Drivers who see an HVAC warning lamp or notice that the automatic climate‑control system stops adjusting temperature are often told a “U0660” code has been stored. The code specifically indicates that the vehicle’s control module can no longer talk to the left‑rear temperature actuator B, the sensor that feeds cabin‑temperature data from the rear passenger side. Because the HVAC system relies on accurate temperature feedback to modulate blower speed, blend doors, and heating elements, loss of that signal can leave the climate system stuck in a fixed mode or cycling erratically. Early identification through a scan and a focused communication check prevents prolonged discomfort and avoids unnecessary component replacement.
The temperature actuator is connected to the HVAC control module through a dedicated harness. Pin‑point bends, rodent damage, or exposure to moisture can create high‑resistance joints or open circuits. Corroded terminals impede the low‑level signal the actuator uses, causing the module to register a loss of communication.
Connectors at the actuator or module can develop bent pins, broken clips, or debris. A misaligned pin‑out prevents the module from receiving the sensor’s voltage reference, leading the diagnostic scanner to set U0660.
The actuator contains a thermistor or resistance‑based temperature sensor and a small driver circuit. If the thermistor opens or the driver burns out, the module sees an out‑of‑range value and flags a communication loss. The actuator may still appear physically intact, making the fault hard to spot without a bench test.
Modern HVAC modules store calibration tables for each temperature sensor. Corrupted flash memory or an incomplete previous reprogramming can make the module ignore the left‑rear actuator B, even though the hardware is sound. The error persists until the module is re‑flashed or replaced.
Circuit board cracks, failed voltage regulators, or damaged communication transceivers inside the HVAC control module can break the data link to the actuator. In such cases, the module reports a loss of communication despite clean wiring and a functional actuator.
– Connect a manufacturer‑specific scanner. Confirm that U0660 is present and note any related codes (e.g., U0661 for the left‑rear actuator A) that may indicate a broader communication issue.
– Access the HVAC module’s live data stream. Verify whether the left‑rear temperature sensor B shows a valid voltage or resistance reading. A “‑” or “N/A” indicates a communication break.
– Locate the left‑rear actuator harness. Look for chafed insulation, exposed conductors, or water intrusion.
– Disconnect the actuator connector and inspect pins for corrosion, bending, or missing contacts. Clean with electrical contact cleaner and reseat.
– With the ignition off, use a multimeter to check continuity from the module’s pin to the actuator connector. Open circuits point to wiring damage.
– Measure the actuator’s resistance at the connector (typically 2 kΩ ± 10 %). Values far outside this range suggest a failed sensor.
– Some scan tools offer a “communication test” that sends a request to the actuator and waits for a response. Failure confirms a loss of data exchange, not just a sensor fault.
– If wiring and the actuator test good, reflash the module with the latest factory software. This clears corrupted tables and restores proper sensor mapping. Reprogramming costs $150‑$250 for the flash plus $100‑$150 labor at a qualified shop.
– When the module fails the communication test after reprogramming, internal hardware damage is likely. Repair options (e.g., board‑level component replacement) are limited and often temporary.
– Replace the HVAC control module with a VIN‑matched unit. The replacement is programmed to the vehicle’s specific software version, ensuring all climate functions operate correctly. Expect a part cost of $400‑$700 plus $150‑$250 labor.
– After repair or replacement, clear the DTCs and perform a functional test: engage automatic climate control, adjust rear temperature, and confirm the HVAC warning light remains off.
Service Recommendation: Most issues related to this fault are diagnosed and corrected through inspection, wiring repair, and calibration rather than module replacement. For modules not typically replaced through aftermarket suppliers, diagnosis and repair should be performed by a certified automotive technician with access to factory service information and tooling.