Powertrain Control Module (PCM)

WARNING: This page is about a different car, the 2003 Mazda 6. However, it is still accessible from the selected car via links, so may be relevant.

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.