1. Introduction
Diagnostic Trouble Code (DTC) P0A85 is defined as Hybrid Battery Pack Cooling Fan 1 Control Circuit High. This is a generic powertrain code that applies to OBD-II equipped hybrid vehicles, including Toyota Prius, Lexus CT200h/RX450h, Hyundai Ioniq, Kia Niro, Nissan Leaf, and various Ford and GM hybrids.
The code sets when the hybrid Battery ECU (also referred to as the Battery Energy Control Module or BECM) detects a higher-than-normal voltage in the control circuit for Battery Cooling Fan 1. Since Cooling Fan 1 is responsible for circulating air through the high-voltage (HV) battery pack, abnormal voltage can cause incorrect fan speed, overheating risks, and reduced hybrid/EV performance.
System Operation
The speed of the battery cooling blower assembly is controlled by the HV control ECU (or hybrid vehicle control ECU). Power to the blower assembly is supplied when the FCTL terminal of the control ECU activates the battery blower relay. The control ECU sends command signals (S0, S1, S2 or SI) to the blower assembly to achieve the fan speed corresponding to the HV battery temperature. Voltage information applied to the blower assembly (VM) is sent back to the control ECU as a monitor signal via serial communication through the battery smart unit.
2. Diagnostic Conditions
2.1 Conditions for Running the DTC
The diagnostic monitor for P0A85 runs under the following conditions:
- Ignition is in RUN or CRANK position
- The engine is running (for some vehicle applications)
- The hybrid/EV battery pack cooling fan is enabled
2.2 Conditions for Setting the DTC
The DTC sets when the following conditions are met simultaneously:
For certain vehicle models (e.g., Toyota with INF code P0A8100), the DTC sets when:
- The HV battery temperature is high and the cooling fan speed calculated based on the blower output frequency (FP0) is not within the target control speed range
- The blower assembly speed is excessively high when a speed below a certain level is requested
For INF code P0A85-123, the DTC sets when the output voltage of the battery cooling blower assembly (VM) is too high compared to the target control voltage range (1 trip detection).
2.3 Action Taken When the DTC Sets
- The hybrid system may be disabled or power may be limited
- The Battery ECU restricts output as a precaution
- The system may enter limp mode under sustained high battery temperature
- The service engine soon light (or MIL) illuminates
2.4 Conditions for Clearing the DTC
The DTC clears when the fan speed control feedback returns to normal.
3. Diagnostic Test Procedures
3.1 Safety Precautions
WARNING: Before inspecting or servicing the high-voltage system, mandatory safety measures include:
- Wear insulated gloves and protective goggles
- Remove the service plug grip to prevent electrical shock
- Carry the removed service plug grip in your pocket to prevent other technicians from accidentally reconnecting it while you are working
- Wait at least 5 to 10 minutes after disconnecting the service plug grip before touching any high-voltage connectors or terminals – this allows the high-voltage capacitor inside the inverter to discharge
- Do not turn the power switch to READY after removing the service plug grip unless instructed by the repair manual
- After waiting, verify voltage at the inverter inspection point is 0V before beginning work
3.2 Required Tools
- OBD-II scan tool with hybrid functions (e.g., Techstream, GTS, or intelligent tester)
- Digital multimeter
- Back-probe pins
- Trim removal tools
- Infrared thermometer
- Compressed air
3.3 Step-by-Step Diagnostic Procedure
Step 1: Check DTC Output
- Connect the diagnostic scan tool to the DLC3 (Data Link Connector)
- Turn the ignition switch to the ON position
- Navigate to: Powertrain → Hybrid Control (or HV Battery) → Trouble Codes
- Read all output DTCs
NOTE: Check whether DTC P0A1F is also output. If yes, follow the DTC chart for P0A1F first.
Step 2: Check the BATT FAN Fuse
- Remove the BATT FAN fuse from the engine room relay block
- Measure the resistance of the fuse
- If resistance is not within specification, repair or replace the harness/connector and replace the fuse
Step 3: Check Harness and Connector (VM Voltage)
- Disconnect the cable from the negative (-) battery terminal
- Confirm the service plug grip is removed
- Remove the No. 4 HV battery intake duct (or upper hybrid battery cover)
- Reconnect the negative battery cable
- Connect the scan tool to the DLC3
- Turn the power switch to ON (IG)
- Using the scan tool’s Active Test function, select “Driving the Battery Cooling Fan”
- Operate the blower assembly at each air volume mode (1 to 6)
- Measure voltage at the VM terminal
Standard Voltage: 5V or less (with power switch ON)
If voltage exceeds specification, there is a short-to-power condition in the wiring or a faulty fan motor.
Step 4: Visually Inspect Wiring and Connectors
Step 5: Test Fan Operation with Scan Tool
- Using the scan tool’s special function, command the hybrid cooling fan ON and OFF several times
- Observe whether the fan responds to commanded speeds or stays stuck at maximum speed
- If the fan does not respond or operates incorrectly, the fan motor or its control circuit is faulty
Step 6: Test Fan Current Draw
- Measure the current draw of the cooling fan motor
- Excessively high current may cause control voltage to spike
Step 7: Inspect the Cooling Duct
Step 8: Check HV Battery Temperature Sensors
- Test the battery temperature sensors
- Verify the intake air temperature sensor is functioning correctly
4. Trouble Areas (Fault Locations)
Based on diagnostic findings, the following components may be the source of the fault:
Common specific causes include:
- Short-to-power in Cooling Fan 1 control wiring
- Faulty fan motor drawing excessive voltage or locked at high speed
- Corroded or moisture-damaged fan module or connectors
- Rodent damage to wiring harness
- Fan blades jammed or obstructed, forcing high current spikes
5. Repair and Resolution Procedures
5.1 Repair Steps
Based on the diagnostic findings, perform the appropriate repairs:
If wiring/connector issues are found:
- Repair or replace the damaged wiring harness
- Clean or replace corroded connectors
- Ensure all connections are secure and free of resistance
If the cooling fan assembly is faulty:
If the fuse is blown:
- Replace the BATT FAN fuse
- Inspect the circuit for underlying causes that may have caused the fuse to blow
If the battery smart unit is faulty:
If the integration relay is faulty:
If the power management control ECU is defective:
If cooling duct blockages are found:
5.2 Post-Repair Verification
- Clear all DTCs using the diagnostic scan tool
- Perform the All Readiness check procedure to verify the vehicle has returned to normal operation
- Command the fan ON and OFF using the scan tool to verify proper operation
- Perform a road test to confirm the DTC does not return
- Monitor battery temperature to ensure the cooling system is functioning correctly
5.3 Important Note on DTC Clearing
Do not clear the DTC by simply disconnecting the negative battery cable. Always use the specified scan tool procedure for the vehicle make and model to properly clear the codes and reset monitor readiness.
6. Common Symptoms
Drivers may experience the following symptoms when P0A85 is present:
- Fan runs at full speed constantly, even when battery temperature is low
- Harsh or noisy fan operation
- Service engine soon light or master warning light illuminated
- Reduced EV or hybrid performance – Battery ECU restricts output as a precaution
- Battery overheating warnings
- Increased cabin noise, especially from rear seat or trunk area
- System enters limp mode under sustained high battery temperature
- High-voltage system may not function properly
