OBD-II diagnostic code indicating a fault in the hybrid battery temperature sensor circuit. The Battery ECU monitors pack temperature via multiple NTC (Negative Temperature Coefficient) thermistors embedded in the module stack. When the sensor signal falls outside the expected range — either too high, too low, or erratic — P0AC4 is stored. Left untreated, this can lead to battery overheating, accelerated degradation, and in rare cases thermal runaway.
P0AC4 is a generic OBD-II code for "Hybrid Battery Pack Temperature Sensor A Circuit." It is set by the Battery ECU when the signal from one of the primary pack temperature thermistors is implausibly high, implausibly low, or shows a rate of change that is physically impossible for the battery pack to experience.
Toyota hybrid systems use multiple NTC thermistors (typically 3–5 depending on generation) placed at strategic locations within the battery pack: one near the intake air vent, one in the middle of the module stack, and one near the exhaust side. These sensors feed real-time temperature data to the Battery ECU, which uses it to control the battery cooling fan speed, adjust charge/discharge limits, and trigger over-temperature protection.
On Gen 2 Prius (2004–2009), the most common cause of P0AC4 is a failed intake air temperature sensor located near the battery fan intake duct. The sensor sits in the air stream and is exposed to cabin dust, moisture, and temperature cycling. Over time, the thermistor can drift out of calibration or develop an open/short circuit. Gen 3 Prius (2010–2015) improved sensor placement but still sees failures from the fan control module relay sticking on or off.
If P0AC4 is accompanied by a cooling fan that does not run, the battery pack can overheat during aggressive driving or hot weather. NiMH batteries permanently lose capacity when exposed to temperatures above 65°C (149°F). If the pack smells hot or you notice reduced performance, pull over and let the battery cool before driving further. Always follow HV safety procedures: disconnect 12V, remove service plug, wait 10 minutes.
Connect a hybrid-capable scan tool and access live battery temperature data. With the vehicle cold-soaked (engine off for 4+ hours), all temperature sensors should read within 1–2°C of each other and close to ambient temperature. A sensor reading −40°C or 200°C+ indicates an open or short circuit. A sensor that reads 5°C+ different from others after a drive indicates a drifting or mis-calibrated thermistor.
Sensor 1 (TH1): Intake air side, near fan inlet. Sensor 2 (TH2): Middle of pack, between modules 14 and 15. Sensor 3 (TH3): Exhaust side, near outlet duct. P0AC4 typically refers to Sensor 1 or the primary intake sensor.
Before diving into sensor testing, verify that the battery cooling system can actually move air. Locate the battery cooling fan intake vent (usually in the rear passenger side trim, near the C-pillar on Prius). Check the intake screen for lint, pet hair, leaves, and debris. Turn the vehicle to READY mode and command the cooling fan on with the scan tool (or by heating up the battery with a long drive). Listen for the fan in the rear cargo area. Check the exhaust vent for airflow.
A clogged intake filter is the #1 cause of battery overheating and P0AC4 on Prius. The intake screen can pack full of dust and pet hair over 3–5 years, reducing airflow by 50%+ and causing the pack to run 15–25°C hotter than normal.
Put on Class 0 gloves. Disconnect the 12V battery and remove the HV service plug. Wait 10 minutes. Locate the temperature sensor connector near the Battery ECU or on the pack enclosure. Disconnect the sensor connector and measure the resistance across the two thermistor pins with your multimeter set to ohms. Also measure the actual ambient temperature with a digital thermometer. Compare the resistance reading to the manufacturer's thermistor chart.
At 0°C (32°F): ~29 kΩ. At 25°C (77°F): ~10 kΩ. At 50°C (122°F): ~3.6 kΩ. At 80°C (176°F): ~1.2 kΩ. If your reading is infinite (open circuit) or 0 ohms (short circuit), the sensor has failed. If it is off by more than 20% from expected, it has drifted out of calibration.
If the thermistor resistance checks out good when disconnected but the code returns when connected, the fault is in the wiring harness or the Battery ECU input. Visually inspect the sensor wiring harness from the thermistor all the way to the Battery ECU connector. Look for chafed insulation, pinched wires, or corrosion in the connector pins. Measure the resistance of each wire end-to-end — both should be less than 1 ohm. Also check for 5V reference voltage at the ECU side connector with the ignition on.
The Battery ECU provides a 5V reference voltage through a pull-up resistor inside the module. If you measure 5V on one pin and 0V on the other with the sensor disconnected, the ECU side is good and the sensor is the problem. If you don't see 5V, the fault is in the ECU or its power supply.
If the temperature sensor is reporting correctly but the battery still overheats and triggers P0AC4, the cooling fan or its control module may be faulty. Locate the battery cooling fan in the rear cargo area (on top of or next to the battery pack). Disconnect the fan motor connector. Use a fused 12V jumper wire to directly power the fan motor from the 12V auxiliary battery. If the fan spins freely and quietly, the motor is good — the problem is in the fan control module or the ECU command. If it doesn't spin or makes noise, the fan motor needs replacement.
On 2004–2009 Prius, the fan speed is controlled by a dedicated fan control module (transistor / power amplifier) mounted near the fan. This module takes a PWM signal from the Battery ECU and switches high current to the fan motor. The module is a known failure point — symptoms include fan only working on one speed, or not working at all.
Once you have identified the faulty component (thermistor, wiring, fan module, or fan motor), proceed with replacement. For thermistor replacement on Prius, the intake air sensor is usually accessible from the fan duct without full pack disassembly. Gently pull the old sensor out of its rubber grommet, disconnect the connector, and install the new thermistor. While you're in there, vacuum all dust and debris from the intake duct, fan blades, and exhaust vent. Clean the intake screen thoroughly.
Ensure the replacement thermistor has the correct 25°C resistance rating (typically 10 kΩ for Toyota) and the correct Beta value (usually around 3435K). Using the wrong thermistor will cause inaccurate temperature readings and may trigger the same code again.
Reinstall all trim panels and connectors. Reinstall the HV service plug and reconnect the 12V battery. Start the vehicle in READY mode. Clear all DTCs with the scan tool. Monitor live temperature data while you go for a 20–30 minute test drive in warm conditions or with the heater on (to raise cabin temperature). Verify that all temperature sensors track together and that the cooling fan activates at the correct temperature (typically around 35–40°C pack temperature).
A successful repair shows all temperature sensors tracking within 1–2°C of each other, the cooling fan cycling on and off normally, pack temperature staying below 50°C under normal driving, and no recurrence of P0AC4 after at least 50 miles of mixed driving.
P0AC4 has many potential root causes. Seek professional help if any of the following apply:
The battery pack is actually overheating (temps above 65°C) and you cannot quickly determine why — continued driving risks permanent battery damage.
The fault is inside the Battery ECU (no 5V reference, sensor input circuit failure) — ECU replacement may require dealer-level programming.
You smell a burning or hot plastic odor from the rear battery area — this could indicate a more serious thermal event in progress.
You do not own 1000V-rated insulated gloves, a CAT III multimeter, and a hybrid-capable scan tool with temperature data.
Cooling fan replacement requires removing the full battery pack on your model (some Lexus and Ford hybrids have fan assemblies mounted deep inside the pack).
The vehicle is still under the hybrid battery warranty (8 years / 100,000 miles — do not void it with DIY disassembly).
Battery SOH below 70% threshold. Chronic overheating from a failed cooling system is a leading cause of accelerated battery degradation.
Individual module voltage sensor fault. Similar sensor-circuit diagnosis approach — but for voltage taps instead of thermistors.
Inverter / converter system malfunction. Inverter cooling problems can also trigger temperature-related codes and reduce hybrid performance.
Open the master specification sheet for your generation. Includes NTC thermistor resistance charts, OEM fan motor part numbers, cooling system airflow specifications, and step-by-step fan module replacement procedures.