Summary
P0720 means the transmission control module (TCM) can't read the output shaft speed sensor signal. This sensor measures how fast the transmission's output shaft is spinning — which directly correlates to vehicle speed. Without it, the TCM can't calculate gear ratios, manage shift points, or verify that shifts completed successfully. The leading cause is a failed speed sensor (45%), followed by wiring or connector damage (25%) and sensor contamination from metallic debris (15%).
Severity: Moderate to High — affects shift control and may disable speedometer
Safe to drive: Drive with caution — erratic shifting and limp mode likely
Repair cost: $30–$800+ depending on cause
DIY difficulty: Moderate
What does P0720 mean?
The output speed sensor (OSS) — also called the vehicle speed sensor (VSS) on some platforms — is mounted on the transmission's output shaft housing or tail shaft. It reads rotational speed using a magnetic pickup or Hall-effect sensor and sends that data to the TCM. The TCM compares output speed to input speed to determine the actual gear ratio, manage shift scheduling, and monitor torque converter lockup.
When the TCM receives no signal, an erratic signal, or a reading outside the expected range from the output speed sensor circuit, it sets P0720. This is a circuit-level code — it means the electrical signal is missing or abnormal, not just that a ratio is off.
Symptoms vary by vehicle but commonly include: no or erratic speedometer reading, harsh or delayed shifts, failure to shift at all, limp mode activation, and on some vehicles, the cruise control and ABS systems may also be affected since they rely on vehicle speed data.