P0720 Code: Output Speed Sensor Circuit

What the P0720 code means

A modern automatic transmission decides its shifts by comparing two speeds: the input speed, which comes from the engine, and the output speed, which goes to the wheels. The relationship between the two tells it which gear is really engaged and whether the clutches are slipping.

The P0720 code is stored when the signal from the output sensor is missing or not coherent. Without that data the transmission loses its main reference and cannot manage the shifts safely.

The system responds by going into limp mode, normally locking in a single gear. That is a deliberate protection: without knowing the real speed, a badly calculated shift could over rev the engine or lock the wheels.

It is worth telling it apart from P0500, which refers to the vehicle speed signal in general. On many models both come from the same sensor and the two codes appear together. On others they are different sensors and only one is involved.

It helps to know that on many vehicles this sensor does two jobs at once, and that explains how wide the symptoms are. Besides informing the transmission, its signal feeds the speedometer, the cruise control and sometimes variable power steering. When it fails, the driver notices half a dozen different problems and tends to suspect a serious electrical fault, when the usual cause is a single thirty dollar sensor or its connector.

There is a test that separates the sensor from the tone wheel without opening anything. Lift the drive wheels, let the transmission turn with the engine running and watch the signal. If it shows up but with regular, periodic dropouts, the problem is a damaged tooth on the tone wheel, not the sensor. If the signal does not show up at all, or wanders with no pattern, the sensor or its circuit is responsible.

P0720 code symptoms

• Transmission in limp mode, often locked in a single gear.
• Harsh shifts, late shifts or no shifts at all.
• Speedometer dead or erratic on vehicles where it shares the signal.
• Check engine light and transmission warning light on.
• Cruise control disabled.
• Unstable idle when you come to a stop.
• A P0500 code may be stored along with it.
• The signal shows regular, periodic dropouts as the shaft turns.

Likely causes of the P0720 code

1. Failed output speed sensor. The most frequent cause.
2. Corroded connector or one with moisture, because of its exposed location.
3. Damaged wiring along its run to the transmission.
4. Metal shavings stuck to the sensor tip, coming from internal wear.
5. Output shaft tone wheel damaged or with broken teeth.
6. Wrong air gap after fitting a sensor that is not specific to the vehicle.
7. Blown fuse in the circuit.
8. Poor ground in the circuit.
9. Failed transmission module. The last possibility.
10. Damaged tooth on the output shaft tone wheel, which produces a dropout on every turn.

How to diagnose the P0720 code step by step

1. Watch the output speed in live data while you drive slowly. It should rise smoothly and match the real speed.
2. Compare it with the input speed. The ratio between them should match the gear that is engaged.
3. Remove the sensor and clean its tip of metal shavings. It costs nothing and solves quite a few cases.
4. Note how many shavings there are. A heavy buildup points to internal wear and affects the repair.
5. Measure the sensor resistance if it is an inductive type, or check its supply if it is a Hall effect type.
6. Inspect the connector for corrosion and moisture.
7. Inspect the wiring along its whole run.
8. Check the fuse and the ground of the circuit.
9. Lift the drive wheels and watch the signal as they turn. Regular, periodic dropouts mean a damaged tooth. A complete absence of signal, or a wandering one, points to the sensor.

How to fix the P0720 code

• Clean the sensor tip and its bore before you consider replacing anything.
• Replace the sensor if its measurements are out of range or it generates no signal.
• Clean or replace the corroded connector and protect it with dielectric grease.
• Repair damaged wiring, following the original routing.
• Check the air gap when you fit the new sensor.
• Replace the fuse and find out why it blew.
• Consider an internal inspection if the shavings are heavy.
• Replace the tone wheel if the dropouts are periodic, because a new sensor will not correct a broken tooth.

After the repair, check that the output speed follows the real speed across the whole range and that the transmission gets all its gears back. Then clear the P0720 code and confirm it does not return.

Technical reference data

• Inductive sensors: typical resistance between 200 and 2000 ohms.
• Hall effect sensors: 5 or 12 volt supply and a square wave signal whose frequency rises with speed.
• Usual air gap: 0.5 to 1.5 millimeters (about 0.020 to 0.060 inch).
• The ratio between input and output speed identifies the gear that is really engaged.
• A heavy buildup of shavings on the tip points to internal wear in the transmission.
• Regular, periodic dropouts: damaged tooth. Missing or wandering signal with no pattern: sensor or circuit.

Brand-specific notes

Metal shavings stuck to the sensor tip are two things at once: an easy cause to fix and an indicator of the internal condition of the transmission. Cleaning them off can clear the code, but if there are a lot of them, they come from wear of the internal parts and they will come back.

If the code appears along with a P0500, check whether both come from the same sensor on your model. On many vehicles they do, and then it is one repair and not two.

Frequently asked questions about P0720