OBD-II code U0660 indicates that a control module (typically the HVAC or front control module) has stopped receiving messages from the left rear temperature actuator "B" over the in-vehicle communication network (usually CAN or LIN). The actuator is a small motor that drives the blend-air door for the rear-left climate zone, and when its signal drops out, the controller cannot command or verify its position.
The actuator's internal motor, potentiometer, or driver electronics wear out; this is the most common single failure for a lost-communication code on a specific actuator.
Rear HVAC actuators are often mounted behind trim panels that get wet, and connector corrosion, bent pins, or a partially seated plug will cut off power, ground, and network signaling.
Chafed, pinched, or broken wires in the harness to the rear HVAC box are common, especially where the harness flexes around rear seat or HVAC housing edges.
Loss of the dedicated communication circuit prevents the module from polling the actuator; this can be wiring damage, a splice failure, or high resistance in the data circuit.
The module that polls the rear actuator can develop a driver or network-interface fault that prevents communication with this specific device, even though other functions still work.
Listed last because it is rare for this code: a gateway or network issue higher in the network can knock the actuator off-line, but the actuator and its wiring should be ruled out first.
No. The PCM/ECM almost never controls a rear temperature actuator and is rarely the cause of this code. The actuator itself, its connector, and its wiring are the realistic culprits, and drivers are sometimes incorrectly told to replace the computer first. Modules should only be suspected after the actuator and its circuit have been verified.
Yes, in most cases the vehicle will continue to run normally. The trade-off is reduced or lost rear climate control; in cold or hot conditions this can be a comfort issue but is not a safety or drivability problem.
No. Clearing the code only erases the stored fault; if the actuator is not communicating, the code will return as soon as the controller polls it again on the next drive cycle.
It generally indicates a second or variant actuator at the left-rear position (for example, in a dual-blend or multi-zone rear HVAC system). Diagnosis is the same; you are simply targeting the specific actuator labeled "B" in the vehicle's HVAC configuration.