Limit to low-speed local use. Repair urgently. P0720 means the output shaft speed sensor (OSS) signal is absent or out of range while the vehicle is in motion -- the ECM or TCM expected a valid speed signal from the transmission output shaft but did not receive one.
What P0720 means
The output speed sensor (also called OSS, NSS, or vehicle speed sensor depending on platform) reads the rotational speed of the transmission output shaft using a toothed tone wheel and a magnetic or Hall-effect sensor. The ECM and TCM use this signal for gear ratio calculation, automatic shift scheduling, torque converter lock-up control, speedometer input (on some architectures), and idle speed control during deceleration. P0720 is stored when the output shaft speed signal is absent or implausible during conditions where a valid signal is expected -- typically when the vehicle is in a forward drive range and moving above a threshold speed of approximately 10-15 km/h. The specific threshold and timing vary by manufacturer but a common detection window is absence of pulses for 0.5 to 2 seconds while in gear with throttle applied. Without a valid output speed signal, the transmission cannot accurately calculate the current gear ratio against the input speed, shift timing becomes inaccurate, and lock-up control is disabled or degraded. The TCM may enter a fixed-gear or limited-range limp-home mode. P0720 does not distinguish whether the failure is in the sensor itself, the tone wheel, or the wiring between the sensor and controller.
Symptoms
- Check engine light on
- Transmission shifting incorrectly -- missed shifts, late upshifts, or downshift hunting
- Speedometer may be inaccurate or non-functional if the output speed sensor is also the speedometer source on the specific platform
- Torque converter lock-up not engaging at highway speed
- Possible limp-home mode with restricted gear selection
- ABS and VSC warning lights may illuminate if the output speed sensor also feeds the ABS module on this architecture
Common causes
- Failed output speed sensor -- internal open winding, broken Hall-effect element, or shorted sensor; most common cause
- Damaged or missing tone wheel teeth on the output shaft -- physical damage from road debris, severe shaft impact, or corrosion removes the teeth that generate the signal pulses
- Open or short in the OSS wiring harness between the sensor and the TCM or ECM connector
- Corroded or damaged sensor connector -- common on rear-wheel-drive transmissions where the output sensor is exposed to road spray and contamination
- Sensor tip physically damaged or sensor mounting hardware loose, creating excessive air gap between the sensor pickup and tone wheel
- Transmission fluid contamination coating the sensor tip -- metallic debris from wear can coat magnetic sensors and attenuate the signal below detection threshold
Severity & driving advice
Severity: High — Transmission control significantly degraded. Incorrect shifts and possible limp mode. Avoid highway driving until repaired.
Can I drive? Limit to low-speed local use. Repair urgently.
Diagnostic approach
- Observe vehicle speed in scan tool Data List while driving — Connect a scan tool and navigate to transmission Data List or powertrain Data List. Observe the output speed or vehicle speed value while driving above 15 km/h. If the value reads zero while the vehicle is moving, the signal is confirmed absent. Compare to the speedometer reading if available -- a working speedometer with a zero OSS reading suggests the sensor and speedometer use different signal sources and the fault is in the transmission sensor circuit specifically.
- Locate and visually inspect the output speed sensor — The output speed sensor location varies by transmission type. On most rear-wheel-drive transmissions it is mounted in the extension housing at the rear of the gearbox. On transaxles it may be on the differential cover or the output axle housing. Inspect the sensor and connector for corrosion, physical damage, and correct seating. Magnetic sensors attract metallic debris -- wipe the sensor tip clean and inspect for debris accumulation that might be bridging the gap to the tone wheel.
- Test sensor resistance and wiring continuity — Disconnect the sensor connector. For passive magnetic sensors, measure resistance across the two signal terminals -- healthy values are typically 200-2,000 ohm depending on the application (consult the specific service manual). An open circuit (infinite resistance) confirms a failed sensor winding. For Hall-effect sensors, the test involves measuring supply voltage (typically 5 V or 12 V) at the supply pin and verifying signal output voltage toggles when the output shaft is rotated. Also measure each signal wire to body ground -- should be 10 kilohm or higher to confirm no shorts.
- Inspect the tone wheel for damaged or missing teeth — If the sensor and wiring pass all electrical tests but P0720 persists, the tone wheel is suspect. Drain the transmission fluid (or remove the transmission pan/extension housing as required) to gain visual or borescope access to the output shaft tone wheel. Look for missing, chipped, or corroded teeth, or debris packed between teeth. A missing tooth creates a periodic dropout that the TCM detects as a speed absence fault. A damaged tone wheel requires removal of the output shaft for repair or replacement.
Make & model notes
General Motors: 4L60-E and 4L80-E transmissions (Silverado, Sierra, Tahoe, Suburban, Camaro): the output speed sensor is externally mounted at the extension housing and is easily accessible without removing the transmission. P0720 on these platforms is commonly caused by connector corrosion from road spray. Inspect and clean the connector before replacing the sensor.
Ford: 4R70W and 5R55S transmissions (Mustang, F-150, Explorer): the output shaft speed sensor is externally mounted and straightforward to replace. P0720 on Ford platforms occasionally results from the tone ring (press-fit on the output shaft) slipping axially, moving away from the sensor pickup. If the sensor and wiring are good, inspect the tone ring position before removing the transmission.
Chrysler/Dodge: 45RFE and 545RFE transmissions (RAM 1500, Grand Cherokee, Durango): the output speed sensor is on the rear of the transmission extension housing. These transmissions use two speed sensors (input and output); both must be functional for correct shift control. P0720 alongside P0715 or P0717 on a Chrysler platform suggests wiring or connector damage to the transmission sensor harness as a whole rather than isolated sensor failures.
FAQ
Can P0720 cause the ABS light to turn on?
On some vehicle architectures, yes. If the output speed sensor signal is also used by the ABS or VSC module to calculate wheel speed or vehicle speed reference, loss of the OSS signal will trigger ABS and stability control warning lights in addition to the check engine light. On platforms where ABS has independent wheel speed sensors (which is the most common modern layout), P0720 affects the transmission controller only and the ABS light will not be triggered.
Is P0720 the same as a broken speedometer?
Not always. On older vehicles, the output speed sensor was the sole speed input for both the TCM and the speedometer. On modern vehicles the speedometer typically uses ABS wheel speed sensors rather than the transmission OSS, so P0720 may be present with a fully functional speedometer. If your speedometer is also inoperative alongside P0720, it suggests the vehicle is one of the older architectures sharing the same sensor signal.
What happens if I keep driving with P0720?
The transmission will continue to shift based on throttle position and engine speed estimates alone, without accurate output speed data. This causes incorrect shift timing, potential for over-revving between shifts, and loss of torque converter lock-up efficiency. Extended driving in this condition accelerates transmission clutch wear. Repair promptly to avoid secondary damage.