AutoCodes – Generic – P077C

P077C Code - Transmission Output Speed Sensor Circuit Low Voltage

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: P077C indicates Transmission Output Speed Sensor Circuit Low Voltage.
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 5,890

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
Comments
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Code P077C 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 the vehicle speed sensor signal circuit voltage is less than 0.25 V for 0.8 s.

OBDII Code P077C - Transmission Output Speed Sensor Circuit Low Voltage - AutoCodes.com — P077C Code - Transmission Output Speed Sensor Circuit Low Voltage

What does this mean?

What are the Possible Causes of the DTC P077C?

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 P077C?

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 P077C 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.

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Information for Specific Makes

P077C Buick — Transmission Output Speed Sensor Circuit Low Voltage
P077C Cadillac — Transmission Output Speed Sensor Circuit Low Voltage
P077C Chevrolet — Transmission Output Speed Sensor Circuit Low Voltage
P077C GMC — Transmission Output Speed Sensor Circuit Low Voltage

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.