P0720 — Output Speed Sensor Circuit

Quick answer

• What the code means: The Powertrain Control Module (PCM) has detected an abnormal signal from the output speed sensor, indicating a potential issue with measuring the transmission's output speed.
• Severity: Get towed or drive home carefully. Depending on the severity, shifting issues may occur, and prolonged driving could damage the transmission.
• Most common cause: A faulty output speed sensor.

What the P0720 code means

The P0720 diagnostic trouble code signifies "Output Speed Sensor Circuit Malfunction." In simpler terms, your vehicle's computer (the Powertrain Control Module or PCM) isn't getting a reliable signal from the sensor that measures how fast the transmission's output shaft is spinning. This output speed directly correlates to your vehicle's road speed.

The output speed sensor plays a crucial role in the proper operation of your automatic transmission. It provides critical data to the PCM for precise shift scheduling, torque converter lock-up control, and even speedometer accuracy. When this sensor circuit malfunctions, the PCM struggles to determine the correct gear, leading to various drivability problems.

Common symptoms

• Illuminated Check Engine Light
• Erratic or harsh shifting
• No speedometer or inaccurate speedometer reading
• Transmission slipping
• Vehicle stuck in "limp mode" (often third gear)
• Loss of anti-lock brake system (ABS) function in some vehicles
• Cruise control not working

Common causes

1. Faulty Output Speed Sensor: This is by far the most common culprit. Sensors can fail electronically or physically due to wear, debris, or internal shorts.
2. Damaged Wiring or Connector: Frayed wires, corrosion in the connector, or a loose connection to the output speed sensor can disrupt the signal.
3. Low Transmission Fluid Level or Contaminated Fluid: While less direct, extremely low or dirty transmission fluid can sometimes affect sensor readings due to altered internal transmission conditions.
4. Faulty Transmission Control Module (TCM) or PCM: In rare cases, the control module itself might be at fault for incorrectly interpreting the sensor signal or for internal circuit issues. This is usually diagnosed after all other possibilities have been ruled out.

How a technician diagnoses it

1. Verify the complaint and perform a visual inspection: The technician will first confirm the symptoms and look for obvious issues like damaged wiring, loose connectors, or signs of fluid leaks around the transmission.
2. Scan for DTCs and freeze-frame data: A professional scan tool will be used to confirm the P0720 code and check for any other related codes. Freeze-frame data provides a snapshot of vehicle conditions (engine speed, vehicle speed, engine load, etc.) at the moment the code was set, which can offer valuable clues.
3. Check output speed sensor readings: Using a scan tool, the technician will monitor the live data stream from the output speed sensor while test driving the vehicle. A good sensor will show a smooth, consistent speed reading that matches the vehicle's actual road speed. A faulty sensor might show erratic readings, no reading at all, or a constant (stuck) reading.
4. Inspect wiring and connector integrity: The technician will physically inspect the wiring harness leading to the output speed sensor for any signs of damage, chafing, or corrosion. They will also check the sensor connector for a secure fit and clean terminals.
5. Perform continuity and resistance checks: Using a multimeter, the technician will check the output speed sensor itself for proper resistance values (referencing manufacturer specifications) and test the wiring for continuity to ensure there are no open circuits or shorts.
6. Check transmission fluid level and condition: Ensuring the transmission fluid is at the correct level and isn't severely degraded or contaminated is a standard diagnostic step.

Typical repair cost

The cost to repair a P0720 code can vary based on the specific cause, vehicle make and model, and labor rates in your area. Here