P0720: Output Speed Sensor Circuit Malfunction

What P0720 tells you, and what it does not

The sensor that measures how fast the transmission output shaft is turning, which the computer uses along with wheel speed for shift timing and speedometer accuracy, is not sending a usable signal.

The code sits in the transmission output speed sensor area and is reported by the comprehensive component monitor. It names the circuit or the condition where the computer saw a reading outside its expected range. It does not name a failed part, and several genuinely different faults set this same code, which is the whole reason the causes below are ordered rather than listed.

What people usually notice

• Speedometer inaccurate or reads zero
• Harsh or mistimed shifts
• Cruise control disabled
• Transmission warning light on

What causes P0720?

The causes below are ordered by how often they turn out to be the answer, cheapest and most common first. That order matters more than the list itself: the expensive component named in the code text is very rarely the first thing to replace, and working down from the top is what stops a transmission output speed sensor fault becoming an expensive guess.

Corroded connector at the sensor is the one to rule out before anything else. The most common and cheapest cause to check first.

Working out which cause it is

Start with freeze frame. Check the output shaft speed and vehicle speed stored together when the code set. That snapshot is taken at the instant the code set, and it is the single most useful piece of evidence you have, which is also why clearing the code before reading it is such a costly habit.

This code is diagnosed by watching values change rather than by reading the number again, so the tool has to stream live data and hold freeze frame. Live data on the output speed signal compared to actual road speed isolates the sensor from a wiring fault.

The order worth working in

• 1. Corroded connector at the sensor: The most common and cheapest cause to check first.
• 2. Wiring fault between sensor and computer: A break in this wiring prevents the correct signal from reaching the computer.
• 3. Damaged tone ring: A damaged ring gives the sensor nothing consistent to see.
• 4. Contaminated transmission fluid: Debris in the fluid can interfere with the sensor.
• 5. Failing output speed sensor: The sensor itself is a wear item.

What P0720 means for driving the car

Speedometer accuracy and shift timing can both be affected, drive cautiously since you may not have a reliable speed reading.

This is a severity 2 of 4 code. It will not usually strand you, but it will fail an emissions test while it is present, and the underlying fault rarely gets cheaper by being left alone.

What does it cost to ignore P0720?

An unreliable speedometer and inconsistent shift timing left unaddressed adds wear and reduces confidence in what speed you are actually going.

There is also the diagnostic cost of waiting. Freeze frame data, pending code history and the readiness monitor state all describe the fault as it was when it set, and all of it is lost the moment somebody clears the code to make the lamp go out.