P0AA9Hybrid/EV Battery Voltage Isolation Sensor Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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.
- What you may notice
- Check engine and EV system warning lights
- Reduced power or speed limit on some models
- Likely causes
- LikelyIsolation sense line shorted to ground or open input
- PossibleFailed isolation measuring circuit in the battery controller
- RareController software misreading electrical noise (update available on some models)
- Repair
- Workshop only · Usually a job for a workshop.Typical cost: €100 – €2,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.