Output Speed Sensor Circuit
The Powertrain Control Module (PCM) uses the Output Shaft Speed (OSS) sensor to determine the rotational speed of the transmission output shaft. Code P0720 is set when the PCM detects an erratic, missing, or irrational voltage signal from the OSS sensor circuit.
This code requires immediate attention because it directly impairs the transmission's ability to shift gears correctly. Continued driving can result in unpredictable vehicle behavior, limp mode, or internal transmission damage.
Common symptoms
- Check Engine Light illuminated
- Erratic, harsh, or delayed transmission shifting
- Transmission stuck in 'limp mode' (failsafe gear)
- Speedometer operates erratically or reads zero
- Engine stalling or hesitation when coming to a stop
Likely causes
- Faulty Output Shaft Speed (OSS) sensor
- Damaged, corroded, or loose wiring and connectors in the OSS circuit
- Metal debris on the magnetic tip of the OSS sensor
- Dirty or contaminated transmission fluid
- Faulty Powertrain Control Module (PCM) or Transmission Control Module (TCM)
What to check before replacing parts
Inspect the OSS sensor wiring harness and connector
Look for: Look for chafed wires, melted insulation, or corroded pins that could interrupt the sensor signal.
Check the transmission fluid level and condition
Look for: Check for heavily degraded fluid or excessive metal shavings, which can interfere with the magnetic sensor's ability to read the reluctor wheel.
Monitor the OSS sensor data PID using an advanced scan tool
Look for: While driving, look for a smooth, progressive increase in output speed; dropouts or erratic spikes indicate a failing sensor or circuit issue.
Test the OSS sensor resistance with a digital multimeter
Look for: Compare the measured resistance across the sensor terminals to the manufacturer's specifications; an open or out-of-range reading confirms a dead sensor.
Check the sensor signal output using an oscilloscope
Look for: Look for a clean, consistent square wave (Hall effect) or AC sine wave (VR sensor) that increases in frequency with vehicle speed, without any dropouts or noise.
This page explains the generic P0720 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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