What Does P077B Mean?
The TCM has detected a direction error from the output speed sensor circuit. Unlike a loss of signal, this code indicates the sensor is reporting direction information that contradicts other data the TCM is receiving (such as input speed, gear state, or vehicle speed). This could mean the sensor is reading backward rotation when forward is expected, or the signal phasing is incorrect, suggesting a sensor, wiring, or installation problem.
Common Causes
30%
Output speed sensor installed incorrectly (rotated or reversed connector wiring)
25%
Swapped or crossed wires in the OSS circuit reversing the direction signal
20%
Faulty output speed sensor generating an incorrect phase relationship
15%
Damaged reluctor ring with missing or deformed teeth causing erroneous direction readings
10%
TCM internal fault misinterpreting the direction signal
Diagnostic Steps
1
Check if the OSS was recently replaced or serviced — incorrect installation orientation is the most common cause of direction errors.
2
Verify the wiring pinout at both the sensor connector and the TCM connector to ensure signal wires are not swapped.
3
Use an oscilloscope to view both OSS signal channels simultaneously — verify the phase relationship matches the expected direction of rotation.
4
Inspect the reluctor ring for damage, contamination, or misalignment that could produce an ambiguous direction signal.
5
Compare the OSS direction reading in live data against the transmission's commanded gear and input speed to confirm the mismatch.
6
If the sensor and wiring are correctly installed and undamaged, suspect a TCM fault and check for software updates before replacement.
Estimated Repair Cost
$100 - $600
Parts + labor, varies by vehicle and location
The TCM has detected a direction error from the output speed sensor circuit. Unlike a loss of signal, this code indicates the sensor is reporting direction information that contradicts other data the TCM is receiving (such as input speed, gear state, or vehicle speed). This could mean the sensor is ...
The most common cause of P077B (Output Speed Sensor Circuit - Direction Error) is: Output speed sensor installed incorrectly (rotated or reversed connector wiring)
Typical repair costs for P077B range from $100 to $600, depending on the vehicle, location, and whether you do it yourself or go to a shop.
A direction error could cause the TCM to make incorrect shift decisions, potentially engaging the wrong gear. While the TCM usually enters a protective mode to prevent damage, the vehicle should be diagnosed promptly to prevent potentially dangerous shifting behavior.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P077B to identify the root cause.
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Quick Info
Category
Powertrain
System
Automatic Transmission
Difficulty
Type
Generic (SAE)
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