Take the appropriate action and verify that the warning light turns off.

Taking Action
Message Indicated on DisplayMazda 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: Master cylinder disassembly/assembly
Caution
If the master cylinder body is damaged, replace the unit as a
component. When securing the master cylinder in a vise, tighten only the
flange of the master cylinder.
1. Disassemble in the order indicated in the table.
2. Assemble in the reverse order of disassembly.
Stop ...