P1735

What Does Code P1735 Mean?

DTC P1735 indicates that the Powertrain Control Module (PCM), or in some architectures, the Transmission Control Module (TCM), has detected an electrical circuit malfunction within the first gear position switch or its associated wiring. This switch is a critical input for the transmission control system, providing feedback on the current gear engaged (specifically, first gear). The PCM/TCM monitors the voltage signal from this switch. A “circuit malfunction” implies that the received signal is outside the expected operational parameters, such as an open circuit (no signal), a short circuit to ground (constant low voltage), a short circuit to voltage (constant high voltage), or a signal that is implausible relative to other sensor inputs like vehicle speed, engine RPM, or transmission output speed. The control module interprets this discrepancy as an inability to accurately determine if the transmission is in first gear, which can affect shift strategy, torque converter lock-up, and overall transmission performance.

Common Symptoms

• Check Engine Light (CEL) illumination: The primary indicator that a fault has been detected.
• Erratic or harsh shifting: Particularly when engaging or downshifting into first gear.
• Transmission defaulting to ‘limp home’ mode: The transmission may operate only in a few select gears (e.g., 2nd or 3rd) to prevent further damage.
• Inaccurate gear display: If the vehicle is equipped with a gear position indicator, it may display the incorrect gear or no gear at all.
• Inability to engage first gear manually: If equipped with a manual shift mode or a manual transmission.
• Reduced fuel economy: Due to the transmission operating inefficiently or in a default mode.

What Causes the Code P1735?

• Faulty First Gear Position Switch/Sensor: The switch itself may have failed internally, leading to an incorrect or intermittent signal.
• Damaged Wiring Harness: Open circuits, short circuits to ground, or short circuits to voltage within the wiring connecting the first gear switch to the PCM/TCM. This can be due to chafing, corrosion, or physical damage.
• Corroded or Loose Electrical Connectors: Poor contact at the first gear switch connector or the PCM/TCM connector can interrupt or distort the signal.
• Faulty Transmission Range Sensor (TRS): If the first gear signal is integrated into a multi-position Transmission Range Sensor, a failure within the TRS unit could be the cause.
• Internal Transmission Mechanical Issue: Though less common for a “circuit malfunction,” mechanical issues preventing the switch from being properly actuated could lead to an incorrect electrical signal.
• Failed Powertrain Control Module (PCM) or Transmission Control Module (TCM): While rare, an internal fault within the control module itself could cause it to misinterpret the switch signal or fail to provide proper reference voltage/ground.

How to Diagnose and Troubleshoot

Diagnosis of P1735 requires careful electrical testing and reference to vehicle-specific wiring diagrams.

1. Visual Inspection:
   • Begin with a thorough visual inspection of the wiring harness leading to the first gear switch (typically located on or within the transmission casing). Look for signs of chafing, cuts, or corrosion.
   • Inspect the electrical connectors at the switch and the PCM/TCM for bent pins, corrosion, or looseness.
2. OBD-II Scanner Data Analysis:
   • Connect an advanced OBD-II scanner and check for any other related Diagnostic Trouble Codes (DTCs), especially other transmission-related codes.
   • Access live data parameters. Monitor the “Gear Position Sensor” or “Transmission Range Sensor” input while cycling through all gear selections (P, R, N, D, 1, 2, 3, etc.) with the engine off, ignition on (KOEO). Observe if the scanner accurately reports the selection of first gear and if the signal state changes as expected.
   • Note if the first gear signal remains static, indicates an open circuit, or reads an implausible value when first gear is physically engaged.
3. Digital Multimeter (DMM) Testing:
   • Locate the First Gear Switch: Consult a vehicle-specific service manual to pinpoint the exact location and wiring diagram for the first gear switch.
   • Check Reference Voltage and Ground: With the ignition on (KOEO), backprobe the switch connector to verify the presence of the specified reference voltage (typically 5V or 12V) and a good ground signal from the PCM/TCM. A lack of either indicates a wiring or control module issue.
   • Test Switch Continuity/Resistance: Disconnect the switch connector. Use the DMM to test the switch itself. Depending on the switch type (normally open or normally closed), check for continuity or resistance changes when the transmission is physically shifted into first gear. Compare readings to manufacturer specifications. An open circuit when it should be closed, or vice versa, indicates an internal switch failure.
   • Check for Shorts in Wiring: Disconnect both the switch and the PCM/TCM connectors. Use the DMM to check for continuity between the signal wire and ground (short to ground) and between the signal wire and battery voltage (short to voltage). Also, check for continuity through the signal wire from the switch connector to the PCM/TCM connector to ensure no open circuit.
   • Wiggle Test: While monitoring live data or performing continuity tests, gently wiggle sections of the wiring harness and connectors. An intermittent signal change indicates a loose connection or damaged wire.

Recommended Repairs and Solutions

Once the diagnostic procedure pinpoints the source of the malfunction, the following repairs are commonly indicated:

• Replace the First Gear Position Switch/Sensor: If testing confirms the switch itself is faulty, replacement is the primary solution. Ensure the new switch is an OEM-equivalent part for proper function and longevity.
• Repair or Replace Damaged Wiring: If an open circuit, short, or damaged insulation is found in the wiring harness, the affected section should be professionally repaired using soldering and heat-shrink tubing, or the entire sub-harness replaced if damage is extensive.
• Clean or Replace Corroded Connectors: If corrosion is present, carefully clean the connector pins with electrical contact cleaner and a small brush. If corrosion is severe or pins are bent/damaged, replace the connector shell and/or terminal pins.
• Replace the Transmission Range Sensor (TRS): If the first gear signal is integrated into the TRS and it is determined to be faulty, replace the entire TRS unit.
• PCM/TCM Replacement: This should be considered a last resort, only after all other potential causes (switch, wiring, connectors) have been thoroughly tested and ruled out. PCM/TCM replacement often requires specialized programming or relearn procedures, so it should be performed by a qualified technician.

Mechanic’s Tip: After any repair, always clear the DTCs from the PCM/TCM memory and perform a road test under various driving conditions, including engagement of first gear, to confirm the repair has been successful and the code does not return. Additionally, always ensure the transmission fluid level and condition are correct, as extremely low or contaminated fluid can sometimes contribute to transmission sensor issues or poor shift quality, though less directly related to a circuit malfunction.