CUSTOMER CONCERN Cabin heat output is very low with DTCs P277300 and P277400 stored. CAUSE The PTC heater assembly had an internal open circuit; supporting checks use connector M87 terminals 6-to-7 and 8-to-9. CORRECTION Disable the high-voltage system, remove and replace the PTC heater assembly, clear DTCs P277300 and P277400, and verify normal cabin heat output.