If your vehicle rapidly closes in on the vehicle ahead because the vehicle applies the brakes suddenly while you are traveling in headway control, the warning sound activates and the brake warning is indicated in the display. Always verify the safety of the surrounding area and depress the brake pedal while keeping a safer distance from the vehicle ahead.
Additionally, always keep a safer distance from the vehicles behind you.

Mazda Radar Cruise Control (MRCC) Display Indication
Setting the SystemMazda 6 Owners Manual: Manually Shifting Up
You can shift gears up by operating the selector lever or the steering shift
switches (some models).
M1 → M2 → M3 → M4 → M5 → M6
Using selector lever
To shift up to a higher gear, tap the selector lever back (+) once.
Using steering shift switch (some models)
To ...
Mazda 6 Owners Manual: No.8 brake system warning light stays on more than 4 seconds with ignition
switch on
Diagnostic procedure
When performing an asterisked (*) troubleshooting inspection, shake the
wiring harness and connectors while doing the inspection to discover whether
poor contact points are the cause of any intermittent malfunction. If there
is a problem, check to make sure co ...