We had the opportunity to look at a 2021 Toyota Highlander Hybrid for a no start, gasoline engine will not run, and the vehicle will not go into READY mode. The vehicle was towed in, and the customer stated that the CEL and HV warning light had come on while he was driving, a message appeared on the dash telling him to stop driving and pull over. The customer kept driving as the vehicle continued to operate. He was able to drive the vehicle with the warning lights on for a few days when the vehicle finally would not READY up. At which point the vehicle was towed on for repair.

System isolation failure

Step one was to check for codes to get an idea of where the problem was. I scanned the vehicle and retrieved a code P0AA6 with subcode 49. The code description was System Isolation Internal Electronic Failure, and the failure criteria was listed as Insulation resistance between the high-voltage circuit and the body. What this means is that the High Voltage (HV) monitoring system has picked up a HV leak to the body of the vehicle, basically a short in the HV system to the body of the vehicle. While there are many codes for HV leakage, and most will point you to a particular component or system the P0AA6-49 is a general fault with no specific component listed.

When you look at the trouble chart for this code, all HV components and cables are listed as potential fault areas. Testing all these components with the short present, can be daunting and was made worse in our case because as soon as we cleared the codes the vehicle returned to normal operation with no codes present.

Road test

When faced with this type of issue, a road test performed in the proper manner can be used to identify the problem area. To do this you’ll want to monitor the following data bit Short -Wave Highest Value (Fig1). This data bit (PID) is how Toyota monitors HV leakage. The PID (depending on the vehicle) can be displayed a few different ways but on this Highlander Hybrid the PID had four states. Normal, Not Judged, Insulation Lower Level 2 and Insulation Lower Level 3. Normal is self explanatory with each level after that showing a higher level of leakage with Level 3 being almost a dead short.

A road test was performed while monitoring the Short-Wave Highest Value as well as MG1 and MG2. How this works, start by clearing all the codes, then while watching the warning lights and scan tool put the vehicle in Ignition On mode. This turns the Battery Control Module on but does not connect the HV battery to the resto of the vehicle. If the fault sets during this time the problem is in the HV battery pack. We then put the vehicle in READY mode and shifted to Neutral. This connects the HV battery to the Inverter Assembly but lock out the Motor Generators. Next, we pushed on the gas pedal to get the HV system to start the ICE. This makes MG1 work. Then, it was time to drive the vehicle. This engages MG2 and in our case for some acceleration and braking, also engages the rear axle Drive motor. On the road test the vehicle continually set the code while accelerating and braking. You can see the Data change as the vehicle is braked and accelerated. (Fig 1). Since MG2 is working all the time the vehicle is in motion but the Rear Drive Motor on works while accelerating and braking, we decided to look at the Rear Drive Motor first.

Corroded cables

Upon inspection of the Rear Drive Motor HV cables I noticed that the rear cover looked to be slightly distorted and dirty. (fig 2). I then had the connector taken apart so we could get a better look at the cabling. We found severe corrosion and deterioration of the mounting bracket and shielding. (Fig 3)

The cable was priced out, over $2000.00 plus installation. Fortunately for the customer the vehicle had a 96-month, 160,000 km warranty on Hybrid related components. The dealer is going to cover the repair under warranty, and we found out that they stock this cable. Obviously, this problem is known to the dealership. All in all, a good result for the customer.