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DTC codes / Ford codes / P1640

P1640 · FordPowertrain DTCs Available in Another Module

P1640 on Ford vehicles means the powertrain control module (PCM) has detected that one or more other modules on the vehicle's controller area network (CAN) have stored diagnostic trouble codes (DTCs) of their own. The PCM is not reporting a fault in its own circuitry; it is acting as a messenger, telling you that a companion module such as the transmission control module (TCM), anti-lock brake system (ABS) module, or body control module (BCM) has logged a fault that needs attention.

SystemPowertrain
Code typeFord specific
SeverityMedium
Fix difficultyProfessional
AreaComputer and output circuits

Symptoms you may notice

  • Illumination of the check engine light (malfunction indicator lamp, MIL) on the instrument cluster
  • Often no symptoms besides the warning light, because the fault lies in another module and may not affect drivability
  • Multiple warning lights on the dash, such as ABS, traction control, or airbag, if the affected module controls those systems
  • Erratic transmission shifting or a limp-home mode if the transmission control module is the module with stored codes
  • Reduced engine performance or stalling if the fault in the other module interferes with powertrain coordination
  • Intermittent electrical glitches, such as flickering lights or inoperative accessories, when the body control module is involved

Common causes, most likely first

  1. A stored diagnostic trouble code in the transmission control module (TCM) or another powertrain-related module
  2. A stored diagnostic trouble code in the anti-lock brake system (ABS) module or traction control module
  3. A stored diagnostic trouble code in the body control module (BCM) affecting network communication
  4. Low battery voltage or a weak battery that causes multiple modules to log communication faults
  5. Corroded, loose, or damaged wiring and connectors on the controller area network (CAN) bus
  6. A faulty or failing module that is unable to communicate reliably on the network
  7. A software or calibration issue in one of the modules that sets spurious codes

How to diagnose and fix it

  1. Connect a scan tool capable of reading all modules, not just the engine, and retrieve the specific codes stored in every module on the network
  2. Record all codes and freeze-frame data from each module before clearing anything, so you can trace the root cause
  3. Check battery voltage and perform a load test; a weak battery is a common cause of multiple module communication codes
  4. Inspect the controller area network (CAN) wiring and connectors for corrosion, chafing, or loose pins, especially at the modules that stored codes
  5. Diagnose and repair the underlying fault in the module that stored the original code, following the specific diagnostic procedure for that code
  6. Clear all codes from all modules and verify that P1640 does not return after a test drive
  7. If the code persists with no other codes present, check for a faulty PCM or a network communication issue that prevents the PCM from reading other modules

P1640 is a messenger, not the fault

  • P1640 is often misdiagnosed as a PCM failure, but the PCM is simply reporting that another module has set a code; replacing the PCM will not fix the underlying problem.
  • A scan tool that only reads engine codes will miss the stored codes in the transmission, ABS, or body modules that actually triggered P1640.
  • Low battery voltage or a poor ground connection can cause multiple modules to log communication codes simultaneously, setting P1640 without any real component failure.
  • On some Ford models, a fault in the transmission control module (TCM) or its wiring can set P1640 along with harsh shifting, leading owners to replace the transmission unnecessarily.
  • After repairing the underlying fault in the other module, you must clear codes from all modules; P1640 will not clear itself until the PCM sees a clean network.

Can I keep driving with P1640?

You can generally drive the vehicle short distances to a repair shop, but avoid heavy loads or high speeds until the underlying module fault is identified. If the vehicle enters limp-home mode or multiple warning lights are on, have it towed to prevent damage to the transmission or other systems.

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

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