P0720

What Does Code P0720 Mean?

The diagnostic trouble code P0720, “Output Speed Sensor Circuit Malfunction,” indicates that the Powertrain Control Module (PCM) or Engine Control Module (ECM) has detected an electrical anomaly within the circuit of the transmission’s Output Speed Sensor (OSS). The OSS is a crucial component responsible for measuring the rotational speed of the transmission’s output shaft, which directly correlates to the vehicle’s speed. The PCM utilizes this input data for a multitude of critical functions, including precise transmission shift scheduling, torque converter clutch (TCC) engagement and disengagement, speedometer and odometer operation, cruise control, and overall vehicle speed regulation. A “circuit malfunction” means the PCM is receiving an implausible signal, no signal at all, or a signal that falls outside of the expected voltage and frequency parameters for the sensor, suggesting an electrical fault rather than necessarily a mechanical failure of the sensor itself.

Common Symptoms

• Inaccurate or Inoperative Speedometer/Odometer: The most direct and noticeable symptom, as the PCM cannot accurately determine vehicle speed.
• Erratic or Harsh Transmission Shifting: The transmission may shift late, early, harshly, or experience delayed engagements due to incorrect speed data.
• Transmission Entering “Limp Mode”: To prevent damage, the transmission may revert to a default gear (often 2nd or 3rd) and limit engine RPM.
• Illuminated Malfunction Indicator Lamp (MIL): The “Check Engine” light will be illuminated.
• Inoperative Cruise Control: Systems reliant on accurate vehicle speed will often be disabled.
• Reduced Fuel Economy: Incorrect shift points and TCC operation can lead to inefficient power transfer.
• Hesitation or Stalling at Low Speeds: The engine and transmission may not properly synchronize at very low speeds or during stops.

What Causes the Code P0720?

• Faulty Output Speed Sensor (OSS): The sensor itself may have an internal electrical fault, such as an open circuit in its windings or a degraded internal component.
• Wiring Harness Damage: Open circuits, short circuits to ground, or shorts to voltage within the OSS signal, power, or ground wires. This can result from chafing, corrosion, or physical damage to the harness.
• Corroded or Damaged Connector: The electrical connector for the OSS may have bent, corroded, or loose pins, leading to an intermittent or complete loss of signal integrity.
• Internal Transmission Fluid Contamination: Excessive metallic debris or contaminated fluid can sometimes interfere with magnetic pick-up type sensors, though this typically manifests as an implausible signal rather than a circuit malfunction.
• Damaged Reluctor Wheel: If the sensor reads a reluctor wheel on the output shaft, damage to the teeth (e.g., chipped or missing) can cause an erratic or incorrect signal that the PCM interprets as a circuit fault.
• Faulty Powertrain Control Module (PCM): While less common, an internal fault within the PCM’s driver circuit for the OSS can also trigger this code. This should be considered only after all other possibilities have been thoroughly investigated.

How to Diagnose and Troubleshoot

Diagnosing P0720 requires a methodical approach, utilizing an OBD-II scanner, a digital multimeter (DMM), and visual inspection.

1. Initial Scan and Live Data Analysis:
   • Connect an OBD-II scanner and retrieve all stored Diagnostic Trouble Codes (DTCs). Note any related codes (e.g., P0721, P0722 for OSS range/performance; P0500 for general vehicle speed sensor malfunction).
   • Monitor live data for the “Output Shaft Speed” (OSS) or “Vehicle Speed Sensor” (VSS) parameter while operating the vehicle. Compare the OSS reading to the Input Speed Sensor (ISS) and actual speedometer reading. Look for a zero reading, erratic fluctuations, or a reading that does not correspond to actual vehicle speed. If the reading is consistently zero or erratic, it strongly indicates an OSS or circuit issue.
2. Visual Inspection:
   • Locate the Output Speed Sensor, typically on the transmission case near the output shaft.
   • Inspect the sensor and its electrical connector for visible damage, cracks, fluid leaks, or contamination.
   • Carefully trace the wiring harness from the OSS connector back to the PCM. Look for any signs of chafing, pinching, melting, corrosion, or exposed wires. Pay close attention to areas where the harness passes near hot engine components or sharp edges.
   • Disconnect the OSS connector and inspect the terminals for corrosion, bending, or spreading. Clean with electrical contact cleaner if necessary.
3. Electrical Testing with DMM:
   • Sensor Resistance (for passive sensors): With the ignition OFF and the OSS disconnected, measure the resistance across the two terminals of the OSS itself. Compare the reading to the manufacturer’s specifications (typically 200-2500 ohms, but varies significantly). An open circuit (OL) or a reading significantly outside specifications indicates a faulty sensor.
   • Reference Voltage/Power Supply: Reconnect the OSS. With the ignition ON (engine OFF), back-probe the OSS connector terminals to check for the proper reference voltage (often 5V or 12V from the PCM) and a good ground. Refer to the vehicle’s wiring diagram for specific pinouts.
   • Continuity Test (Wiring Harness): Disconnect both the OSS connector and the PCM connector. Using the vehicle’s wiring diagram, identify the OSS signal, power, and ground wires at the PCM connector.
     • Test for continuity between the corresponding pins at the OSS connector and the PCM connector. An open circuit (OL) indicates a break in the wire.
     • Test each wire for continuity to ground and to battery voltage. Any continuity indicates a short circuit.
   • Dynamic Signal Test (Oscilloscope Recommended): While difficult without specialized equipment, if possible, back-probe the signal wire at the OSS connector (or PCM connector) with a DMM set to AC voltage (for passive sensors) or an oscilloscope. With the vehicle safely raised and the transmission in drive (if possible, extreme caution required), or by manually rotating the output shaft, observe the signal. Passive sensors will generate an AC voltage that increases in frequency and amplitude with speed. Active (Hall effect) sensors will produce a square wave signal. An absence of signal or an erratic, non-uniform signal despite shaft rotation indicates a sensor or reluctor issue.
4. Reluctor Wheel Inspection: If the sensor and wiring test good, remove the OSS and visually inspect the reluctor wheel (if accessible) inside the transmission for missing, bent, or damaged teeth. This may require specific tools and potentially draining transmission fluid.

Recommended Repairs and Solutions

The repair strategy for P0720 depends directly on the results of the diagnostic process:

• Replace the Output Speed Sensor (OSS): This is the most common repair if the sensor itself fails the electrical resistance test or if dynamic signal testing confirms a lack of proper signal generation. Always opt for an OEM or high-quality aftermarket sensor to ensure reliability and proper function.
• Repair or Replace Wiring Harness: If visual inspection or continuity testing reveals damaged wiring, repair the affected section of the harness. Use appropriate soldering techniques, heat-shrink tubing, and ensure proper insulation. For extensive damage, replacement of the entire harness segment may be necessary. Ensure proper routing to prevent future damage.
• Clean or Repair Connector: If the OSS connector is found to have corrosion or bent pins, carefully clean the terminals with electrical contact cleaner and a small brush. Use a pick to straighten bent pins. If the damage is severe, replace the connector shell and terminals.
• Address Damaged Reluctor Wheel: If the reluctor wheel is found to be damaged, this is a more involved repair, often requiring transmission disassembly to access and replace the component. This typically warrants professional transmission shop intervention.
• PCM Replacement/Reprogramming: If all other components (sensor, wiring, connectors) test good, and vehicle-specific diagnostic procedures point to an internal PCM fault, then PCM replacement or reprogramming might be necessary. This is a rare occurrence for P0720 and should only be undertaken by a qualified technician with the correct programming tools.

Mechanic’s Tips: After any repair, clear the DTCs and perform a thorough test drive under varying conditions to confirm the fix and ensure that all readiness monitors set. Always check the transmission fluid level and condition, as very low or severely contaminated fluid can indirectly contribute to transmission sensor issues. When testing connectors, use proper back-probing tools to avoid damaging the terminals, which can lead to further electrical issues.