The P1605 diagnostic trouble code (DTC) indicates a loss of communication between the vehicle's Powertrain Control Module (PCM) and the Heating, Ventilation, and Air Conditioning (HVAC) control module. This lack of communication can prevent the HVAC system from functioning correctly, leading to discomfort for the vehicle's occupants. Understanding the causes, symptoms, and troubleshooting steps for this code is crucial for effective diagnosis and repair.

| Topic | Description | Potential Causes | | Component | Description a. Component Type | Description only to find it was a simple wiring issue, or a faulty HVAC control module. |

Detailed Explanations

1. Definition of P1605:

The P1605 code, defined as "Loss of HVAC Serial Data," indicates a communication breakdown between the PCM and the HVAC control module. This code signals that the PCM is not receiving the necessary data from the HVAC system to function correctly. Essentially, the PCM is "blind" to the requests and status of the HVAC system, leading to potential functional problems.

2. Function of HVAC Serial Data:

HVAC serial data is the information exchanged between the HVAC control module and other vehicle modules, primarily the PCM. This data includes information such as the desired temperature setting, blower motor speed, A/C compressor status, and other relevant HVAC parameters. This data allows the PCM to make adjustments to engine performance, such as idle speed, to compensate for the load placed on the engine by the A/C compressor. Without this data, the PCM cannot optimize engine performance for fuel efficiency and emissions control while the HVAC system is in use.

3. Symptoms of P1605:

Several symptoms can indicate a P1605 code. The most obvious is a malfunctioning HVAC system. This can manifest as a complete failure of the A/C or heating, intermittent operation, or the inability to adjust temperature or blower speed. Other symptoms might include the check engine light illuminating, and potentially other warning lights related to the HVAC system or engine management. In some cases, the vehicle might also experience poor fuel economy or rough idling, especially when the A/C compressor is engaged.

4. Common Causes of P1605:

Several factors can contribute to a P1605 code.

  • Faulty HVAC Control Module: A defective HVAC control module is a primary suspect. If the module itself is malfunctioning, it may not be able to transmit data correctly.
  • Wiring Issues: Damaged, corroded, or loose wiring in the communication circuit between the PCM and the HVAC control module is a common culprit. This includes shorts, opens, and high resistance in the wiring harness.
  • CAN Bus Problems: The Controller Area Network (CAN) bus is the communication network that allows various modules in the vehicle to communicate. Problems with the CAN bus, such as a faulty terminating resistor or a shorted wire, can disrupt communication between the PCM and the HVAC control module.
  • Faulty PCM: While less common, a malfunctioning PCM can also cause a P1605 code. If the PCM is unable to receive or process data correctly, it may misinterpret the lack of communication as a fault.
  • Poor Ground Connections: Inadequate or corroded ground connections for the HVAC control module or PCM can also disrupt communication.
  • Connector Issues: Corroded or damaged connectors at the HVAC control module, PCM, or along the wiring harness can interrupt the data flow.
  • Software Glitches: Rarely, a software glitch in either the PCM or HVAC control module can cause communication errors.

5. Diagnostic Steps:

A systematic approach is essential for diagnosing a P1605 code.

  • Scan for Codes: Use a scan tool to confirm the presence of the P1605 code and check for any other related DTCs. Document all codes present.
  • Visual Inspection: Carefully inspect the wiring harness and connectors between the PCM and the HVAC control module for any signs of damage, corrosion, or looseness. Pay close attention to any areas where the wiring harness is routed near sharp edges or hot components.
  • Check Power and Ground: Verify that the HVAC control module and PCM are receiving proper power and ground. Use a multimeter to check the voltage and ground connections at both modules.
  • CAN Bus Testing: Use a scan tool or multimeter to test the CAN bus communication circuit. Check for proper resistance and voltage levels. Look for shorts to ground or voltage.
  • Module Communication Test: Some scan tools have the capability to perform a module communication test. This test can help determine if the PCM and HVAC control module are able to communicate with each other.
  • Wiring Continuity Test: Use a multimeter to perform a continuity test on the wiring between the PCM and the HVAC control module. This test can help identify any open circuits or high resistance in the wiring.
  • Component Testing: If the wiring and power/ground connections are good, test the HVAC control module itself. This may require specialized equipment or consulting a factory service manual. Similarly, while less likely, the PCM should be tested if other possibilities have been ruled out.

6. Tools Needed for Diagnosis:

Several tools are necessary for diagnosing a P1605 code.

  • Scan Tool: A scan tool is essential for reading and clearing DTCs, as well as monitoring data from the PCM and HVAC control module. A professional-grade scan tool with CAN bus diagnostic capabilities is highly recommended.
  • Multimeter: A multimeter is needed for checking voltage, resistance, and continuity in the wiring harness and components.
  • Wiring Diagram: A wiring diagram for the vehicle's HVAC system and CAN bus is crucial for identifying the correct wires and connectors to test.
  • Basic Hand Tools: Basic hand tools, such as screwdrivers, pliers, and wire strippers, are needed for accessing and inspecting the wiring harness and components.
  • Test Light: A test light can be used to quickly check for power and ground at the HVAC control module and PCM.
  • CAN Bus Tester (Optional): A dedicated CAN bus tester can be used to more accurately diagnose problems with the CAN bus communication circuit.

7. Potential Repair Procedures:

The repair procedure will depend on the identified cause of the P1605 code.

  • Repair or Replace Wiring: If damaged or corroded wiring is found, repair or replace the affected sections of the wiring harness.
  • Clean or Replace Connectors: Clean corroded connectors with electrical contact cleaner. Replace damaged connectors.
  • Repair Ground Connections: Clean and tighten loose or corroded ground connections.
  • Replace HVAC Control Module: If the HVAC control module is found to be faulty, replace it with a new or remanufactured unit.
  • Replace PCM: If the PCM is determined to be the cause of the problem (after other components are ruled out), it may need to be replaced or reprogrammed. This is a complex procedure and should be performed by a qualified technician.
  • CAN Bus Repair: If the CAN bus is faulty, repair any shorts or opens in the wiring. Replace faulty terminating resistors.
  • Software Update: If a software glitch is suspected, try updating the software in the PCM or HVAC control module. This may require a trip to the dealership.

8. Importance of Proper Diagnosis:

Proper diagnosis is crucial for resolving a P1605 code effectively. Replacing parts without properly diagnosing the problem can be costly and time-consuming. A systematic approach, starting with a visual inspection and progressing to more advanced testing, is essential for identifying the root cause of the problem. This will ensure that the correct repairs are made and that the P1605 code is resolved permanently.

9. Related Diagnostic Trouble Codes:

Several other DTCs may be related to P1605, depending on the specific vehicle and the nature of the problem. These might include:

  • U-codes: Codes starting with "U" typically indicate communication problems on the CAN bus. Examples include U0001 (CAN Communication Bus) or U0164 (Lost Communication with HVAC Control Module).
  • HVAC-specific codes: Codes related to specific HVAC components, such as the blower motor, A/C compressor, or temperature sensors. These codes may be triggered as a result of the loss of communication.
  • PCM-related codes: Codes related to engine performance, such as idle speed control or fuel trim, which may be affected by the loss of HVAC serial data.

10. Vehicle Makes and Models Affected:

The P1605 code can occur on various vehicle makes and models, particularly those with more complex HVAC systems that rely heavily on data communication. It is important to consult the vehicle's service manual or a reliable online database to determine if P1605 is a relevant code for a specific make and model. Some common makes where this code has been reported include (but are not limited to): GM, Ford, Chrysler, and Toyota vehicles. The specific model year can also be a factor.

Frequently Asked Questions

  • What does P1605 mean? P1605 indicates a loss of communication between the PCM and the HVAC control module. This means the PCM isn't receiving necessary data from the HVAC system.

  • Can I still drive my car with a P1605 code? Yes, you can usually drive your car, but the HVAC system may not function correctly. It's best to diagnose and repair the issue