Code
P0720
HUMMER
P — Powertrain
Output Speed Sensor Circuit Malfunction
Views:
UK: 22
EN: 33
RU: 34
AI status
Completed
Completed
100%
Causes
- Damaged or failed output speed sensor (OSS)
- Open, shorted or corroded wiring/connectors between OSS and PCM
- Loose, bent or contaminated sensor connector pins
- Damaged or missing reluctor/magnetic tone wheel on the transmission output shaft
- Intermittent connector contact from vibration or corrosion
- Low/reference supply voltage or poor ground to the sensor circuit
Symptoms
- Check Engine MIL illuminated with P0720 stored
- Speedometer or odometer incorrect or intermittent
- Erratic transmission shifting, harsh shifts or transmission stuck in gear/limp mode
- Cruise control may be disabled or behave erratically
- Transmission torque converter lockup problems or reduced fuel economy
- ABS/wheel speed systems may show inconsistent speed readings if vehicle speed comparison is used
What to check
- Retrieve freeze frame data and stored transmission-related codes with a scan tool
- Compare OSS (output speed) live data with vehicle speed and wheel speed sensors while driving and at various speeds
- Visually inspect OSS connector and harness for damage, corrosion, loose pins or signs of rubbing/chafing
- Check transmission fluid level and condition (metal debris may indicate internal damage)
- Backprobe sensor connector: measure supply voltage, ground continuity and sensor signal while cranking and driving
- Measure sensor resistance (if specified) and continuity to PCM pins; check for shorts to battery or ground
Signal parameters
- Sensor types vary by model: either variable reluctance (VR) producing an AC sine/saw waveform or a Hall-effect/CMOS producing a square 0–5 V pulse
- VR sensor typical open-circuit resistance: manufacturer specific (often hundreds of ohms); produces AC voltage increasing with shaft speed (mV to volts)
- Hall-type sensor: provides 0–5 V square wave; reference supply typically 5 V and a ground; output toggles with shaft rotation
- Expected behavior: no output at rest (engine off), steady increasing pulse frequency with vehicle speed, clean amplitude without dropouts or large noise
- If available, use oscilloscope: clean repetitive waveform, consistent amplitude and frequency proportional to shaft speed
Diagnostic algorithm
- Read and record DTCs and freeze-frame data; note vehicle speed when code set.
- Use a scan tool to view live data: monitor OSS (output speed) and compare to wheel speed/vehicle speed sensors at idle and during road test. Look for mismatch or dropouts.
- Visually inspect OSS connector, harness routing and transmission connector for corrosion, damaged insulation, pin deformation or unseated terminals.
- With ignition ON, backprobe connector: check for required reference voltage (if Hall-type), good ground, and expected sensor signal while rotating output shaft or during road test.
- Measure sensor resistance (per service manual). For VR sensors, check for continuity and reasonable resistance; for Hall sensors verify reference and signal switching to 0/5 V.
- Use a scope (preferred) to inspect waveform quality while rotating the output shaft. Verify frequency increases with speed and waveform is clean and consistent.
- Wiggle harness and connector during monitoring to locate intermittent faults. Repair any chafed wiring, corroded connectors or loose pins.
- Inspect reluctor/tone wheel for missing teeth, debris or damage; repair or replace as needed.
- If wiring and sensor test good, check continuity from sensor connector to PCM pin and verify PCM ground(s).
- After repair or replacement, clear codes and perform road test to confirm proper operation and that P0720 does not return. If problem persists, consider PCM diagnosis and consult manufacturer technical service information/TSBs.
Likely causes
- Failed OSS (most common)
- Wiring break or short between OSS and PCM
- Corroded/loose sensor connector or pin(s)
- Damaged/shifted reluctor ring or missing teeth
- Poor ground at transmission harness or chassis ground
Fault status
Status
PCM detected a malfunction in the transmission output speed sensor circuit — signal missing, intermittent, or out of expected range.
Repair difficulty: Medium
Diagnostic time: 1.0 - 3.0 hours
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