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This is actually a "known problem" and Mazda has issued a re-designed coolant resevoir tank (that has the level sensor built in) to correct the problem. The coolant level sensor float in the coolant reserve tank (A) is deteriorated and absorbs coolant. The float gets heavy, and sinks, causing the the coolant level warning light comes on. As a mass-production change, the design of the float has been modified.