P0AC0

P0AC0 indicates a circuit fault on hybrid battery pack air-temperature sensor A — the BECM is reading an out-of-range voltage (open circuit, short to ground, or short to power) from the thermistor that monitors air entering or within the pack. Most NiMH and lithium hybrid packs use 2-4 air-temperature sensors distributed through the pack to detect thermal stratification. Cheapest-first diagnostic ladder: pull live data and read all pack temperature sensors simultaneously — a healthy set should agree within 3C at rest after a cold soak; if sensor A reads -40C (open circuit pegged low) or 215C (shorted high) while the others read 22C ambient, the sensor or its wiring is the fault. The thermistors are NTC-type with nominal 10K ohms at 25C. Caveat: P0AC0 alone does not disable the hybrid system but it does force the BECM into a conservative thermal management mode (cooling fan on more aggressively, charge/discharge current limited), which on Prius Gen 2 reduces fuel economy by 3-5 MPG until fixed.

Vehicle-specific patterns: 2004-2009 Toyota Prius Gen 2 (NHW20) — pack temp sensor A harness chafe on the pack lid edge causes intermittent P0AC0 that worsens with vibration; repair harness routing and add loom protection. 2010-2015 Toyota Prius Gen 3 — revised harness routing reduced this failure but the sensor itself sometimes fails internally; Toyota P/N G9281-47010. 2005-2012 Ford Escape Hybrid — BECM internal multiplexer failure on the temperature input throws P0AC0 + P0AC4 together; Ford TSB 12-7-15. 2003-2011 Honda Civic Hybrid — IMA pack temp sensor failure typically presents with IMA light plus reduced assist. Estimated repair: $180 to $1,600.