P1704

What Does Code P1704 Mean?

DTC P1704 indicates that the Powertrain Control Module (PCM) or Engine Control Module (ECM) has detected an anomaly in the state transitions of the Digital Transmission Range Sensor (TRS) during its self-diagnostic tests, specifically conducted in Key On, Engine Off (KOEO) or Key On, Engine Running (KOER) modes. The Transmission Range Sensor, often referred to as the Neutral Safety Switch, is a multi-position switch that informs the PCM of the current gear selector lever position (e.g., Park, Reverse, Neutral, Drive, 2nd, 1st). Unlike an analog sensor that provides a variable voltage output, a digital TRS typically uses multiple discrete switches or an encoder to send a specific combination of voltage signals (high or low) to the PCM for each gear position. The PCM performs internal tests during KOEO and KOER to verify the TRS’s ability to accurately and consistently switch between these states as the gear selector is moved. If the PCM commands a transition (or expects one based on user input during the test) and does not observe the correct corresponding change in the digital input signals from the TRS within a specified timeframe, or if it detects an illogical or invalid state, code P1704 is set. This directly affects the transmission control subsystem, impacting gear selection and potentially engine starting.

Common Symptoms

• Malfunction Indicator Lamp (MIL) illumination: The “Check Engine” light will be on.
• Transmission shifting irregularities: Hard shifts, delayed shifts, inability to shift into certain gears, or the transmission may operate in “limp home” mode, locking into a single gear (e.g., 3rd or 2nd).
• No-crank or hard-start condition: The engine may not crank in Park or Neutral, or conversely, may crank in gears other than Park or Neutral due to incorrect TRS input.
• Reverse lights not illuminating or staying on: Incorrect signaling from the TRS regarding the “Reverse” position.
• Incorrect gear display: The instrument cluster’s gear indicator (if equipped) may show the wrong gear or no gear at all.
• Cruise control inoperable: Many systems disengage or prevent activation of cruise control if transmission position data is unreliable.

What Causes the Code P1704?

• Faulty Digital Transmission Range Sensor (TRS): Internal wear, corrosion, or electrical failure within the multi-position switch preventing proper state transitions.
• Damaged or corroded wiring harness: Open circuits, short circuits (to ground or voltage), or high resistance in the wiring between the TRS and the PCM/TCM.
• Poor electrical connections: Loose, corroded, or damaged terminals at the TRS connector, PCM/TCM connector, or inline connectors.
• Mechanical misalignment of the TRS or shift linkage: If the TRS is externally mounted and adjustable, improper alignment can prevent it from physically registering the correct gear positions. Issues with the shift cable or lever binding can also contribute.
• Transmission fluid contamination or ingress: Fluid intrusion into the TRS can cause internal shorting or contact issues, particularly if the sensor is submerged or not properly sealed.
• Internal PCM/TCM failure: Although less common, a faulty input circuit within the PCM or Transmission Control Module (TCM) could misinterpret the TRS signals or fail to properly execute the KOEO/KOER tests.

How to Diagnose and Troubleshoot

Diagnosis of P1704 requires a systematic approach, often involving a digital multimeter (DMM) and an OBD-II scan tool capable of monitoring live data.

1. Verify the DTC and Freeze Frame Data: Connect an OBD-II scanner and confirm P1704. Note any other related DTCs. Review freeze frame data to understand engine conditions (RPM, speed, temperature, etc.) when the code was set. This can provide context, such as if the error occurred during a specific KOEO/KOER test condition.
2. Visual Inspection:
   • Inspect the Digital TRS and its electrical connector for obvious signs of damage, corrosion, bent pins, or fluid contamination.
   • Trace the wiring harness from the TRS to the PCM/TCM, looking for chafing, cuts, signs of rodent damage, or areas where the harness might be pinched.
   • Check the mechanical shift linkage for proper adjustment, excessive play, or binding that might prevent the TRS from fully engaging each position.
3. TRS Circuit Electrical Tests (with DMM):
   • Disconnect Battery: Always disconnect the vehicle’s battery before performing resistance or continuity checks to prevent damage to electrical components.
   • Disconnect TRS Connector: Access the TRS and disconnect its electrical connector.
   • Power and Ground Verification: With the ignition in the KOEO position (after reconnecting the battery), use a DMM to verify the presence of the reference voltage (typically 5V or 12V) and a good ground at the appropriate pins of the harness side of the TRS connector. Refer to the vehicle’s wiring diagram for pin assignments.
   • TRS Internal Resistance/Continuity Check: With the TRS connector disconnected, and referring to the vehicle-specific service manual, measure the resistance or continuity across the various pins of the TRS itself while manually moving the gear selector through all positions (P, R, N, D, 2, 1). A digital TRS will typically show open or closed circuits (or very low resistance) for specific pin combinations depending on the selected gear, corresponding to its digital outputs. An open circuit where continuity is expected, or vice-versa, indicates an internal sensor fault.
   • Wiring Harness Integrity Check: With both the TRS and PCM/TCM connectors disconnected, perform continuity checks on each wire from the TRS connector to its corresponding pin at the PCM/TCM connector. Look for open circuits (infinite resistance). Also, check each wire for shorts to ground (continuity to chassis ground) and shorts to voltage (continuity to a B+ source).
4. Live Data Monitoring (with Scan Tool):
   • Reconnect all components and the battery. With the scan tool connected, monitor the Transmission Range Sensor PIDs (Parameter IDs) in live data.
   • Slowly cycle the gear selector through all positions (P, R, N, D, 2, 1) while observing the live data. The displayed gear position should accurately and immediately reflect the physical selector position.
   • Pay close attention to any delays in transition, “skipped” positions, or illogical readings (e.g., displaying “Neutral” when in “Drive”). If the sensor states do not transition cleanly or accurately, it points to a TRS or wiring issue.
5. Shake Test: While monitoring live data, gently wiggle wiring harnesses and connectors, especially near the TRS and PCM. A sudden change in readings or the flickering of the reported gear indicates an intermittent connection.

Recommended Repairs and Solutions

Based on the diagnostic findings, the following repairs and solutions are typically recommended:

• Replace the Transmission Range Sensor (TRS): If the electrical tests or live data monitoring clearly indicate an internal fault with the TRS (e.g., failed continuity, incorrect state transitions), replacement is the primary solution. Ensure to purchase an OEM equivalent or genuine part, as aftermarket sensors can sometimes have calibration or reliability issues.
• Repair or Replace Wiring Harness: If damaged wiring (opens, shorts, chafing) or corroded terminals are identified, repair the harness using proper automotive wiring techniques (solder, heat shrink tubing, appropriate gauge wire) or replace the damaged section of the harness. Clean and apply dielectric grease to connectors as a preventative measure.
• Clean and Secure Electrical Connections: For loose or corroded connections, thoroughly clean the terminals with electrical contact cleaner and a small brush, then apply dielectric grease before re-securing them.
• Adjust Shift Linkage: If the mechanical shift linkage is out of adjustment, follow the vehicle manufacturer’s specific procedure to correctly adjust it, ensuring the TRS can accurately register all gear positions.
• PCM/TCM Replacement or Reprogramming: If all TRS and wiring integrity checks pass, and the symptoms persist, a faulty PCM/TCM might be the cause. This is a last resort and should only be considered after exhaustive testing rules out external components. PCM/TCM replacement typically requires programming to the specific vehicle, which should be performed by a qualified technician using manufacturer-specific tools.
• Clear DTCs and Test Drive: After any repair, clear the DTCs using the OBD-II scanner and perform a comprehensive test drive, including multiple KOEO/KOER cycles and various driving conditions, to ensure the code does not return and that the transmission operates correctly.