P0AC4

P0AC4 is the same fault class as P0AC0 but on hybrid battery pack air-temperature sensor B — the second of the pack's distributed thermistors, typically positioned at the pack exhaust or rear, used by the BECM to calculate thermal gradient across the pack and command cooling fan speed accordingly. Cheapest-first diagnostic ladder: same as P0AC0 — pull live data on all pack temperature sensors simultaneously, look for the one reading -40C or 215C while the others show ambient, then back-probe and measure NTC resistance against the standard 10K-at-25C curve. If P0AC4 is set alongside P0AC0 (both sensors faulted at once), the failure is almost certainly the BECM's internal temperature input multiplexer rather than two coincidental sensor failures — one bad sensor is plausible, two simultaneous is a BECM issue 90% of the time. Caveat: like P0AC0, this code biases the BECM into conservative thermal management; if you have a Prius Gen 2 throwing P0AC4 in winter only, the sensor harness may be intermittently open from cold-shrink contact issues.

Vehicle-specific patterns: 2004-2009 Toyota Prius Gen 2 (NHW20) — rear pack temp sensor (sensor B) intermittent open at BECM connector from cowl water corrosion, often paired with P0AC0. 2010-2015 Toyota Prius Gen 3 — internal sensor failure more common than harness; Toyota P/N G9281-47020. 2005-2012 Ford Escape Hybrid — BECM mux board failure throws P0AC4 + P0AC0 together. 2007-2011 Toyota Camry Hybrid — pack temp sensor B sits near the trunk-mounted blower intake, gets dust contamination that destabilizes the reading. Estimated repair: $140 to $1,600.