The diagnostic trouble code (DTC) P0A23 for a 2008 Toyota Highlander Hybrid relates to the Inverter A Coolant Pump Control Circuit. This code indicates an issue with the circuit that controls the inverter's coolant pump, which is critical for maintaining the appropriate temperature of the inverter and motor.

Here's a more detailed explanation and steps you might consider for troubleshooting:

1. **Purpose of the Inverter Coolant Pump:**
The inverter in a hybrid vehicle like the Toyota Highlander Hybrid converts DC from the battery to AC for the electric motors. It generates a substantial amount of heat, and therefore, it requires a cooling system. The inverter coolant pump circulates coolant to keep the inverter and motors at safe operating temperatures.

2. **Potential Causes for P0A23:**
- **Faulty Inverter Coolant Pump:** The pump itself might have failed electronically or mechanically.
- **Electrical Connection Issues:** Wiring problems, such as an open or short circuit in the pump's electrical circuit.
- **Poor Electrical Connections:** Corroded or loose connectors can cause poor contact.
- **Faulty Power Control Module (PCM):** Although less common, a fault in the PCM might misreport or misinterpret the circuit’s operation.

3. **Troubleshooting Steps:**
- **Check Coolant Levels:** Ensure that the coolant levels in the inverter cooling system are adequate. Low coolant might not trigger this specific code directly, but it's good maintenance.
- **Inspect the Coolant Pump:** Check the pump for obvious signs of failure such as leaks or physical damage. You can also test it directly by supplying power to see if it operates.
- **Examine Wiring and Connectors:** Look for any visible damage to wiring and connectors. Use a multimeter to check for continuity, resistance, and potential shorts to identify electrical problems.
- **Consult a Repair Manual:** A detailed repair manual for your vehicle can provide specific instructions for accessing and testing the inverter coolant pump control circuit.
- **Perform a Coolant Pump Functional Test:** If possible, use diagnostic equipment to issue commands to the pump and verify its operation through a scan tool.

4. **Considerations:**
- Working with hybrid systems can be dangerous due to the high voltage present. Ensure that you follow all necessary safety precautions.
- If the pump is confirmed faulty, it will need to be replaced. It is recommended to use an OEM part for reliability.

Addressing this issue promptly is important, as insufficient cooling can lead to overheating and potential damage to hybrid drive components, which can be costly to repair.