P0A05: Motor Electronics Coolant Pump Control Circuit / Open
Severity: high
What it means: Hybrids have a separate cooling system, with its own small electric pump, for the inverter (the box that powers the electric motors). P0A05 means the hybrid computer sees a problem, such as an open circuit, in the control circuit for that pump. In practice owners often find the pump itself has died.
Safe to drive? Drive only a short distance to get it checked. Without coolant flow the inverter can overheat, and the hybrid system may cut power.
Common symptoms
- Check Hybrid System warning, often with Check VSC
- Loss of power or reduced power
- Inverter coolant not moving in the reservoir with the car on
Likely causes, ranked
- Failed inverter coolant pumpDIY or shop
- Wiring or connector problem at the pumpDIY or shop
- Blown fuse or relay for the pumpDIY
Vehicle-specific notes
- 2006–2008 Lexus RX 400h: On ClubLexus, RX 400h owners with P0A05 and Check Hybrid/Check VSC warnings found failed inverter coolant pumps. One paid $282 total. Read the full answer · Source
DIY or shop?
DIY for checking fuses and whether coolant is moving. Pump replacement is a moderate job, and you must bleed the inverter cooling system afterward.
Before you replace parts
- Read all hybrid codes with a scanner that reads hybrid systems.
- With the car on, look for coolant moving in the inverter coolant reservoir.
- Check the fuse and the pump connector.
- Replace the pump if it isn't running, then bleed the inverter cooling system.
Safety / mechanic note
Verify with a mechanic. An overheating inverter can shut off hybrid power while driving. A code points to a circuit or system, not always a specific part.