Vehicle: 2003 Toyota Prius, L4-1.5L (1NZ-FXE) Hybrid, Automatic Transmission/Transaxle

Mileage: 166,050

Problem: The Prius was brought to the shop with the malfunction indicator light (MIL) on. The owner said that the vehicle’s performance seemed normal.

Case Details: The technician connected a scan tool and found diagnostic trouble code (DTC) P3009, Electric Leak from High Voltage System. There is a laundry list of items that can trigger this DTC, such as issues with the system main relay, battery ECU, transaxle cables, battery temperature sensor, or the HV battery assembly, just to name a few.

Using the DTC P3009 diagnostic procedure in ALLDATA, the technician inspected the hybrid wires and components and found the insulation resistance on the cables to the transaxle assembly were definitely out of specifications. He also noted that the transmission fluid color and viscosity was closer to water than hydraulic fluid.

Confirmed Repair: After replacing the transmission fluid, the technician decided to clear the DTC and recheck the cable insulation resistance to the transaxle assembly. The resistance was back in specifications. After a good test drive, the MIL did not return and no DTCs were pending.

So, how can changing the transmission fluid solve an electrical issue? Toyota’s Hybrid transaxle fluid is hygroscopic which causes it to pull moisture from the atmosphere and hold it. At a whopping 281K miles, the fluid looked like it had never been changed and the viscosity was very thin. In this state, the fluid had become electrically conductive. Any time the diagnostics points you to a transmission/electrical problem like this, check the fluid first. Odd problem solved!