• Honda (Accord)
• Hyundai
• Kia (Sorento)
• Mercedes-Benz
• Peugeot (308)
• Skoda (Octavia)
• Ssangyong (Actyon, Kyron)
• Volkswagen (Jetta, Passat, Polo, Tiguan, Touareg)

Fault code P0721 can sometimes be found with other errors. The most common are the following: P0491, P0716, P0720, P0722, P0723, P072C, P0730, P0740, P0791, P2432.

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