Powertrain, generic · Hybrid propulsion

P0AA5 Hybrid/EV Battery Negative Contactor Circuit Stuck Open

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P0AA5 is set when the negative main contactor stays open or fails to conduct after the controller has commanded it closed, so the negative side of the high-voltage bus never connects. Audi reports it, alone or with P0AA2, on the 2020-2022 Q5 e quattro and 2021-2022 A7 e quattro plug-in hybrids with a no-start complaint; the stated cause is oxidation of the contact surfaces in the battery junction box after very long standing or extreme humidity. Audi's procedure: several ignition cycles and an EV road test with many consumers on, and a new junction box only if the fault returns. GM includes P0AA5 among the Ultium EV high-voltage codes that require pack data retrieval. It is the counterpart of P0AA4 (welded contacts).

Likely causes

Oxidised contact surfaces after long standing or high humidity (Audi)
Failed battery junction box or burnt negative contactor
Contactor coil not energised due to wiring or 12 V supply fault

Symptoms

  • Vehicle does not start or reach ready mode
  • Hybrid or high voltage battery warning
  • Charging not possible

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€100–3,000
Time0.5–6 h

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

Parts involved

hv battery negative contactorbms

Related codes

P0AA4P0AA2P0AA3P0ADD

References: static.nhtsa.gov, static.nhtsa.gov, static.nhtsa.gov.