P077B

What it means

P077B is the generic code for a direction error in the transmission output shaft speed sensor circuit. The control unit uses this sensor to know how fast the output shaft turns and which way it turns, and it has set the code because the direction information is missing, implausible or disagrees with other signals such as wheel speed or the selected gear. Exact sensor type and how the direction is worked out vary by manufacturer and transmission.

Symptoms

• Warning light on, possibly with a transmission warning message
• Gearbox in limp mode or stuck in one gear
• Harsh, delayed or flaring gear changes
• Speedometer erratic or not working
• Hill-hold or reverse-related functions not working on some models

Possible causes

• Faulty output shaft speed sensor, either internal failure or a dual-channel sensor that has lost its direction signallikely
• Damaged wiring or poor connector contact in the sensor circuit (chafing, corrosion, water ingress, open or short circuit)likely
• Contaminated or damaged sensor, or metal debris on the sensor tip or reluctor or encoder ringpossible
• Damaged, cracked or incorrectly fitted reluctor or encoder ring on the output shaftpossible
• Incorrect sensor air gap or poorly seated sensor after previous gearbox workpossible
• Fault in the transmission control unit or its power and earth supplyrare
• Outdated control unit software where a known fault for that model is documentedrare

How to diagnose

1. Read all stored and pending codes and note the freeze frame data (road speed, gear, fluid temperature), as other speed sensor or wheel speed codes help to narrow down the cause
2. Check the transmission fluid level and condition, as the manufacturer specifies, looking for metal debris or contamination
3. Inspect the sensor, wiring and connector for chafing, heat damage, oil soaking, corrosion and bent or pushed-back pins
4. Compare the output shaft speed and direction live data with the wheel speed and vehicle speed readings on a road test or with the drive wheels raised safely
5. Check the sensor's supply voltage, earth and signal wiring for continuity and shorts, using the manufacturer's wiring diagram and specified values
6. Remove the sensor and inspect its tip for debris and damage, and check the mounting and seal
7. Where accessible, inspect the reluctor or encoder ring for damage or missing teeth or magnetic segments
8. Check for relevant technical service bulletins or software updates for the specific model and transmission

How to fix

• Repair or replace damaged wiring and clean or replace corroded connectors
• Replace the output shaft speed sensor with the correct type for the transmission
• Clean debris from the sensor and fit new seals, ensuring it is seated and torqued correctly
• Repair or replace a damaged reluctor or encoder ring, which may require gearbox work depending on the model
• Update the transmission control unit software only if an outdated calibration is a documented cause for that vehicle
• Repair the power or earth supply fault, or replace the control unit if it is proven faulty

Can I keep driving?

Nearby codes

• P077AGenericOutput Shaft Speed Sensor Circuit - Loss of Direction Signal
• P077CGenericOutput Shaft Speed Sensor Circuit Low