P1718

What Does Code P1718 Mean?

DTC P1718 signifies an “In Range Failure High” condition for the Transmission Fluid Temperature (TFT) sensor. This code indicates that the Powertrain Control Module (PCM), or Transmission Control Module (TCM) in some applications, is receiving an input signal from the TFT sensor that suggests an abnormally high transmission fluid temperature. Crucially, the term “in range failure” means the signal is not an outright open circuit or a dead short to ground (which would typically set codes like P0710, P0711, P0712, or P0713), but rather an electrical signal that is plausible to the PCM/TCM yet is significantly higher than expected or rational for the current operating conditions. The TFT sensor is a thermistor, usually a Negative Temperature Coefficient (NTC) type, meaning its electrical resistance decreases as the fluid temperature increases. Therefore, an “in range failure high” condition implies that the sensor’s resistance is lower than it should be, or the circuit integrity causes the PCM/TCM to interpret the signal as a lower resistance, corresponding to an excessively hot transmission fluid. This skewed high temperature reading directly impacts automatic transmission operation, as the module relies on accurate fluid temperature data for optimal shift scheduling, torque converter clutch engagement, line pressure control, and overall transmission protection strategies.

Common Symptoms

• Harsh or erratic shifts: The TCM may adjust shift points or line pressure incorrectly based on the perceived high temperature, leading to uncomfortable or unpredictable gear changes.
• Delayed or early shifts: Incorrect fluid temperature readings can cause the transmission to operate outside its designed thermal range, affecting shift timing.
• Transmission slipping: Improper line pressure control due to false temperature data can result in the clutches and bands not holding effectively.
• Activation of transmission failsafe or limp mode: The PCM/TCM may detect the implausible high temperature as a critical fault and engage a protective mode, limiting available gears or engine power.
• Reduced fuel economy: Incorrect transmission operation due to thermal miscalculation can lead to inefficiencies.
• Check Engine Light (MIL) illumination: Although a transmission-specific code, P1718 will often trigger the MIL.
• Inaccurate transmission temperature gauge reading: If the vehicle is equipped with a transmission temperature gauge that relies on the same sensor data, it may display an excessively high reading.

What Causes the Code P1718?

• Faulty Transmission Fluid Temperature (TFT) Sensor: The most common cause is the sensor itself developing an internal fault, such as a resistance drift or internal short, causing it to consistently report a lower-than-actual resistance value (corresponding to a higher temperature).
• Wiring Harness Issues:
  • Partial short to ground: A frayed or chafed wire in the TFT sensor circuit intermittently or consistently contacting chassis ground can create a lower resistance path, mimicking a high-temperature condition.
  • Partial short to voltage: In some sensor designs, a partial short to a voltage reference line can manipulate the signal interpreted by the PCM/TCM as a high temperature.
  • High resistance or intermittent connection: While typically leading to an open circuit code, an intermittent high resistance could, in rare cases, cause erratic readings that sometimes trend high.
• Corroded or Damaged Electrical Connectors: Corrosion, bent pins, or loose terminals at the TFT sensor connector or the PCM/TCM connector can introduce spurious resistance changes or intermittent shorts into the circuit, leading to incorrect signal interpretation.
• Internal Transmission Fluid Contamination/Actual Overheating (less common for “in-range failure high”): While the code primarily points to a sensor/circuit issue, severe, prolonged actual transmission overheating could theoretically push a sensor to its extreme limits or cause sensor degradation, but P1718 usually points to a sensor or circuit fault rather than the sensor accurately reporting actual catastrophic overheating.
• Faulty Powertrain Control Module (PCM) or Transmission Control Module (TCM): Though rare, an internal fault within the PCM/TCM’s input circuit for the TFT sensor could lead to misinterpretation of the sensor signal, falsely setting P1718.

How to Diagnose and Troubleshoot

Diagnosing P1718 requires systematic testing of the TFT sensor and its associated circuit using an advanced OBD-II scan tool and a digital multimeter (DMM).

1. Preliminary Visual Inspection: Begin by inspecting the transmission fluid level and condition. Dark, burnt-smelling fluid can indicate actual overheating issues. Visually check the external wiring harness to the transmission, looking for signs of chafing, cuts, or corrosion, particularly around the TFT sensor and its connector. Ensure all related connectors are securely seated.
2. Scan Tool Data Analysis:
   • Connect an OBD-II scan tool capable of reading live data. Access the TFT sensor reading.
   • With the engine cold (preferably after sitting overnight), compare the TFT sensor reading to the ambient air temperature sensor (IAT) and engine coolant temperature (ECT) sensor readings. All three should be very close if the engine has not been run. If the TFT reading is significantly higher than ambient, even when cold, it strongly suggests a sensor or circuit fault.
   • Monitor the TFT reading while the engine warms up. Observe if the temperature increases smoothly or if it jumps erratically, or if it remains stuck at an implausibly high value.
   • Check for any pending codes and freeze frame data, which can provide valuable context regarding engine and transmission conditions when the code was set.
3. TFT Sensor Resistance Test:
   • Locate the TFT sensor. It is typically mounted in the transmission pan, integrated into the valve body, or part of the internal transmission wiring harness. Access may require draining fluid and removing the transmission pan.
   • Disconnect the electrical connector from the TFT sensor.
   • Using a DMM set to ohms, measure the resistance across the two terminals of the TFT sensor itself.
   • Compare this reading to the manufacturer’s specifications for the current ambient temperature. A “failure high” condition implies the sensor’s resistance is lower than specified for the actual temperature. For example, if the ambient temperature is 70°F (21°C), the sensor might read a resistance value typical for 200°F (93°C).
   • As the sensor is an NTC thermistor, its resistance should decrease as it is gently warmed (e.g., with a heat gun at a safe distance), and increase as it cools. An unresponsive or erratic change indicates a faulty sensor.
4. Wiring Harness Integrity Test:
   • With the TFT sensor disconnected and the PCM/TCM connector disconnected (if accessible for the TFT circuit), perform continuity checks. Measure resistance between the corresponding pins at the TFT sensor connector and the PCM/TCM connector. Resistance should be less than 0.5 ohms.
   • Check for shorts to ground: Measure resistance from each wire of the TFT sensor circuit (at the PCM/TCM side) to chassis ground. Resistance should be infinite (OL). A low resistance indicates a short to ground.
   • Check for shorts to voltage: With the ignition ON (but PCM/TCM disconnected to prevent damage), measure voltage on each wire of the TFT sensor circuit (at the PCM/TCM side). There should be no significant voltage present unless it’s a reference voltage line.
   • Verify reference voltage and ground: Connect the PCM/TCM, but leave the TFT sensor disconnected. With the ignition ON, check for the presence of the specified reference voltage (typically 5V) and a good ground at the TFT sensor connector terminals. Lack of either can cause incorrect readings.
5. PCM/TCM Diagnosis: If all sensor and wiring tests pass, and symptoms persist, a faulty PCM/TCM could be the cause. This should only be considered after exhaustive testing of the external components and circuit.

Recommended Repairs and Solutions

The repair strategy for P1718 is directly dependent on the diagnostic findings:

• Replace the Transmission Fluid Temperature (TFT) Sensor: If diagnostic tests confirm the TFT sensor itself is providing incorrect resistance readings, it must be replaced. This typically involves:
  • Draining the transmission fluid.
  • Removing the transmission pan.
  • Disconnecting and replacing the sensor (which may be a standalone unit or integrated into the internal wiring harness or valve body).
  • Installing a new pan gasket and refilling with the manufacturer-specified automatic transmission fluid (ATF) to the correct level.
• Repair or Replace Wiring Harness/Connectors: If the diagnosis points to a damaged wiring harness (frayed wires, shorts) or corroded/loose connectors, the affected sections must be repaired or replaced. Use appropriate automotive-grade wire and connectors, ensuring secure, weather-sealed repairs.
• Address Actual Transmission Overheating (if applicable): If initial fluid inspection or other diagnostic data suggests actual transmission overheating (e.g., clogged transmission cooler, low fluid, excessive towing), these underlying mechanical issues must be addressed in addition to any sensor/circuit repair. However, for a P1718 “in-range failure high,” the focus is generally on the sensor or its circuit *misreporting* the temperature.
• Replace the PCM/TCM: If all other components and wiring test perfectly, and the issue remains, the PCM/TCM may require replacement. This is an expensive and less common repair, often requiring reprogramming or specific setup procedures for the new module.

Important Mechanics’ Tips:

• Always use OEM or equivalent quality replacement parts, especially for critical sensors. Aftermarket sensors can sometimes have calibration differences.
• Ensure the transmission fluid is at the correct level and of the specified type after any work involving fluid drainage. Incorrect fluid levels or types can lead to further transmission issues.
• After completing repairs, clear the DTCs with a scan tool and perform a thorough road test to verify the fix. Monitor TFT sensor live data during the road test to ensure accurate and rational temperature readings under various operating conditions.
• Be mindful that in some modern transmissions, the TFT sensor is integrated into the valve body assembly or a larger internal wiring harness, making individual sensor replacement more complex and potentially more expensive.