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DTC codes / Mercedes-Benz codes / P1705

P1705 · Mercedes-BenzStarter Lock-Out Backup Lamp Switch S161

P1705 on Mercedes-Benz vehicles indicates that the transmission control module (TCM) has detected an implausible signal from the starter lock-out backup lamp switch (S161), which is part of the electronic selector lever module. The TCM expects a specific voltage pattern from this switch when the selector lever moves between Park/Neutral and drive ranges, and it sets the code when the signal does not match the lever position or is missing entirely.

SystemPowertrain
Code typeMercedes-Benz specific
SeverityMedium
Fix difficultyProfessional
AreaTransmission

Symptoms you may notice

  • The engine may not crank or start when the selector lever is in Park or Neutral, especially intermittently.
  • The reverse lamps may fail to illuminate when Reverse is selected, or they may stay on continuously.
  • The transmission may not engage the selected gear, or it may default to a limp-home mode with harsh shifting.
  • The instrument cluster may display a warning such as 'Transmission: Visit Workshop' or show the selector lever position incorrectly.
  • The vehicle may not allow the key to be removed from the ignition or may not release the steering lock.
  • Often no symptoms besides the warning light, with the fault stored only in the TCM memory.

Common causes, most likely first

  1. Faulty starter lock-out backup lamp switch (S161) inside the electronic selector lever module, often due to wear or contamination.
  2. Damaged or corroded wiring harness between the selector lever module and the transmission control module, including chafed wires near the shifter.
  3. Loose or corroded electrical connector at the selector lever module or at the TCM.
  4. Internal failure of the electronic selector lever module (ESM) that houses the switch.
  5. Transmission control module software glitch or internal fault causing false interpretation of the switch signal.
  6. Mechanical misalignment of the selector lever mechanism preventing the switch from reaching its correct detent positions.

How to diagnose and fix it

  1. Use a Mercedes-Benz-specific scan tool to read the TCM fault memory and view live data for the starter lock-out backup lamp switch while moving the selector lever through all positions.
  2. Inspect the wiring and connectors at the selector lever module and TCM for corrosion, loose pins, or chafed insulation, repairing as necessary.
  3. Check the mechanical alignment of the selector lever cable and adjust if the lever does not align precisely with the switch detents.
  4. Perform a resistance and continuity test on the S161 switch circuit according to the factory wiring diagram, comparing results with specifications.
  5. If the switch or selector lever module is faulty, replace the electronic selector lever module and perform coding or adaptation as required.
  6. Clear the fault memory and test-drive the vehicle, verifying that the code does not return and that all selector lever functions operate correctly.

Selector switch faults that mimic transmission failure

  • The S161 switch is integrated into the electronic selector lever module on many Mercedes-Benz models, so a faulty switch often means replacing the entire module rather than a separate component.
  • A weak or failing battery can cause voltage drops that make the TCM misinterpret the switch signal, setting P1705 even when the switch itself is healthy.
  • After replacing the selector lever module, some models require reprogramming or adaptation of the TCM to recognize the new component, otherwise the code may persist.
  • P1705 can be set by a mechanical problem such as a stretched selector cable or worn bushings that prevent the switch from reaching its correct position, so always check the mechanical linkage before replacing electrical parts.
  • This code is often accompanied by other transmission codes or communication faults, so diagnosing the root cause requires reviewing all stored codes and live data rather than focusing solely on P1705.

Can I keep driving with P1705?

If the vehicle starts and drives normally, you can drive it to a workshop for diagnosis, but avoid situations where you must rely on the reverse lamps or repeated starting, as the fault may worsen. If the engine fails to crank or the transmission behaves erratically, stop driving and have the vehicle towed to prevent safety risks.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for Mercedes-Benz vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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