Powertrain, generic (extended)

P2C85 Hybrid/EV Battery Voltage External Isolation Fault

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

P2C85 means the isolation monitor has found a fault-level loss of insulation on the vehicle side of the battery, in cables or components connected through the contactors. Mercedes-Benz uses the code in the battery management system with the text vehicle-side insulation fault. A 2024 warranty notice for C 350e, GLC 350e, GLE 550e and S 550e/560e plug-in hybrids lists it for a corroding high-voltage A/C compressor winding; the car could not be restarted once switched off. A 2025 bulletin for the 295, 296 and 297 model series traces it to moisture in a high-voltage PTC heater without a diaphragm. External versus internal follows from where the resistance is measured, as a Magna patent separates battery and component insulation. P2C87 is the earlier warning stage, P0AA6 the system fault.

Likely causes

Insulation breakdown in the high-voltage A/C compressor
Moisture in the high-voltage PTC heater or its connector
Chafed or crushed high-voltage cable touching the body
Coolant or water inside inverter, motor or charger

Symptoms

  • Red high-voltage or battery warning, do not restart message
  • Vehicle cannot be restarted after switching off
  • Electric heater or A/C not working

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€300–4,000
Time1.5–8 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

hv batteryhv wiringisolation monitor

Related codes

P2C87P2C88P2C86P0AA6P0E74P0DAA

References: static.nhtsa.gov, static.nhtsa.gov, patents.google.com.