AutoCodes – GMC – P0721 2018 GMC Sierra

P0721 2018 GMC Sierra Code - Transmission Output Speed Sensor Performance

Quick Answer

The Output Speed Sensor (OSS) is a hall-effect type sensor. Common causes include Faulty Transmission Output Speed Sensor (OSS), Transmission Output Speed Sensor harness is open or shorted, Transmission Output Speed Sensor circuit poor electrical connection. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means: P0721 2018 GMC Sierra indicates Transmission Output Speed Sensor Performance.
Common causes: Common causes include Faulty Transmission Output Speed Sensor (OSS), Transmission Output Speed Sensor harness is open or shorted, Transmission Output Speed Sensor circuit poor electrical connection.
Severity: High - diagnose this code as soon as possible.
Can I drive?: Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

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Code Popularity 230

How popular is this code, among other codes? Low

Repair Importance Level

Urgent (Diagnose now) (3 / 3)

Repair Difficulty Level

Advanced (Pro) (3 / 3)

What is this?

Table of contents Created and updated by William Orellana on Jul 29, 2022

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
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Code P0721 2018 GMC Sierra Description

The Output Speed Sensor (OSS) is a hall-effect type sensor. The OSS mounts to the automatic transmission case assembly and connects to the control solenoid valve assembly through a 2-wire harness and connector. The sensor faces the park gear teeth. The sensor receives battery voltage on the OSS supply voltage circuit from the Transmission Control Module (TCM). As the output shaft rotates, the sensor produces a signal frequency based on the park gear teeth. This signal is transmitted through the OSS signal circuit to the TCM. The TCM uses the OSS signal to determine line pressure, transmission shift patterns, vehicle speed, and gear ratio.
The Diagnostic Trouble Code (DTC) will be set when the TCM detects a noisy output speed signal.

OBDII Code P0721 2018 GMC Sierra - Transmission Output Speed Sensor Performance - AutoCodes.com — P0721 2018 GMC Sierra Code - Transmission Output Speed Sensor Performance

What does this mean?

What are the Possible Causes of the DTC P0721 2018 GMC Sierra?

Faulty Transmission Output Speed Sensor (OSS)
Transmission Output Speed Sensor harness is open or shorted
Transmission Output Speed Sensor circuit poor electrical connection
Faulty Transmission Control Module (TCM)

How to Fix the DTC P0721 2018 GMC Sierra?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the P0721 2018 GMC Sierra code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

Need more information about the P0721 2018 GMC Sierra code?

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About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about P0721 2018 GMC Sierra Code

1. What are the common symptoms of OBDII code P0721 in a 2018 GMC Sierra?

- Common symptoms include erratic speedometer readings, harsh shifting, and the transmission going into limp mode.

2. What does OBDII code P0721 mean for a 2018 GMC Sierra?

- OBDII code P0721 indicates a performance issue with the transmission output speed sensor in the vehicle.

3. What are some possible causes of OBDII code P0721 in a 2018 GMC Sierra?

- Potential causes include a faulty transmission output speed sensor, damaged wiring or connectors, or issues with the transmission control module.

4. How can I diagnose OBDII code P0721 in my 2018 GMC Sierra?

- Diagnosis typically involves using a scan tool to read the code, testing the transmission output speed sensor, and inspecting related wiring for damage.

5. Can I continue driving my 2018 GMC Sierra with OBDII code P0721?

- It is not recommended to continue driving with this code as it can lead to further transmission issues and potential safety concerns.

6. How is OBDII code P0721 in a 2018 GMC Sierra typically repaired?

- Repair often involves replacing the faulty transmission output speed sensor, repairing or replacing damaged wiring, and clearing the code from the vehicle's computer.

7. Are there any DIY tips for addressing OBDII code P0721 in a 2018 GMC Sierra?

- DIY enthusiasts may attempt to replace the transmission output speed sensor themselves, but it is recommended to have the vehicle properly diagnosed by a professional.

8. Can resetting the OBDII code P0721 in a 2018 GMC Sierra fix the issue temporarily?

- Resetting the code may temporarily clear the warning light, but if the underlying issue with the transmission output speed sensor is not addressed, the code is likely to return.

9. How much does it typically cost to repair OBDII code P0721 in a 2018 GMC Sierra?

- Repair costs can vary depending on the extent of the issue, but replacing a transmission output speed sensor can range from $100 to $300, including parts and labor.

10. Is OBDII code P0721 a common issue in 2018 GMC Sierra vehicles?

- While not extremely common, issues with the transmission output speed sensor leading to OBDII code P0721 can occur in 2018 GMC Sierra models and should be addressed promptly to prevent further damage.