P0AC9Hybrid/EV Battery Temperature Sensor B Circuit Intermittent/Erratic
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- P0AC9 catches a battery temperature sensor B signal that changes erratically. Pack temperatures move slowly because of the large thermal mass, so rapid swings point to a bad connection. GM documents the method in its 2016 and 2019 hybrid tables: the controller integrates the absolute change of the sensor value over a 10-sample window at 100 ms per sample; a window fails when the sum exceeds 10, and the code sets after 4 of 5 failed windows, Type B, two trips. The older two-mode and 2013 tables have no erratic code for this sensor. The fault is often intermittent and may clear by itself, so check connector locking and terminal tension at the sensing board.
- What you may notice
- Hybrid warning light that comes and goes
- Sensor B value jumps in live data
- Occasional power limitation
- Likely causes
- LikelyLoose terminal or unlatched connector at the sensing board
- PossiblePartly broken sensor wire inside the pack
- RareInterference from high-voltage switching on the sensor line
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Workshop only · Usually a job for a workshop.Typical cost: €100 – €2,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.