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DTC P2108: Throttle Actuator Control Module Processor Error

WARNING: This page is about a different car, the 2004 Mazda RX-8. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: DTC P2108 Detection Condition & Possible Causes Chart
G01345418Courtesy of MAZDA MOTORS CORP.

Diagnostic Procedure 

Fig 2: DTC P2108 Diagnostic Procedure Chart
G01345419Courtesy of MAZDA MOTORS CORP.