Possible causes – HO2 sensors harness is open or shorted– HO2 sensors circuit fuse– HO2 sensors circuit relay defective– HO2 sensors circuit connectors (wrong connection)– Faulty Engine Control Module (ECM) Symptoms – Engine Light ON (or Service Engine Soon Warning Light) P0040 MAZDA OBD Code Description The heated oxygen sensor (HO2S) monitor determines if the…