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When the blower motor in a vehicle works only on the high setting, it's indicative of a specific issue in the circuit that controls the motor's speed. The correct answer involves an open thermal limiter, which is a component that can protect the blower motor from overheating. Essentially, the thermal limiter is designed to shut off the motor or reduce its functionality in response to excessive heat.

If the thermal limiter is open, it will prevent the blower motor from receiving the lower voltage signals needed for lower speed operations. As a result, the blower will only operate at the high speed, where it draws full voltage. This behavior aligns with the symptoms described in the question, confirming that the limitation is indeed due to the thermal limiter's inability to function properly.

Other choices suggest different possible issues but do not directly cause the blower motor to work only on high speed. For example, a blown 30 amp fuse would typically result in the blower motor not working at all. Similarly, an open in circuit 31, which typically refers to ground issues, would affect the operation of the blower motor completely. Lastly, an open in the P/BK (pink/black) wire could cause intermittent or no operation, rather than restricting it solely to high speed.

Understanding the role of