The P0720 diagnostic trouble code (DTC) is triggered when your vehicle’s engine control module (ECM) detects a malfunction in the transmission output speed sensor circuit. This sensor is critical for monitoring how fast your transmission’s output shaft is spinning, which helps the transmission control module optimize shift timing, torque converter lockup, and fuel efficiency. When this sensor fails or its circuit is compromised, your vehicle may experience rough shifting, poor fuel economy, and reduced performance.
What Does P0720 Mean?
The P0720 code specifically refers to a problem with the Output Speed Sensor (OSS) Circuit, also known as the vehicle speed sensor (VSS) or transmission speed sensor. This sensor generates an electrical signal that tells the transmission computer how fast the transmission output shaft is rotating. The ECM uses this data to:
- Calculate actual vehicle speed
- Determine proper shift points and timing
- Monitor torque converter lockup
- Adjust transmission pressure and flow
- Enable cruise control and speedometer operation
When the P0720 code is set, it means the sensor is either not sending a signal, sending an intermittent signal, or sending a signal that doesn’t match expected values. The transmission control module recognizes this discrepancy and stores the fault code.
Common Symptoms
- Check Engine Light (CEL) or Transmission Warning Light – The most obvious indicator that the code has been triggered
- Harsh or Delayed Shifts – The transmission may shift abruptly or take longer to shift gears because it lacks speed feedback
- Erratic Speedometer Behavior – The speedometer may bounce, read incorrectly, or stop working entirely
- Reduced Fuel Economy – Without proper speed feedback, the transmission can’t optimize shift points for efficiency
- Transmission Stuck in Limp Mode – The transmission may default to a single gear (usually 3rd or 4th) to allow safe driving to a repair shop
- Cruise Control Malfunction – Cruise control may be disabled or operate erratically
- Torque Converter Lockup Issues – You may notice the engine RPM staying higher than normal on the highway
- Difficulty Starting or Rough Idle – In some vehicles, a faulty OSS can affect engine performance
Possible Causes
Listed from most to least common:
- Faulty Output Speed Sensor – The sensor itself has failed due to age, heat damage, or internal electrical failure. This is the most common cause.
- Damaged Sensor Wiring or Connector – Corrosion, loose connections, or damaged wires in the sensor circuit can prevent proper signal transmission.
- Contaminated or Damaged Sensor Reluctor Ring – The tone ring (reluctor ring) that the sensor reads may be cracked, rusted, or covered in debris, preventing accurate readings.
- Transmission Fluid Issues – Low, dirty, or contaminated transmission fluid can affect sensor operation and cause electrical problems.
- ECM or TCM Software Glitch – A temporary malfunction in the transmission control module may cause a false P0720 code.
- Wiring Harness Problems – Pinched, melted, or corroded wiring in the sensor harness can interrupt the signal.
- Internal Transmission Damage – Rarely, internal transmission problems affecting the output shaft can trigger this code.
- Failed Transmission Control Module (TCM) – In rare cases, the TCM itself may fail, preventing proper sensor signal processing.
Diagnostic Steps
Follow these steps to diagnose the P0720 code:
Step 1: Verify the Code and Scan for Related Codes
Use an OBD-II scanner to confirm P0720 is present. Check for related codes such as:
- P0500 – Vehicle Speed Sensor Malfunction
- P0501 – Vehicle Speed Sensor Range/Performance
- P0715 – Input Speed Sensor Circuit
Related codes can help pinpoint whether the issue is with the output sensor, input sensor, or the transmission control module itself.
Step 2: Inspect the Sensor and Connector
Locate the output speed sensor (typically mounted on the transmission housing near the output shaft). Inspect for:
- Physical damage or cracks
- Corrosion or rust on the connector
- Loose or disconnected wiring
- Debris or contamination around the sensor
Clean the connector and ensure it’s fully seated. Sometimes this simple step resolves the issue.
Step 3: Check Transmission Fluid Level and Condition
Low or dirty transmission fluid can cause sensor malfunction. Check the fluid level and color:
- Fluid should be bright red and clear
- Dark red or brown fluid indicates contamination
- A burnt smell suggests internal transmission damage
If the fluid is low, top it off and retest. If it’s dirty, a transmission fluid and filter change may be needed.
Step 4: Test the Sensor with a Multimeter
Disconnect the sensor connector and use a multimeter to check:
- Resistance: Should typically be between 200–900 ohms (varies by vehicle)
- Voltage: With the engine running, AC voltage should be present (typically 0.5–5 volts AC)
- Continuity: Check wiring for breaks or shorts
If readings are outside the manufacturer’s specifications, the sensor or wiring is faulty.
Step 5: Inspect the Reluctor Ring
The reluctor ring (tone ring) on the output shaft must be clean and undamaged. If accessible, inspect for:
- Cracks or chips in the ring
- Rust or corrosion
- Debris buildup
A damaged reluctor ring requires transmission removal and repair or replacement.
Step 6: Check Wiring and Connectors Throughout the Circuit
Trace the sensor wiring from the sensor to the transmission control module, checking for:
- Pinched or melted wires
- Corroded connectors
- Loose connections
- Water intrusion or moisture
Step 7: Clear the Code and Test Drive
After making repairs or cleaning connections, clear the code using your OBD-II scanner and take a test drive. If the code returns, the sensor or related component needs replacement.
Repair Cost Estimates
The cost to repair a P0720 code varies depending on the root cause and your vehicle:
- Output Speed Sensor Replacement: $150–$400
- Parts: $50–$150
- Labor: $100–$250
- Wiring Repair or Connector Replacement: $100–$300
- Parts: $20–$100
- Labor: $80–$200
- Transmission Fluid and Filter Change: $150–$300
- Reluctor Ring Replacement: $500–$1,500+ (requires transmission removal)
- Transmission Control Module Replacement: $800–$2,000+ (rare)
Note: Luxury and high-performance vehicles may have higher labor rates. Always get a diagnostic fee quote before committing to repairs.
Can I Still Drive?
P0720 is classified as a moderate severity code. Here’s what you need to know:
- Short-term driving is generally safe, but you should have the vehicle diagnosed and repaired as soon as possible.
- Your transmission may enter “limp mode,” limiting you to a single gear and reducing power. This protects the transmission from damage.
- Fuel economy will suffer significantly without proper transmission shift optimization.
- Harsh shifting can accelerate transmission wear, potentially leading to more expensive repairs if ignored.
- Avoid towing or hauling heavy loads until the sensor is repaired, as the transmission cannot properly manage load-dependent shift points.
- Highway driving is safe, but expect reduced performance and fuel economy.
Bottom line: You can drive to a repair shop, but don’t delay repairs. Continuing to drive with a faulty output speed sensor can lead to transmission damage and more expensive repairs down the road.
Frequently Asked Questions
Can a bad output speed sensor damage my transmission?
Yes, if left unrepaired for an extended period. Without proper speed feedback, the transmission cannot optimize shift timing and pressure, leading to harsh shifts, slipping, and accelerated wear on internal components. Addressing the code promptly prevents more costly transmission damage.
Will clearing the code fix the problem?
No. Clearing the code temporarily removes the warning light, but it does not fix the underlying problem. The code will return once the sensor detects the same fault condition again. You must repair or replace the faulty component to permanently resolve the issue.
Is the output speed sensor the same as the vehicle speed sensor?
In many vehicles, yes. The output speed sensor (OSS) and vehicle speed sensor (VSS) are the same component. However, in some modern vehicles with multiple speed sensors, they may be separate. Always consult your vehicle’s service manual to identify the correct sensor for your specific make and model.
Can I replace the output speed sensor myself?
In many vehicles, yes. The sensor is typically located on the transmission housing and can be accessed without removing the transmission. However, some vehicles require transmission removal for access. If you’re comfortable with basic automotive work and have the right tools, you can save on labor costs. Always consult your vehicle’s service manual for specific instructions and torque specifications.