Safety Warning

DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.

2003 Mazda 6 s, AutomaticFACTORY OEM SERVICE PROCEDURE

Powertrain Control Module (PCM)

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.

Factory Service Tools

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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.
• Brakes feel soft, pull hard to one side, or make grinding noises.
• The engine overheats, stalls repeatedly, or misfires under load.
• You are missing required tools, torque specs, or safe lifting equipment.
• You are not confident in the next step or safety outcome.

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