P0720 Output Speed Sensor (OSS) Circuit Malfunction

The P0720 – Output Speed Sensor (OSS) Circuit Malfunction diagnostic trouble code (DTC) indicates that the transmission control module (TCM) has detected a problem with the output speed sensor circuit.

The Output Speed Sensor (OSS) monitors the rotational speed of the transmission output shaft and sends this information to the TCM. The controller uses the signal to determine vehicle speed, control gear changes, calculate transmission ratios, monitor slip, and manage torque converter operation.

P0720 is a general circuit malfunction code. It does not necessarily mean that the output speed sensor itself has failed. The problem can be located in the sensor, wiring, connector, power supply, ground, signal circuit, or the control module receiving the signal.

What Does P0720 Mean?

In simple terms:

The TCM has detected a malfunction somewhere in the electrical circuit responsible for the transmission output-speed signal.

The basic operating sequence is:

Transmission output shaft rotates → OSS sensor detects rotation → Sensor generates an electrical signal → Signal reaches TCM → TCM detects a circuit malfunction → P0720 is stored

The exact circuit depends on the vehicle and transmission design.

Some vehicles use a dedicated external OSS sensor, while others use an internal transmission speed-sensor assembly.

What Is the Output Speed Sensor?

The Output Speed Sensor (OSS) measures how quickly the transmission output shaft is rotating.

Depending on the design, the sensor may use:

• Magnetic sensing

• Hall-effect sensing

• A reluctor or tone wheel

• An internal speed-sensor assembly

The TCM uses this information to understand how fast the transmission output is turning.

This information can then be compared with engine RPM and transmission input speed.

Why Does the TCM Need Output Speed?

Output-speed information is important for several transmission functions.

The TCM may use it to control:

• Gear changes

• Shift timing

• Gear-ratio calculations

• Transmission slip monitoring

• Torque converter clutch operation

• Vehicle-speed calculations

• Fail-safe strategies

A malfunctioning OSS circuit can therefore affect much more than the speed sensor itself.

Symptoms of P0720

The symptoms vary depending on the vehicle and the nature of the circuit fault.

Possible symptoms include:

Check Engine Light

The malfunction indicator lamp may illuminate.

Harsh shifting

The transmission may shift harder than normal.

Delayed shifting

Gear changes may become slower or delayed.

Erratic shifting

The transmission may change gears unpredictably.

Transmission stuck in one gear

The TCM may enter a protective operating mode.

Limp mode

Transmission operation may become restricted.

Speedometer problems

Some vehicles use the transmission output-speed signal as part of the vehicle-speed calculation.

Cruise control problems

Cruise control may be disabled if reliable vehicle-speed information is unavailable.

Transmission slipping

Incorrect speed information can interfere with transmission control and slip monitoring.

Incorrect gear selection

The TCM may select an inappropriate gear.

Torque converter clutch problems

Torque converter lock-up may be disabled or operate incorrectly.

What Causes P0720?

There are several possible causes.

Faulty output speed sensor

The sensor may be damaged or internally defective.

Damaged OSS wiring

An open, shorted, or high-resistance circuit can cause P0720.

Loose connector

Poor terminal contact can interfere with the signal.

Corroded connector terminals

Corrosion can increase resistance or interrupt the circuit.

Water or oil intrusion

Contamination inside the connector can affect electrical contact.

Damaged reluctor or tone ring

The sensor may be unable to correctly detect output-shaft rotation.

Incorrect sensor installation

An incorrect sensor position or air gap can cause signal problems.

Metal contamination

Certain magnetic sensors can be affected by excessive metal debris.

Power supply problem

A Hall-effect sensor may require a stable voltage supply.

Ground problem

A poor sensor ground can prevent proper signal generation.

Internal transmission wiring fault

Some transmissions contain the speed sensor and wiring inside the transmission.

TCM wiring problem

The signal may be lost between the sensor and the control module.

CAN communication problem

On some vehicles, speed information is transmitted electronically between modules.

TCM fault

A TCM failure is possible but should normally be considered after the external circuit has been tested.

Can a Bad Output Speed Sensor Cause P0720?

Yes.

The output speed sensor is one of the primary components to inspect.

However, P0720 is a circuit malfunction code, so the sensor should not automatically be replaced.

A defective sensor can cause P0720, but so can:

• Broken wiring

• Short circuits

• Poor connectors

• Missing power

• Poor ground

• Signal circuit problems

A proper diagnosis should determine whether the sensor is actually receiving and producing the required signal.

Can Wiring Cause P0720?

Yes.

Wiring problems are among the most important possibilities.

Inspect for:

• Open circuits

• Short to ground

• Short to voltage

• High resistance

• Chafed insulation

• Broken conductors

• Corroded terminals

• Loose connections

• Water intrusion

Pay particular attention to areas where the transmission harness can move, rub, or become exposed to heat.

Can a Connector Cause P0720?

Yes.

A poor connector can create a circuit malfunction even when the sensor itself is perfectly functional.

Inspect for:

• Bent pins

• Loose terminals

• Corrosion

• Moisture

• Oil contamination

• Damaged connector locks

Terminal tension is especially important. A terminal can appear clean while failing to maintain reliable electrical contact.

Can a Bad Ground Cause P0720?

Yes.

If the OSS sensor requires a ground reference, a poor ground can prevent it from operating correctly.

Check:

• Sensor ground

• Transmission ground

• Engine ground straps

• TCM grounds

• Battery negative connections

A voltage-drop test can help identify a poor ground under operating conditions.

Can a Power Supply Problem Cause P0720?

Yes, depending on the sensor design.

Some OSS sensors require a dedicated voltage supply.

If the supply is missing or unstable, the sensor may not generate a valid signal.

Check the relevant:

• Fuse

• Power supply

• Sensor reference voltage

• Ground

• Wiring

The exact voltage specification must be obtained from the vehicle manufacturer's service information.

Can a Damaged Reluctor Cause P0720?

Yes.

The OSS sensor reads a rotating target.

If the reluctor or tone ring is:

• Damaged

• Cracked

• Bent

• Misaligned

• Missing teeth

• Excessively worn

the sensor may produce an incorrect or unusable signal.

In this situation, replacing the sensor alone may not solve the problem.

Can Transmission Fluid Cause P0720?

Usually not directly.

However, transmission fluid condition can sometimes provide important diagnostic information.

Excessive metal debris may interfere with certain magnetic sensors and may also indicate internal transmission wear.

If the transmission fluid contains significant metallic contamination, the underlying mechanical condition should be investigated.

Does P0720 Mean the Transmission Is Bad?

No.

P0720 does not automatically indicate internal transmission failure.

The problem can be located entirely in the electrical circuit.

A typical diagnostic path is:

OSS sensor → Connector → Wiring → Power/ground → Signal circuit → TCM

Only after these areas have been verified should internal transmission problems become a major consideration.

Can P0720 Cause Incorrect Gear-Ratio Codes?

Yes.

The TCM uses output speed when calculating actual transmission ratios.

If output-speed information is incorrect or unavailable, the TCM may calculate an incorrect ratio.

P0720 can therefore potentially appear together with:

• P0729 – Gear 6 Incorrect Ratio

• P0730 – Incorrect Gear Ratio

• P0731 – Gear 1 Incorrect Ratio

• P0732 – Gear 2 Incorrect Ratio

• P0733 – Gear 3 Incorrect Ratio

• P0734 – Gear 4 Incorrect Ratio

If several ratio codes appear together with P0720, the output-speed circuit should be checked before assuming that multiple transmission components have failed.

Can P0720 Cause Speedometer Problems?

Yes, on some vehicles.

The output speed sensor may be used to determine vehicle speed.

A malfunction can therefore cause:

• Speedometer reading incorrectly

• Speedometer dropping to zero

• Speedometer fluctuation

• Vehicle-speed-related warning lights

• Cruise-control cancellation

Other vehicles use wheel-speed sensors for vehicle speed, so the speedometer may remain normal despite P0720.

Can P0720 Affect Cruise Control?

Yes.

Cruise control requires reliable vehicle-speed information.

If the vehicle depends on the transmission output-speed signal for its speed calculation, P0720 may cause cruise control to be disabled.

This is generally a protective strategy rather than evidence of a cruise-control component failure.

Can P0720 Cause Limp Mode?

Yes.

The TCM may enter fail-safe mode if it cannot reliably obtain output-speed information.

Depending on the transmission, the vehicle may experience:

• Fixed-gear operation

• Limited gear selection

• Harsh shifting

• Disabled torque converter clutch

• Reduced transmission performance

The exact behavior varies by vehicle.

How Is P0720 Diagnosed?

A proper diagnosis should identify which part of the OSS circuit has failed.

1. Scan all control modules

Look for additional codes related to:

• Output speed

• Input speed

• Engine speed

• Vehicle speed

• Gear ratios

• CAN communication

• TCM

• ECM/PCM

2. Check freeze-frame data

Review the conditions when P0720 was stored.

Important information includes:

• Vehicle speed

• Engine RPM

• Transmission output speed

• Selected gear

• Transmission temperature

• Throttle position

• Battery voltage

3. Check OSS live data

Use a scan tool to determine whether the TCM is receiving an output-speed value.

4. Compare output speed with actual vehicle speed

The reported OSS value should change logically as the vehicle accelerates and decelerates.

5. Inspect the OSS sensor

Check its condition, mounting, contamination, and installation.

6. Inspect the connector

Check for corrosion, loose terminals, bent pins, and water intrusion.

7. Test the wiring

Check for:

• Continuity

• Resistance

• Shorts

• Opens

• Voltage drop

8. Verify power and ground

Confirm that the sensor receives the required electrical supply.

9. Inspect the reluctor

If the electrical circuit is correct, inspect the mechanical target.

10. Check communication

If speed information is transmitted between modules, investigate the CAN network.

Why Is Live Data Important for P0720?

Live data helps determine whether the TCM is actually receiving usable output-speed information.

For example:

Vehicle moving → OSS remains at 0 RPM

This suggests that the TCM is not receiving an appropriate output-speed signal.

Another possibility is:

Vehicle speed increases → OSS value remains fixed or behaves abnormally

This can indicate a sensor, signal, or wiring problem.

What If P0720 Appears Immediately After the Vehicle Starts?

If the code appears before the vehicle moves, inspect the electrical circuit first.

Possible causes include:

• Sensor electrical failure

• Open circuit

• Missing power

• Missing ground

• Signal-wire problem

• TCM input problem

The TCM may detect a circuit fault before it even needs a valid rotating speed signal.

What If P0720 Appears Only After the Vehicle Starts Moving?

This can point toward a problem that becomes detectable when the output shaft begins rotating.

Possible causes include:

• Faulty OSS sensor

• Incorrect sensor gap

• Damaged reluctor

• Sensor installation problem

• Signal-quality problem

The exact diagnostic strategy depends on the vehicle's monitoring logic.

What If P0720 Appears When the Transmission Is Hot?

A heat-sensitive sensor or wiring problem is possible.

If the vehicle operates normally when cold but P0720 appears after the transmission warms up, investigate:

• OSS sensor

• Internal wiring

• Connector

• Sensor mounting

Temperature-dependent electrical faults can be difficult to reproduce during a short workshop inspection.

What If P0720 Appears During Acceleration?

Acceleration can cause increased movement and vibration around the transmission.

If P0720 occurs specifically during acceleration, inspect:

• OSS wiring

• Connector

• Harness routing

• Transmission ground

• Sensor mounting

A partially broken wire may work normally at idle and lose contact when the transmission moves under load.

Can an Oscilloscope Help Diagnose P0720?

Yes.

An oscilloscope can show the actual output-speed sensor waveform.

Depending on the sensor design, testing can reveal:

• Missing pulses

• Incorrect pulse frequency

• Weak signal

• Distorted waveform

• Electrical interference

• Irregular pulse spacing

This can help determine whether the problem originates from the sensor, wiring, or mechanical target.

Is P0720 a Serious Problem?

It can be.

The output-speed signal is an important input for transmission control.

If the signal becomes unreliable, the vehicle may experience:

• Harsh shifting

• Incorrect gear selection

• Limp mode

• Incorrect ratio calculations

• Torque converter problems

• Speedometer problems

The vehicle should be diagnosed promptly, especially if transmission operation has changed.

Can You Drive With P0720?

If the vehicle is operating normally, a short drive to a repair facility may be possible.

However, continued driving is not recommended if there is:

• Severe shifting

• Transmission slipping

• Unexpected gear changes

• Limp mode

• Speedometer failure

• Significant transmission performance problems

An unreliable output-speed signal can cause unpredictable transmission behavior.

How Is P0720 Repaired?

The repair depends on the confirmed cause.

Possible repairs include:

• Replacing the output speed sensor

• Repairing damaged wiring

• Repairing connector terminals

• Correcting sensor installation

• Repairing power or ground problems

• Repairing internal transmission wiring

• Repairing or replacing a damaged reluctor

• Correcting CAN communication problems

• Repairing or replacing the TCM when properly diagnosed

The important point is to repair the confirmed fault rather than automatically replacing the sensor or transmission.

Does P0720 Require Transmission Repair?

Not necessarily.

P0720 is primarily a circuit-related code.

A failed sensor, damaged wire, corroded connector, or electrical supply problem can cause the code without any internal transmission damage.

If the sensor is located inside the transmission, accessing or replacing it may require additional labor, but that still does not automatically mean the transmission needs rebuilding.

Common Diagnostic Mistakes

Assuming the transmission is mechanically damaged

P0720 is an output-speed sensor circuit code.

Replacing the OSS sensor immediately

The wiring and connector should also be tested.

Ignoring power and ground

A sensor cannot operate correctly without the required electrical supply.

Ignoring the reluctor

A damaged tone wheel can cause an otherwise good sensor to produce an incorrect signal.

Testing only while parked

Some problems become apparent only when the transmission output shaft is rotating.

Ignoring other DTCs

Input-speed, engine-speed, gear-ratio, and communication codes may help identify the actual problem.

Replacing the TCM without testing the circuit

A TCM should only be considered after the sensor and wiring have been verified.

How Do You Clear P0720?

The underlying fault should be repaired first.

After the repair:

Clear the DTC → Monitor OSS live data → Road test → Check shifting and vehicle-speed information → Rescan

The output-speed signal should change logically with vehicle speed and transmission operation.

If P0720 returns, further testing is required.

Final Thoughts on P0720

P0720 – Output Speed Sensor (OSS) Circuit Malfunction indicates that the TCM has detected a problem in the electrical circuit responsible for the transmission output-speed signal.

The code is broader than P0721 – Range / Performance, P0722 – No Signal, and P0723 – Intermittent. P0720 does not specify exactly whether the signal is missing, implausible, or intermittent. Instead, it tells the technician that the OSS circuit is not operating as expected.

Possible causes include a faulty output speed sensor, damaged wiring, loose or corroded connectors, power or ground problems, incorrect sensor installation, a damaged reluctor or tone ring, internal transmission wiring faults, CAN communication problems, or a TCM issue.

A useful diagnostic approach is to monitor OSS live data and compare it with actual vehicle speed, engine RPM, transmission input speed, and the selected gear. Electrical testing should then be used to determine whether the problem is at the sensor, wiring, power supply, signal circuit, or control module.

A practical diagnostic path is:

Scan all DTCs → Check freeze-frame data → Monitor OSS live data → Compare output speed with vehicle speed → Inspect the sensor → Check connector and terminals → Test wiring → Verify power and ground → Inspect the reluctor/tone ring → Check communication → Repair the confirmed cause → Clear the code → Road test and rescan.

Most importantly, P0720 does not automatically mean the transmission needs to be rebuilt. In many cases, the actual problem is an electrical fault involving the output speed sensor or its circuit, rather than a mechanical failure inside the transmission.