1. Measure the temperature around the water temperature sensor.
2. Measure the resistance between terminals of the water temperature sensor.

Water temperature sensor removal/installation
Refrigerant pressure switch removal/installationMazda 6 Service Manual: Dtc b1318
Caution
When attaching the tester lead to the ABS (ABS/TCS) HU/CM harness
connector, the SST (49 G066 001) must be used. (See ABS (ABS/TCS) HU/CM
INSPECTION.)
Diagnostic procedure
...
Mazda 6 Service Manual: Accessory Sockets
Only use genuine Mazda accessories or the equivalent requiring no greater than
120 W (DC 12 V, 10 A).
Front
The ignition must be switched to ACC or ON.
Center
The accessory sockets can be used regardless of whether the ignition is on or
off.
CAUTION
To prevent accessory socket damag ...