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P1783 GMC Code - Input Speed Sensor Direction Not Plausible Forward

Quick Answer

The P1783 code indicates that the Input Speed Sensor (ISS) is reporting a signal that is not consistent with expected values when the vehicle is in forward gear. Common causes include Faulty Transmission Input Speed Sensor, Open or shorted wiring in the Transmission Input Speed Sensor harness, Poor electrical connection in the Transmission Input Speed Sensor circuit. 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 GMC indicates Input Speed Sensor Direction Not Plausible Forward.
Common causes
Common causes include Faulty Transmission Input Speed Sensor, Open or shorted wiring in the Transmission Input Speed Sensor harness, Poor electrical connection in the Transmission Input Speed Sensor circuit.
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.
What is this?
Table of contents Created and updated by William Orellana on Jun 01, 2026
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Code P1783 GMC Description

The P1783 code indicates that the Input Speed Sensor (ISS) is reporting a signal that is not consistent with expected values when the vehicle is in forward gear. The ISS is a critical component in the transmission system, as it measures the speed of the primary pulley and communicates this information to the Transmission Control Module (TCM). The TCM uses the data from the ISS to manage various functions, including line pressure, shift patterns, and the operation of the Torque Converter Clutch (TCC). When the TCM detects that the voltage from the ISS exceeds 4.75 volts for a duration of 0.8 seconds, it triggers the P1783 code.

OBDII Code P1783 GMC - Input Speed Sensor Direction Not Plausible Forward
P1783 GMC Code - Input Speed Sensor Direction Not Plausible Forward

Common Causes of P1783 GMC

  1. Faulty Transmission Input Speed Sensor
  2. Open or shorted wiring in the Transmission Input Speed Sensor harness
  3. Poor electrical connection in the Transmission Input Speed Sensor circuit
  4. Defective Control Solenoid Valve Assembly
  5. Faulty Transmission Control Module (TCM)

Symptoms of P1783 GMC

  1. Engine Light ON (or Service Engine Soon Warning Light)
  2. Erratic or harsh shifting between gears
  3. Transmission slipping or failing to engage properly
  4. Decreased fuel efficiency
  5. Unusual engine RPM fluctuations during driving

How to Fix P1783 GMC

  1. Perform a visual inspection of the Input Speed Sensor and its wiring harness for damage or corrosion.
  2. Test the Input Speed Sensor using a multimeter to check for proper voltage and signal output.
  3. If the sensor is faulty, replace the Input Speed Sensor with a new one.
  4. Inspect and repair any damaged wiring or poor connections in the sensor circuit.
  5. Clear the diagnostic trouble codes and perform a road test to ensure the issue is resolved.

Cost to Fix P1783 GMC

The cost to diagnose and repair the P1783 code typically ranges from $150 to $600, depending on the specific issue and whether parts need to be replaced. The Input Speed Sensor itself may cost between $50 and $150, while labor charges can vary significantly based on location, make/model, and engine type, with most shops charging between $80 to $150 per hour. Dealerships may charge higher rates, while independent shops might offer more competitive pricing.

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Information for Specific GMC Models

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.

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