OBD-II Code
P0AC4
Severity
Not yet reviewed
Nobody on our side has assessed this code yet, so we are not going to guess at how serious it is.
Can you drive it
We cannot say
Treat it as unknown until a shop reads it with live data. If the check-engine light is FLASHING, stop driving as soon as it is safe to — that means an active misfire, whatever the code.
Indicative cost (USD)
$140–$1,600
From this code's diagnostic notes. Varies by vehicle, diagnosis, parts, and local labor. This is not a repair quote.
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.
How a shop works this
Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed.
Note every code that is set, not just this one.
Codes that appear together usually narrow the cause faster than any one of them read alone.
Read the freeze frame — the conditions the fault was recorded under.
Cold or warm, idle or load, road speed and engine speed. A fault that only appears cold is a different fault from one that only appears under load, even with the same code on the scanner.
Confirm the fault is current rather than history.
Clear the code and see whether it returns under those same conditions. A code that does not come back may have been logged during a repair, a flat battery, or a bad connection that has since been disturbed.
Then follow the diagnostic notes further down this page, in the order they are written.
They are written in the order a shop works this code, cheapest checks first. Where they name a component, inspect its wiring and connector before replacing it.
If the code comes back after a repair, the underlying fault is still there. That is the point to bring in a scanner with live data and freeze-frame, or a shop that has one.
Common symptoms
- Check engine light
Read the full diagnostic procedureP0AC4 is the same fault class as P0AC0 but on hybrid battery pack air-temperature sensor B — the second of the pack's distributed thermistors, typica…
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 patterns2004-2009 Toyota Prius Gen 2 (NHW20) — rear pack temp sensor (sensor B) intermittent open at BECM connector from cowl water corrosion, often paired w…
- 2004-2009Toyota Prius Gen 2 (NHW20) — rear pack temp sensor (sensor B) intermittent open at BECM connector from cowl water corrosion, often paired with P0AC0.
- 2010-2015Toyota Prius Gen 3 — internal sensor failure more common than harness; Toyota P/N G9281-47020.
- 2005-2012Ford Escape Hybrid — BECM mux board failure throws P0AC4 + P0AC0 together.
- 2007-2011Toyota 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.
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