P1640

What Does Code P1640 Mean?

The diagnostic trouble code (DTC) P1640, “Powertrain DTCs Available in Another Module,” is an informational code set by the Powertrain Control Module (PCM), often synonymous with the Engine Control Module (ECM). Unlike most DTCs which indicate a specific fault within the PCM’s directly monitored circuits or systems, P1640 signifies that the PCM has detected or received information via the vehicle’s Controller Area Network (CAN bus) or other communication protocols (e.g., J1850 VPW/PWM, LIN bus) that another control module on the vehicle’s network has stored one or more powertrain-related diagnostic trouble codes. Essentially, the PCM is acting as a “flag bearer,” alerting the technician that a deeper investigation into other control modules is required to identify the root cause of an underlying powertrain issue. This code does not indicate a direct malfunction within the PCM itself, but rather points to an issue being monitored and stored by a secondary module, such as the Transmission Control Module (TCM), Anti-lock Brake System (ABS) module, Body Control Module (BCM), Fuel Pump Control Module (FPCM), or even a Hybrid Control Module.

Common Symptoms

• Malfunction Indicator Lamp (MIL) illumination: The “Check Engine” light will be illuminated on the dashboard.
• Symptoms related to the underlying fault: The actual symptoms experienced by the vehicle will correspond directly to the specific DTC stored in the other module. These can include:
  • Rough idle, stalling, or misfires (if an FPCM or BCM related to fuel delivery has a fault).
  • Harsh or erratic transmission shifts, delayed engagement, or no-shift conditions (if a TCM fault).
  • Reduced engine power or “limp mode” activation (if a critical powertrain-related module has a severe fault).
  • ABS, Traction Control, or Stability Control warning lights (if an ABS module or wheel speed sensor fault is present).
  • Cruise control inoperative.
• No specific symptoms directly attributable to P1640 itself: As an informational code, P1640 does not independently cause driveability issues; it merely indicates the presence of another problem.

What Causes the Code P1640?

• Presence of another active powertrain-related DTC in a secondary control module: This is the most common and direct cause. P1640 is typically set because another module (e.g., TCM, ABS, FPCM) has identified a fault within its own system and has set a corresponding DTC. The PCM then recognizes this via the communication network.
• Communication network issues:
  • Damaged, corroded, or open wiring within the vehicle’s CAN bus or other data communication lines connecting the PCM to other modules.
  • Short circuits or high resistance within the communication network.
  • Faulty termination resistors on the CAN bus, which are critical for signal integrity.
  • Intermittent communication dropouts between the PCM and other auxiliary modules.
• Faulty or failing secondary control module: A control module (e.g., TCM, ABS module, BCM) that is directly responsible for a powertrain-related function may be internally faulty, causing it to set its own DTCs and/or transmit erroneous data across the network.
• Faulty Powertrain Control Module (PCM): While less common for P1640, an internal fault within the PCM could theoretically cause it to misinterpret communication data or incorrectly set this informational code, even if no other module has an active DTC.

How to Diagnose and Troubleshoot

Diagnosing P1640 requires a systematic approach, focusing on the underlying issues:

1. Scan All Control Modules: The absolute first step is to use an advanced OBD-II scan tool capable of communicating with all vehicle control modules, not just the PCM. Access and retrieve all stored, pending, and historical DTCs from every available module (TCM, ABS, BCM, FPCM, HVAC, etc.). This is crucial, as P1640 often coexists with the root cause DTC(s) in other modules.
2. Identify Root Cause DTCs: Analyze the retrieved DTCs. Prioritize the diagnosis and repair of any specific powertrain-related DTCs found in modules other than the PCM. For example, if a U0101 (Lost Communication with TCM) or a P0700 (Transmission Control System Malfunction) is present in the TCM, those are your primary targets.
3. Check for Technical Service Bulletins (TSBs) and Recalls: Research OEM TSBs or recalls pertaining to P1640, communication network issues, or specific DTCs found in other modules for your vehicle’s make, model, and year.
4. Communication Network Integrity Check (If no other specific DTCs are found or if communication issues are suspected):
   • Visual Inspection: Carefully inspect the wiring harness for the CAN bus (typically a twisted pair of wires, often green/yellow or white/blue, but colors vary by manufacturer). Look for signs of chafing, cuts, corrosion, loose connections, or heat damage, especially near connectors and where the harness passes through firewalls or near moving components.
   • Resistance Check: With the vehicle’s battery disconnected, use a Digital Multimeter (DMM) to measure the resistance between CAN High (pin 6) and CAN Low (pin 14) at the OBD-II diagnostic connector. The reading should be approximately 60 ohms (indicating two 120-ohm termination resistors in parallel). A reading of 120 ohms suggests an open circuit (e.g., a break in one CAN line or a faulty module that provides one termination resistor). A reading of 0 ohms indicates a short circuit.
   • Voltage Check: With the ignition ON (engine off), use a DMM to check the voltage on CAN High and CAN Low lines at the OBD-II port or directly at module connectors. Typical operating voltages are around 2.5V to 3.5V for CAN High and 2.5V to 1.5V for CAN Low. Consult manufacturer-specific wiring diagrams and specifications for precise values. Erratic or absent voltage indicates a communication fault.
5. Verify Power and Ground to Suspect Modules: If a specific auxiliary module (e.g., TCM) is suspected due to its own DTCs or lack of communication, use a DMM to confirm it is receiving proper battery voltage and ground at its connector. Refer to wiring diagrams for pin assignments.
6. Perform Module Self-Tests: If available through your scan tool, execute self-tests on the suspected secondary modules to assess their internal health and communication capabilities.

Recommended Repairs and Solutions

The resolution for P1640 almost always involves addressing the underlying cause rather than P1640 itself:

1. Repair/Replace Components Related to Underlying DTCs: The primary solution is to diagnose and repair the specific DTCs found in the other vehicle modules. For example, if the TCM reported a solenoid fault (P0750), repair or replace the faulty solenoid. Once the root cause DTC in the other module is resolved and cleared, P1640 will typically clear on its own or can be cleared manually.
2. Wiring Harness Repair or Replacement: If the diagnosis identifies damaged, corroded, or shorted communication wires, repair or replace the affected section of the wiring harness. Use appropriate automotive-grade wire, crimp connectors, and heat-shrink tubing to ensure durable, weather-resistant repairs. Avoid soldering on CAN lines as it can alter signal impedance.
3. Replace Faulty Control Module: If a specific auxiliary control module (TCM, ABS module, BCM, etc.) is confirmed to be internally faulty after thorough diagnosis and external factors have been ruled out, replacement will be necessary. Always follow OEM procedures for module replacement, which often include programming, calibration, or initialization steps that require specialized scan tools.
4. Address Communication Termination Resistors: If an issue with CAN bus termination resistors is identified (e.g., 120 ohms or 0 ohms resistance), locate and rectify the fault. This might involve replacing the module containing the faulty resistor or repairing wiring.
5. PCM Replacement (Rare): Replacing the PCM should be considered a last resort for P1640 and only after all other possibilities, including underlying DTCs, communication issues, and auxiliary module faults, have been exhaustively ruled out. A faulty PCM causing P1640 without any other identifiable issues is highly uncommon.
6. Clear All Codes and Retest: After any repair, clear all DTCs from all modules. Perform a comprehensive test drive under various operating conditions to confirm that P1640 and any other related codes do not return.

Expert Tip: Always remember that P1640 is a diagnostic indicator, not the problem itself. Do not become fixated on clearing P1640 until the underlying issues in other control modules or the communication network have been thoroughly diagnosed and rectified. A generic OBD-II scanner may only display P1640 but might not be able to access the specific modules holding the true fault codes, necessitating a more advanced bidirectional scan tool.