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.
DTC B278C: Lost Communication with Power Source Control: Procedure
- CHECK HARNESS AND CONNECTOR (POWER MANAGEMENT CONTROL ECU - BATTERY AND BODY GROUND)
- Disconnect the L5 and L6 power management control ECU connectors.
- Measure the resistance and voltage according to the value(s) in the table below.
Standard Resistance
Tester Connection Condition Specified Condition L5-6 (E1) - Body ground Always Below 1 Ω Standard Voltage
Tester Connection Condition Specified Condition L5-1 (AM22) - Body ground Power switch off 11 to 14 V L6-7 (AM21) - Body ground Power switch off 11 to 14 V TEXT IN ILLUSTRATION*1 Front view of wire harness connector
(to Power Management Control ECU)
NG → REPAIR OR REPLACE HARNESS OR CONNECTOR
OK: Go to next step
- CHECK HARNESS AND CONNECTOR (CERTIFICATION ECU - POWER MANAGEMENT CONTROL ECU)
- Disconnect the L62 certification ECU (smart key ECU assembly) connector.
- Measure the resistance according to the value(s) in the table below.
Standard Resistance
Tester Connection Condition Specified Condition L62-29 (LIN) - L6-11 (LIN2) Always Below 1 Ω L62-29 (LIN) - Body ground Always 10 kΩ or higher TEXT IN ILLUSTRATION*1 Front view of wire harness connector
(to Certification ECU (Smart Key ECU Assembly))*2 Front view of wire harness connector
(to Power Management Control ECU)
NG → REPAIR OR REPLACE HARNESS OR CONNECTOR
OK: Go to next step
- REPLACE POWER MANAGEMENT CONTROL ECU
- Replace the power management control ECU. Refer to REMOVAL .
NEXT: Go to next step
- CHECK DTC OUTPUT
- Clear the DTC. Refer to DTC CHECK / CLEAR .
- Recheck for DTCs.
OK
DTC B278C is not output.
NG → See step 5
OK → END (POWER MANAGEMENT CONTROL ECU WAS DEFECTIVE)
- REPLACE CERTIFICATION ECU (SMART KEY ECU ASSEMBLY). Refer to REMOVAL
NO RELATED
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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.