Mazda 6 Service Manual: No.7 slow return to idle

No.6 cranks normally but will not start

No.8 engine runs rough/rolling idle

Other materials:

Mazda 6 Service Manual: Instrument cluster removal/installation
Caution When replacing the instrument cluster, the configuration procedure must be performed before removing the instrument cluster. If the configuration procedure is not performed before removing the instrument cluster, it is possible that the instrument cluster will not operate normally. ...

Mazda 6 Service Manual: Thermostat
THERMOSTAT REMOVAL/INSTALLATION 1. Remove the plug hole plate. 2. Disconnect the negative battery cable. 3. Drain the engine coolant. (See COOLING SYSTEM SERVICE WARNINGS.) 4. Remove the drive belt. (See DRIVE BELT REPLACEMENT.) 5. Remove the washer tank. 6. Remove the P/S oil pump. Note ...