2020 Nissan Leaf Plus

Nissan Leaf Plus P0AA6 Isolation Fault — Cracked Coolant Hose at Heater Core

P0AA6 P1AA0 Published 2025-03-04 Updated 2025-03-04
Nissan Leaf Plus P0AA6 HV isolation coolant hose heater core liquid cooling

2020 Nissan Leaf Plus 62kWh presented with P0AA6 and cabin heater failure simultaneously. HV negative bus isolation 27 kΩ. Coolant hose at the PTC heater core inlet had developed a small crack, allowing coolant to contact an HV surface inside the heater assembly.

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The Problem
2020 Nissan Leaf Plus 62kWh (61,800 miles) brought in with 'Electric System Fault' and P0AA6, plus the customer noted the cabin heater had stopped working the same day. Coolant reservoir slightly low. P0AA6 and P1AA0 (PTC Heater Circuit Fault) confirmed.
Diagnostic Procedure
  1. 1. Confirm P0AA6 and P1AA0. The simultaneous appearance of both codes and the coolant loss strongly suggests a coolant-related HV fault.
  2. 2. HV isolation: negative bus 27 kΩ. Positive bus normal. Negative bus fault.
  3. 3. Segment negative bus: disconnect PTC heater HV connection. Negative isolation jumps to >1 MΩ. Fault confirmed at PTC heater assembly.
  4. 4. Inspect PTC heater coolant connections. Inlet hose at heater has a 15mm longitudinal crack at a molded bend — brittle cracking consistent with age and heat cycling on a 6-year-old hose.
  5. 5. Coolant has been weeping from the crack and migrating into the PTC heater housing through a gap around the inlet fitting. Coolant bridging HV negative terminal inside heater housing.
  6. 6. Replace cracked coolant inlet hose. Flush PTC heater housing with deionized water, dry with compressed air and heat gun.
  7. 7. Refill coolant system, bleed per Nissan procedure.
  8. 8. HV isolation: negative bus >1.5 MΩ. Clear both codes. PTC heater function confirmed — cabin heat restored.
  9. 9. No fault return on 30-mile test drive.
Resolution & Root Cause
Coolant hose replaced, PTC heater housing dried. HV isolation restored. Cabin heater functional. Both codes resolved.
💡 Key Lesson
Simultaneous P0AA6 and P1AA0 (PTC heater fault) on Nissan Leaf Plus almost always points to a coolant intrusion into the PTC heater housing — either a failed hose or fitting. The two codes together with coolant loss is enough to go straight to the heater coolant connections. Hose replacement is a $40 part and 1.5 hours labor versus misdiagnosis as a HV battery or PTC heater replacement.
About This Case

This case was solved remotely by an HVDesk specialist with 15+ years of hands-on experience across major EV platforms including Tesla, Hyundai/Kia, Volkswagen ID series, BMW i-series, and Ford EVs. The procedure was provided as structured remote support to an independent auto repair shop.