P0718

What Does Code P0718 Mean?

DTC P0718 signifies an detected by the Powertrain Control Module (PCM) or Transmission Control Module (TCM). The Input Speed Sensor (ISS), often referred to as the Turbine Speed Sensor (TSS) or Transmission Input Speed Sensor (TISS), is a critical component responsible for monitoring the rotational speed of the transmission’s input shaft. This shaft is directly coupled to the torque converter, and subsequently, to the engine’s crankshaft. The PCM utilizes the ISS signal to determine the rotational speed entering the transmission. This data is indispensable for calculating actual gear ratios, monitoring torque converter slip, and accurately managing transmission shift points, line pressure, and overall clutch engagement. The code P0718 specifically indicates that the PCM has observed an erratic, inconsistent, or intermittent electrical signal from the ISS circuit. Unlike a continuous open or short circuit (which would typically trigger P0715 or P0717), P0718 points to transient signal drops, spikes, or inconsistencies that intermittently fall outside the PCM’s pre-programmed operational parameters and expected correlation with other speed sensors (like the Output Speed Sensor) and engine RPM. The PCM detects this by continuously comparing the ISS signal against expected values based on engine speed and output shaft speed. If the ISS signal intermittently deviates significantly from this expected correlation, often under specific driving conditions (e.g., vibration, temperature changes, or varying load), the P0718 code is set, indicating an unreliable data stream from the sensor.

Common Symptoms

• Check Engine Light (MIL) illumination: The primary and often only initial symptom.
• Erratic or harsh transmission shifting: Unpredictable or delayed gear changes, or excessively firm shifts due to the PCM receiving unreliable speed data.
• Transmission slipping: Inability of the transmission to maintain proper gear engagement, leading to RPM fluctuations without corresponding vehicle speed changes.
• Transmission entering “limp mode”: The transmission may default to a failsafe mode, often limiting available gears (e.g., 2nd or 3rd gear only) to prevent further damage.
• Lack of specific gears or overdrive functionality: The transmission may refuse to engage certain gears, particularly higher gears, or refuse to lock the torque converter.
• Poor fuel economy: Incorrect gear selection and excessive torque converter slip can lead to increased fuel consumption.
• Inconsistent or no speedometer/tachometer readings: While less common for ISS (more typical of VSS), severe ISS data corruption can indirectly affect overall vehicle speed calculations or cause related instrument cluster issues.

What Causes the Code P0718?

• Faulty Input/Turbine Speed Sensor (ISS): Internal electrical failure, demagnetization of the sensor element, or physical damage to the sensor tip.
• Intermittent wiring issues in the ISS circuit: Frayed, corroded, or loose wires within the harness leading to the ISS, often exacerbated by vibration, heat, or moisture.
• Corroded or damaged sensor connector: Poor terminal tension within the ISS connector, oxidation on the pins, or physical damage to the plastic housing preventing a secure connection.
• Contaminated transmission fluid or debris: Excessive metallic debris or sludge in the transmission fluid can intermittently interfere with the magnetic field of the sensor, causing erratic readings, though this is less common for electrical intermittency.
• PCM internal fault: (Rare) A malfunction within the Powertrain Control Module itself, specifically in the input processing circuitry responsible for interpreting the ISS signal.
• Internal transmission mechanical issues: (Very rare direct cause) While not a direct electrical cause, excessive shaft play or bearing wear within the transmission could theoretically cause the ISS to intermittently lose proper alignment with its tone wheel, leading to signal variations. This would typically be accompanied by other severe mechanical noise or performance issues.

How to Diagnose and Troubleshoot

Diagnosing an intermittent fault like P0718 requires meticulous attention to detail and often necessitates dynamic testing. A structured approach is crucial:

1. Initial Scan and Visual Inspection:
   • Connect an OBD-II scanner to confirm P0718 and check for any related or accompanying codes (e.g., P0715, P0716, P0717 for ISS circuit faults; P0720, P0721, P0722 for Output Speed Sensor; P0700 for general transmission control system fault). Record freeze frame data.
   • Perform a thorough visual inspection of the Input Speed Sensor and its entire wiring harness. Look for signs of chafing, cuts, corrosion, exposed wires, or loose connections. Pay close attention to areas where the harness passes near hot exhaust components, sharp edges, or through grommets.
   • Inspect the ISS sensor itself for physical damage, cracks, or signs of impact.
   • Check the transmission fluid level and condition. Dark, burnt fluid, or the presence of metallic particles, warrants further internal transmission investigation, though it’s rarely a direct cause of electrical intermittency.
2. Wiring Harness Integrity Test (Key Off):
   • Locate and disconnect the ISS sensor connector and, if accessible, the corresponding PCM/TCM connector.
   • Using a Digital Multimeter (DMM), perform continuity checks on the signal, reference voltage, and ground wires from the ISS connector back to the PCM/TCM connector. While performing these checks, gently flex, pull, and wiggle the entire wiring harness, especially at bends and connection points, to detect intermittent open circuits. A steady 0.0-0.5 Ohms reading is expected for continuity.
   • Check for short circuits to ground or to power. Measure resistance between each wire in the ISS circuit and a known good chassis ground, and between each wire and battery positive. Resistance should be infinite (“OL” on most DMMs). Wiggle the harness during this test as well.
   • Inspect the terminals within both the ISS and PCM/TCM connectors for corrosion, bent pins, or signs of poor pin tension. Use a small pick or terminal test kit to gently check the grip of each terminal.
3. Dynamic Sensor Output Monitoring (Advanced Scanner & Driving Test):
   • Connect an advanced OBD-II scanner capable of displaying live data PIDs (Parameter Identifiers).
   • Access and monitor the Input Shaft Speed (ISS), Output Shaft Speed (OSS), and Engine RPM PIDs simultaneously.
   • Perform an extended test drive under varying conditions where the fault typically occurs (e.g., acceleration, deceleration, steady cruise, varying engine/transmission temperatures).
   • Observe the ISS reading carefully for sudden drops, erratic fluctuations, or implausible values that do not correlate with the engine RPM (especially when the torque converter is locked) or the OSS. A functional ISS signal should increase smoothly with engine speed and match engine RPM when the torque converter is engaged.
   • If the ISS reading intermittently drops to zero or displays erratic values while engine RPM and OSS remain stable, it strongly indicates a problem with the ISS sensor or its immediate circuit.
4. Voltage and Ground Checks at Sensor Connector (Key On, Engine Off):
   • With the ISS sensor disconnected, turn the ignition key to the “ON” position.
   • Using a DMM, check for the specified reference voltage (typically 5V or 12V, depending on the sensor type) at the appropriate terminal of the ISS connector.
   • Check for a good ground signal (near 0V) at the ground terminal of the ISS connector.
   • If either is missing or erratic, trace back to the PCM/TCM for a wiring issue or a PCM/TCM fault.
5. Sensor Test (if applicable and safe):
   • For some 2-wire variable reluctance type sensors, a DMM set to AC volts can show varying voltage as the tone wheel rotates. For 3-wire Hall-effect sensors, an oscilloscope is ideal to observe the square wave signal. Without specialized tools or safe access, relying on live data is often more practical.

Recommended Repairs and Solutions

Once the diagnostic steps have pinpointed the likely cause, the following repairs are recommended:

• Repair or Replace Wiring and Connectors: Given the intermittent nature of P0718, wiring harness damage or faulty connectors are the most frequent culprits. Carefully repair any frayed, corroded, or broken wires using proper automotive-grade crimp connectors and heat-shrink tubing for environmental protection. If the sensor connector itself is damaged (e.g., cracked housing, corroded pins, poor terminal tension), replace the entire connector assembly. Ensure all connections are clean, secure, and properly seated.
• Replace the Input/Turbine Speed Sensor (ISS): If the wiring and connectors test good, and live data strongly indicates an erratic or intermittent signal directly from the sensor, replacing the ISS is the next logical step. The location of the ISS can vary significantly by vehicle; some are externally mounted on the transmission case, while others are internal, requiring transmission pan removal and fluid drainage. Always use an OEM-quality replacement part for optimal performance and longevity.
• Address Transmission Fluid Issues: If the initial inspection revealed low or severely contaminated transmission fluid, perform a full transmission fluid and filter service. While less likely to directly cause an electrical intermittency, proper fluid level and condition are vital for the overall health and optimal operation of the transmission, which can indirectly affect sensor performance over time.
• Check for PCM/TCM Software Updates: In rare cases, an overly sensitive PCM/TCM calibration might be flagging minor, non-critical signal fluctuations. Check for any Technical Service Bulletins (TSBs) specific to your vehicle make and model regarding P0718, as a PCM/TCM reflash with updated software may be available to adjust diagnostic thresholds or improve signal processing.
• Thorough Post-Repair Test Drive and Verification: After any repair, clear all diagnostic trouble codes. Perform an extensive test drive under the exact conditions that previously triggered the P0718 code. Monitor live data PIDs (especially ISS, OSS, and Engine RPM) to confirm that the ISS signal is now stable, consistent, and correlating correctly with other speed inputs. Ensure the code does not reset and that transmission operation is smooth and normal.
• Professional Tip for Intermittent Faults: The “wiggle test” is invaluable for intermittent faults. With the engine running (if safe) and the scanner connected, gently wiggle, pull, and flex sections of the wiring harness and connectors while observing the ISS PID on live data. A sudden drop or spike in the reading will help pinpoint the exact location of the intermittent connection. Also, consider the effect of temperature; some intermittent faults only manifest when components are hot or cold.