Powertrain, generic · Hybrid propulsion

P0AA9 Hybrid/EV Battery Voltage Isolation Sensor Circuit Low

Check engine lightOn
EmissionsNot relevant
Drive cycleRequired
Limp modePossible

P0AA9 reports that the isolation sensor signal sits below its valid window, typically because the sense line or the measuring input is pulled toward ground or has lost its supply. GM's 2016 and 2019 tables flag it when the sensor reads under 5 V with the bias switch open in 320 of 400 samples (about 5 s), Type B, two trips; the 2013 GM table uses a negative mid-pack reading under 5 V with contactors closed. Mitsubishi includes P0AA9 in its 2018 Outlander PHEV sub-BMU campaign together with P0AA7 and P0AA8, where noise caused false setting. The high counterpart is P0AAA. A low reading is a measuring fault, not proof of an insulation leak; that is P0AA6. Expect warnings and possibly reduced power.

Likely causes

Isolation sense line shorted to ground or open input
Failed isolation measuring circuit in the battery controller
Controller software misreading electrical noise (update available on some models)

Symptoms

  • Check engine and EV system warning lights
  • Reduced power or speed limit on some models

Repair estimate

DifficultyShop_onlyWorkshop job
Parts and labour€100–2,500
Time0.5–4 h

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

Parts involved

hv battery voltage isolation sensorwiring harnessbms

Related codes

P0AAAP0AA8P0AA7P0AABP0AA6

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