Output Speed Sensor Circuit - Loss of Direction Signal
The P077A code indicates that the transmission control module (TCM) has detected a loss of direction signal from the output speed sensor circuit. This sensor monitors the rotational speed and direction of the transmission output shaft. When the signal is missing or erratic, the TCM cannot determine vehicle direction, leading to improper transmission operation and potential safety concerns.
1. Symptoms You Will Notice
- Check Engine Light illuminated
- Transmission shifting erratically or stuck in gear
- Speedometer or odometer malfunction
- Vehicle may not move in forward or reverse
2. Most Common Causes
- Faulty or worn output speed sensor (OSS)
- Damaged reluctor ring or exciter ring on the output shaft
- Open, shorted, or high-resistance wiring in the sensor circuit
- Corroded, loose, or moisture-contaminated sensor connector
- Failed TCM or ECM that processes the sensor signal
- Electromagnetic interference affecting the sensor circuit
3. Step-by-Step Diagnosis
Step 1: Scan for additional codes and record freeze frame data.
Step 2: Visually inspect the output speed sensor and its wiring for damage or corrosion.
Step 3: Test sensor resistance and AC voltage output while rotating the wheels.
Step 4: Check the tone wheel for damage or debris.
Step 5: If sensor and wiring are good, inspect transmission internal components.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
No, driving with P077A can cause unpredictable transmission behavior, loss of drive, or damage to the transmission. It is recommended to have the vehicle towed to a repair shop.
How much does it cost to fix?▼
Repair costs vary from $150 to $400 depending on the cause. A simple sensor replacement may cost $150-$250, while wiring repairs or internal transmission work can exceed $400.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.