P0AA0

P0AA0 means the hybrid/EV battery management system (BMS) detected an out-of-range or implausible signal from the high-voltage battery pack's air-temperature sensor 'A' circuit — the thermistor that monitors intake air for the pack ventilation system. The BMS uses this reading to schedule cooling fan duty, balance cell-charging targets, and de-rate pack power when intake air climbs above safe thresholds. Shop-floor ladder, cheapest first: (1) Scan live data on a hybrid-capable tool (Techstream, FORScan, IDS, GDS) — the air-temp sensor should read within 5-10 deg F of cabin air at cold soak; anything reading -40 F (open circuit) or +302 F (short to ground) is the classic stuck-fault signature. (2) Pull the cabin trim covering the pack intake vent (rear deck on Prius, under second-row seat on Highlander/RX450h) and inspect the thermistor connector for pollen, pet hair, or corrosion — a $0 cleaning fixes a measurable fraction of these. (3) Backprobe the sensor low-voltage signal wire at the BMS connector (NOT the orange HV bus — this is a 5V reference circuit) and confirm 0-5V swing as you heat the sensor with your hand. (4) Resistance check the thermistor at known temp: typical NTC values 10k ohms at 77 F, 2.5k ohms at 150 F. Out of spec by more than 20% = sensor. DO NOT probe HV orange-wire circuits with a standard multimeter. Always service HV with Class 0 1000V gloves, isolation tester (Megger), and DC-rated lugged probes. Pack contactor stuck-closed (P0AF1/P0AF2) is the only condition where the orange-wire bus may be live AFTER the service plug is removed. NEVER assume zero potential — always measure with an HV-rated meter.

Vehicle-specific patterns: 2004-2009 Toyota Prius Gen2 (NHW20) throws P0AA0 almost always from the cabin-air intake vent on the rear passenger-side parcel shelf clogging with cat hair, lint, or pollen — the thermistor lives in the airflow path and reads the recirculated cabin air; a clogged scroll inlet starves the sensor of moving air, the reading drifts off-pattern, and the BMS sets the code. Vacuum the vent and the evaporator-blower scroll housing before condemning the $180 sensor. 2006-2013 Lexus RX400h/RX450h and 2008-2013 Toyota Highlander Hybrid share the same architecture but route intake from under the second-row seat — pet hair under the seat is the #1 cause. 2005-2012 Ford Escape Hybrid / Mercury Mariner Hybrid (Aisin transmission with Sanyo NiMH pack) uses a different thermistor location at the rear hatch trim and tends to fail outright from vibration around 120k miles. 2010-2015 Honda Civic Hybrid / Insight IMA uses a single in-pack air sensor that's harder to reach but rarely fails. Estimated repair: $0 (vacuum the vent) to $350 (sensor + labor).