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Case Study: 2010 Toyota Prius "Check Hybrid System"

If you're reading this, odds are you've most likely encountered the "Red Triangle of Death" or the more common "Service Hybrid System" and "Check Hybrid system" associated with various other warnings including ABS, VSC, etc..

Most of the time these warning lights lead to a failed or failing high voltage battery, as well as a few other hybrid specific components in the powertrain.

But have you ever encountered the elusive DTC or Diagnostic Trouble Code P0AA6?

The Back Story:

Before contacting Evergreen Auto Solutions, this customer previously had the vehicle diagnosed by their local Toyota Dealer who deemed the Inverter to be defective after the "Check Hybrid System" message appeared on the dash and DTC P0AA6 was retrieved. No insulation diagnostic was performed during their inspection. The customer was then quoted almost $4000.00 in labor and part costs.

P0AA6 sets in the vehicle's computers when a loss of isolation (short to chassis ground) is detected, preventing a restart of the vehicle after it has been turned off - as long as the fault is constant.

But this is only the surface. In order to properly identify the root cause within the hybrid high voltage system, you'll need to have a general understanding how the high voltage components are insulated from the rest of the vehicle, such as the incorporation of a floating ground to prevent a catastrophic short to vehicle chassis ground. Chassis ground is what all low voltage components use to return the ground side of their circuity back to the 12 volt battery. A floating ground provides a high voltage path back to the hybrid battery without ever interacting with any low voltage circuits or chassis ground. This is a standard safety feature on all Hybrids and EVs.

Beyond understanding the fundamentals of a floating ground, you must use a compatible vehicle scanner or OEM software to continue your dive deep into the different INF sub codes that assist in pin pointing the exact location of failure.

P0AA6 DTC and INF sub codes:

Pictured: INF sub codes 526, and 611 detailing potential fault areas for P0AA6

Pictured: INF sub codes 612, 613, and 614 detailing potential fault areas for P0AA6

If the above charts detailing potential fault areas seems overwhelming, you're not alone. In fact, this is where most traditional repair shops call in a specialist such as Evergreen Auto Solutions to help diagnose the vehicle. For the average DIYer, this beyond their skillset and toolset to tackle. NOTICE: ONLY ASE L3 CERTIFIED TECHNICIANS SHOULD ATTEMPT THESE PROCEDURES. NOT ADHERING TO ASE L3 AND MANUFACTURER INSTRUCTION MAY LEAD TO INJURY AND DEATH!

The Basics: PPE, Time, Tools, and the Process of Elimination

Pictured: Personal Protective Equipment (PPE) high voltage gloves

Pictured: Fluke 1587 bundle

Before attempting to go any further, you MUST have a Megohmmeter, such as the Fluke 1587 (pictured above) we use at Evergreen Auto Solutions. This is standard equipment for any shop working on Hybrid and EVs. This meter has the capability to send 1000 volts through the component or frame wire that you're testing (most HV components limit testing to 500 Volts, always double check the manufacturer specifications to prevent damage) in order to detected a short to chassis ground. Without doing this, there is no other way to confirm or deny the existence of a short to chassis ground aside from checking the "Short Wave Highest Value" data pid in Techstream, Toyota's OEM laptop-based software.

Pictured: Toyota Techstream laptop bundle

Pictured: Check Short Wave values in Techstream

After establishing which INF sub code set along with the parent code P0AA6, have your Megohmmeter ready, and compatible scan tool hooked up to the vehicle. You can begin to narrow the source further by simply watching live data while turning the vehicle on.

HINT: Perform a full DTC reset in all computers, and drive.

A quick way to identify which high voltage system in the vehicle is the root cause of concern can be conducted by understanding what components actuate during a drive cycle.

Starting the vehicle: Turn off all loads including A/C

As soon as you open the driver door, you'll audibly hear the brake actuator pump charge the high pressure accumulator in the engine compartment, and possibly even the fuel pump priming the fuel rail. Once the start/stop button has been depressed with your foot on the brake pedal, the hybrid battery and inverter perform a system test and actuate the high voltage contractors (system main relays - positive, negative, and precharge) to ensure their state of health. If P0AA6 sets during the initial start up after clearing, it is likely the root cause is located in either the Hybrid Battery pack due to leaking electrolyte from the modules reaching the battery chassis, a damaged high voltage frame wire with water intrusion in the connectors / open insulation, or if the Inverter DC/DC Converter has an internal short via coolant containment loss or some other type of conductive debris. Depending on the area of isolation loss, a second INF code should populate alongside the standard INF 526. If no other INF accompanies 526, get ready to dig deep.

The Drive Cycle: Short Wave Highest Value and Component Actuation

Pictured: Freeze frame for DTC P0AA6-526. Notice the Short Wave value highlighted.

If you're using Tech Stream you can find the Short Wave data pid in the High Voltage / Hybrid Control ECU pictured above. If you're using an aftermarket scanner you should also be able to view that same pid depending on the user interface. All Autel diagnostic tools have this data pid available. When no other INF accompanies 526 this will be your quickest way to narrow your scope.

Here is an excerpt from Toyota's Tech publication, written by Paul Cortez with a quick rundown on how the Short Wave Highest Value algorithm informs the HV ECU where a potential isolation loss may exist:

"Short Wave Highest Value Isolation faults can be intermittent. Sometimes the PID “Short Wave Highest Val” (SWHV) can help find intermittent faults. For a P0AA6 to set, a fault must be present for at least 60 – 90 seconds. If the problem comes and goes, you have difficulty getting codes to set. Monitoring the PID SWHV will show you when a fault occurs, even if it’s not happening long enough to set a code. However, it can be a little tricky, so read the next part carefully. Short Wave Highest Val is a voltage between 0V and 4.98V. Normally it will be over 4V. When its value is 0V, there is a dead short between high voltage and chassis ground. However, it’s only valid when the boost converter isn’t operating. To determine whether the boost converter is active, check the PIDs VL (voltage low) and VH (voltage high). VL is the voltage at the DC cables in the inverter. VH is the voltage on the boost converter output. If the voltages are roughly equal, the boost converter isn’t active, and you can use SWHV for diagnosis. On the other hand, if VH is higher than VL, ignore SWHV because it will look like there is a fault, even though there isn’t. The easiest way to use SWHV is to graph SWHV, VL, and VH, that way you can easily analyze the data from a test drive. "

Pictured: Short Wave Highest Value graphed in Techstream.

If the code has not yet set, put the vehicle in drive and proceed with a road test. Remember to leave the A/C OFF. If P0AA6 sets while driving there is a good chance the fault lies within the Transmission's Motor/Generators or the Inverter DC/DC Converter.

If the code has not set after starting and driving for a full cycle, the final area of potential is the A/C compressor.

HINT: The A/C compressor on a generation 3 Toyota Prius is a hermetically sealed 3 phase AC high voltage scroll type compressor, with an internal inverter. The frame wire leading from the Inverter/Converter to the AC compressor comprises of 2 orange wires sending high voltage DC to then be inverted to 3 phase AC while the compressor is in use. The lubricating oil used in all Hybird and EV electric compressors is specific to their safe operation as it does not conduct electricity while lubricating the internal components. It is imperative that manufacturer specified lubricants or similar such as Mastercool's 92707 HYBRID A/C COMPRESSOR OIL be used while servicing A/C components on any Hybrid or EV. Failure to do so will lead to the P0AA6 isolation fault.

Turn on the A/C to full blast and roll down the windows to induce a full capacity output. If P0AA6 sets while using the A/C compressor, it is almost certain that the compressor and potentially its associated refrigerant carrying components will need to be changed and properly lubricated to prevent any further conduction.

What Now?

Depending on your drive cycle test you may or may not have narrowed down the scope of your search. Either way, you'll need to get very comfortable reading electrical diagrams and their corresponding flow charts set by the manufacturer to begin disassembly and testing.

If you have a hard time understanding these diagrams and/or don't have the proper tools or personal protection equipment, stop what you're doing and contact us to schedule a diagnostic.

Pictured: Location of components within the high voltage system and their respective INF sub code areas

Pictured: Toyota Prius electrical diagram overview of the high voltage system

After studying the diagrams and developing a fundamental understanding of the Hybrid powertrain and it's components, you can begin testing. If the scope has been narrowed by the previous drive cycle, you can use the charts above to see their corresponding locations. If you were unable to narrow down your scope during the previous drive cycle, we suggest starting with a full undercar inspection to determine the state of health of the frame wires running under the vehicle, as well as checking for any physical damage to the transmission housing or the AC compressor housing. If the visual inspection passes, begin with the Hybrid Battery removal and disassembly process.

HINT: Before completely removing the Hybrid Battery you can perform a quick measurement to see if High Voltage is leaking into chassis ground.

Pictured: High Voltage reaching chassis ground via an electrolyte leak within the hybrid battery.

First, remove the Service Plug Grip, or quick disconnect that disables the Hybrid Battery and remove the protective plate that houses the high voltage contactor assembly. Second, take your Fluke 1587 or equivalent Megohmmeter and carefully insert the positive lead into the back of the orange positive cable (back probing it) without disconnecting it from the high voltage contactor assembly. Third, place the negative lead to a hybrid battery chassis and switch your meter over to Voltage DC. Fourth, reinsert the Service Grip Plug and check your meter for a reading. Any sort of significant reading above 0.5 Volts will set DTC P0AA6, however a higher severity/higher voltage reading will trigger an immediate response from the Hybrid Control Unit. If the fault does not immediately appear during a lower voltage short to chassis ground, it may take a few drive cycles to set. Regardless, if you're seeing voltage between the hybrid battery and chassis ground of any sort you must remove the hybrid battery and begin disassembly in order to examine and identify the source of the electrolyte leak.

Beyond the Battery:

If the Hybrid Battery passed testing and no INF sub codes or drive cycle testing yielded the results you were looking for, you must now adhere to the Toyota Flow Charts associated with testing each component until a defect has been identified. Evergreen Auto Solutions recommends using Techstream with an active subscription to Techstream servers in order to receive the latest and most accurate information available. If you are using a J2534 or equivalent bidirectional OBD2 cable you will also be able to reflash and code any module or component to the latest update or calibration available based on the VIN.

Pictured: An example from Toyota's OEM Diagnostic Flow Charts for DTC P0AA6

Pictured: Fluke 1587 in insulation test mode, sending 500 Volts through the 3 phase Motor/Generator cables to determine state of health.


The customer's vehicle was never able to successfully reproduce DTC P0AA6 or any other INFs during our extensive testing. However, by observing the Short Wave Highest Value data pid we were able to identify small pieces of debris in the transmission fluid conducting High Voltage to chassis ground, creating this extremely intermittent and illusive fault. After flushing the transmission fluid, the vehicle was successfully repaired and the customer saved over $3000.00 + in misdiagnosed repair costs, from the prior diagnostic performed by their local Toyota Dealer. If your car is displaying codes or warning lights and you're not sure what to do, contact Evergreen Auto Solutions to set up a professional diagnostic by our certified technicians.


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