P1720

What Does Code P1720 Mean?

DTC P1720 indicates a detected malfunction within the vehicle speed (meter) circuit. The Engine Control Module (ECM), often referred to as the Powertrain Control Module (PCM), monitors the electrical signal generated by the Vehicle Speed Sensor (VSS) to determine the vehicle’s road speed. This signal is critical for a multitude of vehicle operations, including automatic transmission shift scheduling, cruise control, speedometer operation, and ABS/traction control systems. When P1720 is set, the ECM/PCM has identified an implausible or absent signal specific to the circuit that supplies vehicle speed information, particularly as it relates to the instrument cluster’s speedometer display or the broader vehicle speed information network. This can mean the signal itself is erroneous, intermittent, or completely lost before it reaches the necessary modules that interpret or display speed. The ECM/PCM typically compares the VSS input with other speed-related data (such as wheel speed sensors, if equipped) and, if a discrepancy or absence of signal is detected in the primary VSS circuit feeding the ‘meter’ function, it will trigger P1720.

Common Symptoms

• Inaccurate or erratic speedometer readings: The speedometer may jump, stick, or display incorrect speeds.
• Non-functional speedometer: The speedometer needle remains at zero regardless of vehicle speed.
• Malfunctioning odometer/trip meter: These may not record distance travelled.
• Transmission shifting irregularities: Hard shifts, delayed shifts, or incorrect gear engagement due as the transmission control module relies on accurate vehicle speed data.
• Inoperative cruise control: The system may fail to engage or disengage unexpectedly due to lack of speed input.
• Illuminated Anti-lock Brake System (ABS) or Traction Control System (TCS) warning lights: While P1720 is not primarily an ABS code, a complete loss of vehicle speed information can impact these systems if they rely on the same VSS signal or cannot reconcile speed data from other sensors.

What Causes the Code P1720?

• Faulty Vehicle Speed Sensor (VSS): The sensor itself may be internally shorted, open, or otherwise damaged, leading to an incorrect or absent signal output.
• Damaged or corroded wiring/connectors: Breaks, chafing, shorts to ground or power, or corroded terminals in the VSS circuit can disrupt the signal path between the sensor, PCM, and instrument cluster.
• Intermittent electrical connection: Loose connections at the VSS, PCM, or instrument cluster can cause erratic signal transmission.
• Instrument cluster malfunction: The speedometer unit or its internal circuitry may be faulty, preventing it from correctly interpreting or displaying the VSS signal, even if the signal reaching the cluster is accurate.
• PCM/ECM internal fault: While less common, a faulty PCM/ECM could incorrectly interpret the VSS signal or fail to process it properly, though this usually manifests with other DTCs.
• Tone ring or reluctor wheel damage: If the VSS relies on a magnetic reluctor wheel (e.g., on the transmission output shaft), damage to this wheel can prevent the VSS from generating an accurate signal.

How to Diagnose and Troubleshoot

Diagnosing P1720 requires a methodical approach, utilizing an OBD-II scanner, a digital multimeter (DMM), and thorough visual inspection.

1. Verify the Code and Check for Other DTCs: Connect an OBD-II scanner and confirm P1720. Check for any other related codes (e.g., P0500 VSS Malfunction, P0720 Output Speed Sensor Circuit Malfunction) as they may point to a more specific issue. Record freeze frame data for contextual information.
2. Perform Visual Inspection: Carefully inspect the VSS and its associated wiring harness and connectors. Look for signs of physical damage, chafing, corrosion, or loose connections. Pay close attention to the harness routing near exhaust components, sharp edges, or moving parts. Check the VSS mounting for looseness.
3. Monitor Live Data: With the OBD-II scanner, access live data streams. Observe the “Vehicle Speed Sensor” parameter while test driving the vehicle. If the VSS reading is erratic, stuck at zero, or displays an implausible value compared to actual road speed (e.g., GPS speed), it confirms a signal integrity issue.
4. Check VSS Power and Ground: Disconnect the VSS electrical connector. Using a DMM, check for proper voltage supply (typically 5V or 12V, refer to service manual) and a good ground connection at the VSS harness connector. Backprobe the connector with it attached and the ignition on to verify the presence of power and ground under load.
5. Test VSS Output (Analog Signal):
   • For 2-wire (AC generator type) VSS: With the sensor connected and the vehicle safely raised and supported (drive wheels suspended), manually rotate the drive wheels (or engage drive and slowly accelerate). Set your DMM to AC voltage. Backprobe the VSS signal wires. You should observe a fluctuating AC voltage that increases in frequency and amplitude with wheel speed. Absence of AC voltage indicates a faulty VSS or tone ring.
   • For 3-wire (Hall effect or magnetoresistive) VSS: These typically output a digital (square wave) signal. While backprobing, you’ll be looking for a pulsating DC voltage (e.g., toggling between 0V and 5V). An oscilloscope provides the most accurate visualization of this waveform. A DMM set to DC voltage might show an average voltage that fluctuates with speed, but a frequency counter or oscilloscope is ideal.
6. Check Wiring Continuity and Resistance: With the ignition off and relevant connectors disconnected (VSS, PCM, instrument cluster if necessary), use your DMM to check for continuity between the VSS connector and the PCM/instrument cluster connector terminals according to the wiring diagram. Look for open circuits or shorts to ground/power. Also, check for excessive resistance in the VSS signal wire, which could attenuate the signal.
7. Inspect Tone Ring/Reluctor Wheel: If the VSS is mounted to a transmission output shaft or similar rotating component, visually inspect the tone ring or reluctor wheel for damage, missing teeth, or excessive debris accumulation.
8. Instrument Cluster Test: Some vehicles have a built-in instrument cluster self-test or diagnostic mode. Consult the service manual to activate this to check for speedometer functionality independent of the VSS input.

Recommended Repairs and Solutions

Based on the diagnostic findings, the following repairs are commonly indicated for P1720:

• Replace the Vehicle Speed Sensor (VSS): This is the most common resolution if diagnostics confirm a faulty sensor (no output, incorrect output, or internal resistance out of specification). Always use a high-quality OEM or equivalent aftermarket sensor for reliable operation.
• Repair or Replace Wiring Harness: If damaged, corroded, or shorted wiring is identified, repair the specific section using appropriate automotive-grade wire, solder, and heat-shrink tubing. For extensive damage, replacing the entire harness segment may be necessary. Ensure all connections are clean, tight, and corrosion-free.
• Clean VSS Connector: If corrosion or dirt is present, clean the connector terminals thoroughly with electrical contact cleaner and a small brush. Apply dielectric grease to prevent future corrosion.
• Replace Instrument Cluster: If all VSS circuit components (sensor, wiring, PCM input) test good, and the VSS signal is confirmed to be reaching the instrument cluster correctly, but the speedometer still malfunctions, the instrument cluster itself may require replacement or repair. This often involves programming the new cluster to the vehicle.
• Repair/Replace Tone Ring/Reluctor Wheel: If the VSS itself is good but the tone ring it reads from is damaged, the rotating component (e.g., transmission output shaft, axle) containing the tone ring may need to be replaced.

Mechanic’s Tips:

• Always clear the DTCs after completing any repair and perform a test drive to confirm the fault does not return.
• Pay attention to the specific location and type of VSS. Some vehicles use multiple speed sensors (input, output, wheel speed) for various functions; P1720 points specifically to the circuit providing speed data to the meter/general vehicle speed calculation.
• When working on wiring, always disconnect the battery to prevent accidental shorts or damage to electronic components.
• After replacing a VSS, ensure it is properly seated and secured to prevent future issues.
• For digital VSS signals, an oscilloscope provides a much clearer picture of signal integrity than a DMM. Consider using one if troubleshooting becomes complex.