A low level of electrolyte fluid will cause the battery to discharge quickly.

Inspect the electrolyte level at least once a week. If it is low, remove the caps and add enough distilled water to bring the level between the upper and lower level (illustration).
Do not overfill.
Examine the specific gravity of the electrolyte with a hydrometer, especially during cold weather. If it is low, recharge the battery.
Battery Maintenance
Battery RechargingMazda 6 Owners Manual: Procedures for determining the location of a malfunction
System Wiring Diadram
PCM
1. Check the display of DTC U0121 and/or U0155, using the SST (WDS or
equivalent). (See DTC TABLE.)
2. Referring to the following table, determine the malfunctioning part of the
CAN system.
ABS/TCS HU/CM or DSC HU/CM
1. Accec and monitor the PCM MSG and IC MSGo ...
Mazda 6 Owners Manual: Bonnet switch inspection
Note
Bonnet switch has a built-in bonnet lock.
1. Open the bonnet.
2. Disconnect the negative battery cable.
3. Remove the radiator grille.
4. Disconnect the bonnet switch connector.
5. Check the continuity between the bonnet switch terminals A and B using an
ohmmeter.
If there is ...