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DTC codes / GM codes / P1646

P1646 · GMDriver 1 Line 6

P1646 on GM vehicles indicates that the powertrain control module (PCM) has detected a fault on the class 2 serial data line that it uses to communicate with driver 1 devices, specifically line 6 of that network. The PCM monitors the voltage and message traffic on this line and sets the code when the signal is out of range, shorted, or missing entirely for a calibrated period.

SystemPowertrain
Code typeGM specific
SeverityMedium
Fix difficultyProfessional
AreaComputer and output circuits

Symptoms you may notice

  • The service engine soon light illuminates and the vehicle may enter reduced power mode or default to a fixed operating strategy.
  • The driver information center may display a 'Service Vehicle Soon' or 'Reduced Engine Power' message.
  • Other modules on the same data bus, such as the body control module or instrument cluster, may show erratic behavior or lose communication.
  • The engine may start and run normally, with no noticeable performance change besides the warning light.
  • Intermittent no-start or crank-no-run conditions can occur if the data line fault disrupts the PCM's ability to authenticate with the theft deterrent system.
  • Multiple unrelated warning lights may illuminate simultaneously because several modules share the affected data line.

Common causes, most likely first

  1. A chafed, pinched, or corroded wire on the class 2 serial data circuit between the PCM and the driver 1 devices, often near a connector or where the harness passes through the firewall.
  2. A failed or failing module connected to line 6 that is pulling the data bus voltage out of its normal range, such as the body control module, instrument cluster, or a door module.
  3. Loose or corroded terminals at the PCM connector or at any inline connector on the affected data line.
  4. A short to ground or short to voltage on the serial data wire caused by insulation damage from road debris, rodent damage, or previous repair work.
  5. An internal fault in the PCM's communication driver for that particular data line.
  6. Aftermarket accessories wired into the class 2 bus without proper isolation, such as remote starters or alarm systems, that corrupt the signal.

How to diagnose and fix it

  1. Connect a scan tool and record all stored codes from every module, then clear codes and verify whether P1646 returns immediately or only after a drive cycle.
  2. Inspect the class 2 serial data wiring harness visually for chafing, corrosion, or loose connectors, paying close attention to areas near the PCM and any inline connectors on line 6.
  3. Measure the voltage on the affected data line with a digital multimeter with the key on and engine off; a healthy class 2 bus should rest around 0 to 7 volts and fluctuate as modules communicate.
  4. Disconnect modules one at a time from the data bus and recheck for the code to isolate which device is pulling the line out of range.
  5. Perform a resistance check on the data line from the PCM connector to each driver 1 device to confirm continuity and identify opens or excessive resistance.
  6. If the wiring and all connected modules test good, substitute a known-good PCM and recheck; only replace the PCM after confirming power, ground, and data line integrity.
  7. After any repair, clear codes and complete a full drive cycle with multiple key cycles to confirm the code does not reset.

What most technicians miss on P1646

  • A common misdiagnosis is replacing the PCM first, when the actual fault is often a chafed wire or a failing body control module that is dragging the class 2 bus voltage down.
  • The class 2 serial data bus is a single-wire network, so a fault on line 6 can affect communication with multiple modules and set unrelated codes that distract from the real problem.
  • Aftermarket electronics tapped into the data bus, such as remote starters or GPS trackers, frequently corrupt the signal and set P1646 without any factory component being defective.
  • Intermittent P1646 codes that appear only in damp weather often point to a connector with water intrusion or a wire rubbed through where the harness flexes.
  • A weak or failing battery or poor ground connection can cause the PCM to misread data line voltage and set P1646 even though the wiring and modules are healthy.

Can I keep driving with P1646?

The vehicle is usually safe to drive short distances to a repair shop, but avoid long trips or heavy load because the fault can cause reduced power mode or communication loss with safety systems. If the engine runs poorly or multiple warning lights are on, stop driving and have the vehicle towed.

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 GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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