AutoCodes – GMC – P1783 2021 GMC Sierra

P1783 2021 GMC Sierra Code - Input Speed Sensor Direction Not Plausible Forward

Quick Answer

The Input Speed Sensor detects the primary pulley revolution speed and sends a signal to the Transmission. Common causes include Faulty Transmission Input Speed Sensor, Transmission Input Speed Sensor harness is open or shorted, Transmission Input 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: P1783 2021 GMC Sierra indicates Input Speed Sensor Direction Not Plausible Forward.
Common causes: Common causes include Faulty Transmission Input Speed Sensor, Transmission Input Speed Sensor harness is open or shorted, Transmission Input 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 13

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 Jun 01, 2026

Description
Possible Causes
How To Fix
Cost to Diagnose
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Code P1783 2021 GMC Sierra Description

The Input Speed Sensor detects the primary pulley revolution speed and sends a signal to the Transmission. The Input Speed Sensor (ISS), also known as the transmission turbine speed sensor, is a hall-effect type sensor. The ISS mounts to the control valve upper body assembly and connects to the control solenoid (w/body and Transmission Control Module (TCM)) valve assembly through a wire harness and connector. The sensor faces the 1-2-3-4 and 3-5-R clutch housing machined teeth surface. The sensor receives 8.3-9.3 volts on the Input/Output Speed Sensor (ISS/OSS) supply voltage circuit from the Transmission Control Module (TCM). As the 1-2-3-4 and 3-5-R clutch housing rotates, the sensor produces a signal frequency based on the machined surface of the 1-2-3-4 and 3-5-R clutch housing. This signal is transmitted through the ISS signal circuit to the control solenoid (w/body and TCM) valve assembly. The control solenoid (w/body and TCM) valve assembly uses the ISS signal to determine line pressure, transmission shift patterns, Torque Converter Clutch (TCC) slip speed, and gear ratio. The Diagnostic Trouble Code (DTC) will be set when the TCM detects the ISS signal circuit voltage is greater than 4.75 V for 0.8 s.

OBDII Code P1783 2021 GMC Sierra - Input Speed Sensor Direction Not Plausible Forward - AutoCodes.com — P1783 2021 GMC Sierra Code - Input Speed Sensor Direction Not Plausible Forward

What does this mean?

What are the Possible Causes of the DTC P1783 2021 GMC Sierra?

Faulty Transmission Input Speed Sensor
Transmission Input Speed Sensor harness is open or shorted
Transmission Input Speed Sensor circuit poor electrical connection
Faulty Control Solenoid Valve Assembly

How to Fix the DTC P1783 2021 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 P1783 2021 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 P1783 2021 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.