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Powertrain Control Module (PCM)

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Check display of DTC U0121 and/or U0155, using scan tool. See Fig 1.

Fig 1: DTC Table (Multiplex Communication System)
Courtesy of MAZDA MOTORS CORP.
Referring to PCM Malfunction Locator Table, determine malfunctioning part of CAN system. See Figure and Fig 2 .

Fig 2: PCM Malfunction Locator Table

Courtesy of MAZDA MOTORS CORP.