U0165 is a generic OBD-II network communication diagnostic trouble code indicating that communication with the Rear HVAC Control Module has been lost. The rear HVAC module manages or coordinates climate-control functions for the rear passenger area on vehicles equipped with separate rear HVAC systems.
The rear HVAC control module may be identified by names such as RHVACCM, Rear HVAC Module, Rear Climate Control Module, or another manufacturer-specific designation. The exact module name, location, wiring, communication network, and functions vary by vehicle make, model, and model year.
U0165 Code Details
Code: U0165
Meaning: Lost Communication With HVAC Control Module - Rear
Type: Generic OBD-II Network Communication Code
Main System: Rear HVAC / Climate Control / Vehicle Communication Network
What Is the Rear HVAC Control Module?
The Rear HVAC Control Module is an electronic controller used on vehicles equipped with an independent or electronically controlled rear climate-control system.
Depending on the vehicle, the module may receive temperature commands, sensor information, and network messages and may control or communicate with rear blower motors, temperature actuators, mode actuators, air-distribution components, and other HVAC systems.
Rear HVAC Control Module Abbreviations
Rear HVAC Control Module
Rear Heating, Ventilation and Air Conditioning
Climate Control Module
HVAC Module
Electronic Climate Control
Electronic Automatic Temperature Control
OEM Note: These names are not universal. Some manufacturers use a dedicated rear HVAC controller, while others integrate rear climate-control electronics into another HVAC assembly. Always confirm the exact module identification using the vehicle-specific scan tool and OEM service information.
Symptoms of U0165
- Rear climate-control functions may stop operating.
- Rear HVAC controls may become unresponsive.
- Rear blower operation may be unavailable or intermittent.
- Rear temperature control may not respond correctly.
- Rear airflow direction may become unavailable.
- Rear climate-control display or controls may stop responding.
- Multiple network communication DTCs may be stored.
- The scan tool may be unable to communicate with the rear HVAC module.
- U0165 may be stored intermittently.
- The rear HVAC system may operate in a default or backup mode.
Common Causes of U0165
- Low battery or system voltage.
- Blown fuse supplying the rear HVAC module.
- Missing battery or ignition power supply.
- Poor or damaged module ground.
- Open CAN-H or CAN-L circuit.
- Short circuit in the communication network.
- Damaged communication wiring.
- Loose or corroded electrical connector.
- Damaged or pushed-back connector terminals.
- Water intrusion.
- Another control module interfering with the communication network.
- Software or configuration problem.
- Internal failure of the rear HVAC control module.
How Serious Is U0165?
U0165 is generally a moderate communication fault. The effect depends on the vehicle and the functions controlled by the rear HVAC system.
In many vehicles, the fault mainly affects rear climate-control functions and does not directly prevent the engine from operating. However, the importance increases if the HVAC fault affects windshield defrosting, visibility, or other climate-control functions required for safe driving.
Can You Drive With U0165?
In many vehicles, the vehicle can still be driven with U0165 because the rear HVAC system is normally not required for basic engine or transmission operation.
However, the driver should not ignore the fault if the HVAC problem affects windshield defrosting, visibility, or other important climate-control functions.
How to Diagnose U0165
1. Scan All Control Modules
Perform a complete vehicle scan and record U0165 together with all other active, pending, and stored DTCs.
Pay particular attention to other communication codes, battery-voltage codes, HVAC codes, and network-related faults. If several modules report communication problems, investigate the common network before replacing the rear HVAC module.
2. Check Battery and Charging Voltage
Verify battery condition and charging-system voltage. Low or unstable voltage can cause control modules to reset or disappear from the vehicle communication network.
3. Check Rear HVAC Module Fuses
Use the vehicle-specific fuse diagram to identify all fuses supplying the rear HVAC control module.
Do not rely only on visual inspection. Test the fuse and verify the appropriate voltage at the module power circuit.
4. Test Module Power and Ground
Verify all required battery and ignition power supplies and test the rear HVAC module ground circuits according to manufacturer specifications.
A missing power or ground circuit can make a functioning module appear to be defective.
5. Inspect Connectors and Wiring
Inspect the rear HVAC module connector for corrosion, water intrusion, loose terminals, damaged locking mechanisms, bent pins, pushed-back terminals, and poor terminal tension.
Inspect the surrounding harness for chafing, crushed wires, previous repairs, damaged insulation, or wiring that may have been affected during interior or HVAC repairs.
6. Check CAN Communication
If the rear HVAC module communicates through CAN, test the CAN-H and CAN-L circuits according to the manufacturer's specifications.
Check for open circuits, short circuits, abnormal resistance, damaged wiring, and communication-network termination problems.
7. Attempt Direct Module Communication
Use the scan tool to determine whether the rear HVAC control module can be accessed directly.
If the scan tool cannot communicate with the module, verify power, ground, and network circuits before condemning the module.
8. Check Other Modules on the Same Network
If several control modules report communication faults, determine whether another module or network circuit is affecting communication with the rear HVAC module.
A communication fault does not automatically prove that the rear HVAC module itself has failed.
9. Check HVAC Inputs and Outputs
If communication with the rear HVAC module is restored, inspect available live data and actuator commands. Check rear temperature sensors, blower commands, actuator positions, and other HVAC parameters supported by the vehicle.
10. Confirm Module Failure
The rear HVAC control module should normally be considered defective only after power, ground, fuses, connectors, wiring, and communication circuits have been properly tested.
U0165 Oscilloscope Signal Patterns
An oscilloscope can be useful when U0165 involves an intermittent or distorted CAN communication signal. It is not required for every U0165 diagnosis.
Normal / Active
Normal: Stable communication activity with consistent signal transitions.
Abnormal / Distorted
Abnormal: Noise, distortion, irregular transitions, or intermittent communication activity.
Inactive / No Communication
Inactive: Expected network communication is absent at the tested network point.
Important: These are simplified diagnostic illustrations, not manufacturer-specific waveform specifications. Actual CAN-H and CAN-L voltage levels, timing, termination, and network behavior must be compared with the specifications for the exact vehicle.
U0165 Diagnostic Signal Flow
↓
Rear HVAC Control Module
↓
CAN-H / CAN-L
↓
BCM / Gateway / Other Control Modules
↓
Rear Blower / Temperature / Air Distribution Functions
U0165 OBD Live Data
Available live-data parameters depend on the vehicle and scan tool. Where supported, useful parameters may include rear HVAC communication status, system voltage, rear temperature request, actual temperature, blower command, actuator positions, air-distribution status, and HVAC system status.
Live data should be combined with direct electrical testing because an intermittent connector, power-supply problem, or CAN wiring fault may temporarily disappear while the vehicle is stationary.
Common Repairs for U0165
- Repair missing power supply to the rear HVAC module.
- Repair poor or damaged module ground.
- Replace a failed fuse after identifying the cause.
- Repair damaged CAN-H or CAN-L wiring.
- Repair loose or corroded connectors.
- Repair damaged electrical terminals.
- Correct water intrusion.
- Repair another control module affecting the communication network.
- Repair or replace the rear HVAC control module after internal failure is confirmed.
- Perform required programming, coding, configuration, or calibration after module replacement.
U0165 Repair Cost
- Electrical/network diagnosis: approximately $100–$250+.
- Fuse or wiring repair: approximately $75–$300+.
- Connector/terminal repair: approximately $80–$250+.
- CAN wiring repair: approximately $100–$350+.
- Rear HVAC module repair: approximately $150–$600+.
- Module replacement: approximately $300–$1,500+.
- Programming/configuration: approximately $100–$350+.
These are general estimates only. Actual repair costs depend on the vehicle, module design, parts availability, labor rates, programming requirements, and whether the original module can be repaired.
Common Repair Mistakes
- Replacing the rear HVAC module before checking power and ground.
- Ignoring low battery voltage.
- Failing to check the module fuses.
- Replacing the module without testing CAN communication.
- Ignoring connector corrosion or water intrusion.
- Failing to inspect wiring near the rear HVAC system.
- Assuming every U0165 means the module itself is defective.
- Ignoring other communication codes stored in other modules.
- Failing to check whether another module is disrupting the network.
- Installing a replacement module without required programming or configuration.
Related OBD-II Codes
- U0163 – Lost Communication With Navigation Control Module.
- U0164 – Lost Communication With HVAC Control Module.
- U0166 – Lost Communication With Auxiliary Heater Control Module.
- U0100 – Lost Communication With ECM/PCM.
- U0121 – Lost Communication With Anti-Lock Brake System (ABS) Module.
- U0140 – Lost Communication With Body Control Module.
- U0155 – Lost Communication With Instrument Panel Cluster.
- U0073 – Control Module Communication Bus "A" Off.
FAQ About U0165
What does U0165 mean?
U0165 means Lost Communication With HVAC Control Module - Rear. It indicates that communication with the rear HVAC control module has been lost or is unavailable. 1
What is the RHVACCM?
RHVACCM commonly refers to the Rear HVAC Control Module, the controller responsible for rear climate-control functions on vehicles equipped with a separate rear HVAC system.
Can a weak battery cause U0165?
Yes. Low or unstable system voltage can cause electronic modules to reset or temporarily disappear from the vehicle communication network.
Can a CAN wiring problem cause U0165?
Yes. An open, shorted, damaged, or poorly connected CAN circuit can prevent the rear HVAC module from communicating with other control modules.
Does U0165 mean the rear HVAC module is bad?
No. Power, ground, fuses, connectors, wiring, and network communication should be tested before replacing the module. Loss of power or ground is a common diagnostic direction for this code. 2
Can an oscilloscope help diagnose U0165?
Yes. An oscilloscope can help identify abnormal CAN activity, electrical noise, distorted signals, or intermittent communication problems when conventional electrical tests do not identify the cause.
Can another module cause U0165?
Yes. A faulty control module, gateway, or damaged network circuit can interfere with communication and cause multiple modules to report network communication faults.
Does a replacement rear HVAC module need programming?
Some vehicles require programming, coding, configuration, initialization, or calibration after replacement. The exact procedure is manufacturer-specific.
Final Takeaway
U0165 is a generic OBD-II network communication code indicating lost communication with the Rear HVAC Control Module.
The most reliable diagnosis starts with battery voltage, fuses, module power and ground, connectors, wiring, and CAN communication circuits.
Do not replace the rear HVAC control module simply because U0165 is stored. The actual cause may be a missing power supply, poor ground, blown fuse, damaged wiring, connector corrosion, network interference, or another control module affecting communication.
Always use manufacturer-specific OEM service information to confirm the exact module location, pinout, network architecture, voltage specifications, diagnostic procedure, and programming requirements for the vehicle being repaired.