Powertrain, generic · Hybrid propulsion

P0AA4 Hybrid/EV Battery Negative Contactor Circuit Stuck Closed

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

P0AA4 reports that the negative main contactor still conducts while it should be open, usually through welded contacts, so the pack is no longer fully isolated on the negative side. GM tests it once all contactors have been commanded open for more than 120 s, or the bus has already decayed below 9 % of pack voltage: over 30 V on the negative bus in 4 of 6 samples at 12.5 ms sets Type B (two trips, 19OBDG02). GM's 2017 Bolt EV bulletin prescribes contactor cycling and then a new battery disconnect unit; the 2019 Volt update links welded contactors to P0AA1, P0AA4 and P0C76. Jeep cures false sets on the 2023 Grand Cherokee 4xe by software, and Mitsubishi calls it main contactor (-) seizure on the i-MiEV.

Likely causes

Negative contactor contacts welded after high current or arcing
Battery or hybrid controller software (GM Volt 2019, Jeep 4xe updates)
Faulty battery disconnect unit (GM Bolt EV bulletin)
Negative bus voltage sensing error

Symptoms

  • Vehicle will not start or switch into ready mode
  • Check engine light and high voltage system message
  • Often stored together with P0AA1 or the slow discharge code P0C76

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€80–3,000
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

P0AA5P0AA1P0AA3P0C76

References: gsi.ext.gm.com, static.nhtsa.gov, static.nhtsa.gov, static.nhtsa.gov.