What Does P077A Mean?
The TCM has detected a loss of the direction signal from the output speed sensor (OSS). The output speed sensor provides the TCM with information about both the speed and the direction of rotation of the transmission output shaft. When the direction signal is lost, the TCM cannot determine whether the vehicle is moving forward or in reverse, which is critical for proper shift scheduling and torque converter lockup control.
Common Causes
35%
Faulty output speed sensor unable to generate a reliable direction signal
25%
Damaged or corroded wiring in the OSS circuit affecting the direction signal wire
15%
Excessive air gap between OSS sensor and reluctor ring due to wear or mounting issues
15%
Damaged or missing teeth on the output shaft reluctor ring
10%
TCM input circuit fault preventing direction signal interpretation
Diagnostic Steps
1
Monitor live data for output speed sensor signal — verify if speed is being read but direction is missing, or if the entire signal is absent.
2
Inspect the output speed sensor connector for corrosion, moisture, bent pins, or damaged seals that could affect signal integrity.
3
Check the OSS wiring for continuity and proper resistance on all signal wires, especially the direction signal pin.
4
Use an oscilloscope to view the OSS waveform — a dual-channel sensor should produce two phase-shifted signals for direction detection; verify both channels are present.
5
Inspect the reluctor ring for damaged, missing, or contaminated teeth that could prevent the direction signal from being generated.
6
If the sensor and wiring are intact, test the TCM input circuit to verify it can properly receive and process the direction signal.
Estimated Repair Cost
$100 - $600
Parts + labor, varies by vehicle and location
The TCM has detected a loss of the direction signal from the output speed sensor (OSS). The output speed sensor provides the TCM with information about both the speed and the direction of rotation of the transmission output shaft. When the direction signal is lost, the TCM cannot determine whether t...
The most common cause of P077A (Output Speed Sensor Circuit - Loss of Direction Signal) is: Faulty output speed sensor unable to generate a reliable direction signal
Typical repair costs for P077A range from $100 to $600, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Without a valid direction signal, the transmission may default to a safe operating mode, potentially disabling certain shift strategies or torque converter lockup. The vehicle is typically drivable but with reduced transmission functionality. Repair promptly to restore full shift control.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P077A to identify the root cause.
OBDHut Mobile App
Scan codes directly from your car with the OBDHut app.
Coming Soon
Quick Info
Category
Powertrain
System
Automatic Transmission
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
A quality scan tool helps you read codes, view live data, and clear faults.
