P077A
Output Shaft Speed Sensor Circuit - Loss of Direction Signal
The output shaft speed sensor uses a quadrature output (two channels phase-shifted by 90°) so the TCM can determine direction of rotation. The direction channel is missing — TCM cannot tell whether the vehicle is rolling forward or backward, blocking neutral-idle and rollback-protection strategies.
- Check engine light: Yes
- Emissions related: No
- Repair difficulty: medium
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
Common causes
Ordered by how often each turns out to be the cause.
- Open direction-channel wireCommon
- Failed quadrature sensor elementPossible
Parts involved
- Output Shaft Speed Sensor
- Sensor Wiring
P077A is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.