It's a known problem and is apparently cured by a "Citric Acid Flush". We've had reports of of the heater in a 32,000-mile 2013 Mazda CX-5 2.2d stopping working. The Mazda dealer said the cooling system needed a citric-acid flush. While waiting for this, within a couple of weeks, the dash went from occasionally showing an overheating warning to severe overheating and the EMC unit putting the vehicle into “limp mode”, requiring immediate return to the dealer. The dealer finally carried out the citric-acid flush and all appeared back to normal.