Hi everyone, we’ve been getting a lot of 2004-2009 Prius models coming into the shop lately with brake actuator failure. The brake actuator is one of the “Big 3” costly replacements in the Prius and I’d argue that it’s the most dangerous repair to ignore.
The most common scenario we hear from our customers experiencing brake actuator failure is that some or all of the dash warning lights will appear, but no noticeable symptoms occur right away. If the warnings are ignored, a long persistent buzzer sound may come and go. The buzzer is letting you know that you need to stop driving immediately.
What is a brake actuator and why is it failing?
The brake actuator/accumulator assembly consists of a high pressure motor, a pressure tank and valves that open and close to direct brake fluid to the wheels as needed.
As time progresses small leaks in the system will cause the pressure in the tank to drop which results in the motor having to run more often to maintain the pressure. Most of our customers say they’ve had no indication that there was a problem with the brakes until the warning lights came on, but in our experience when the pump can be heard about every 15 seconds or so, it may be close to failing. You can actually hear the pump trying to pressurize while the car is in ready mode (foot off the brake). Here’s what a failure unit sounds like.
If you already have warning lights on the dash its important to stop driving the car until a complete diagnostic is performed. The most common diagnostic code for brake actuator failure is C1256.
When C1256 code is logged the car will enter a fail-safe mode which will disable antilock brakes, electronic braking force distribution and regenerative braking. (The first two are obvious safety concerns) but another area of issue is how this condition affects the hybrid battery.
Normally, the engine and regenerative braking work together to charge the battery while driving, but when an ABS code is logged, the regen braking is disabled which can lead to an overly discharged hybrid battery.
This is such a common problem that we often see actuator codes accompanied by battery failure codes. If you have both brake and battery codes, it is in your best interest to address the brakes first.
If you have this code, your brake actuator will need to be replaced. Unfortunately, this is not an easy DIY repair and it can be quite costly. We no longer recommend using used or remanufactured parts for the replacement due to reliability concerns.
At our shop we almost always try to offer a used or refurbished part option to our customers, but this is one area where safety overrides savings. Sites like eBay and craigslist are flooded with used actuators of varying conditions, but you’re honestly not going to know if those used parts work until after you’ve completed the lengthy installation. We only recommend purchasing a new OEM part directly from Toyota.
Toyota formerly had a warranty enhancement for the 2004-2009 Prius brake actuator, but it expired in December of 2017. If you have a 2010-2015 your actuator may be covered under warranty enhancement program ZJB which can be found here.
Our price: OEM part plus labor $1895
Toyota Price: $2500-$3800
In short, the hybrid inverter/converter is responsible for boosting the voltage from the DC hybrid battery from 200v to 500v in order to create the three phase power needed for MG1 (motor generator) and MG2. The inverter also supports voltage lowering conversion from the hybrid battery to the 12v system. It’s a lot more complicated than what can be explained in a few sentences, but its safe to say the inverter/converter system is the brains of all voltage conversion within hybrid vehicles.
Determining if the inverter has failed can be tricky, but we’ll try to break down the symptoms, diagnostic codes and some money saving strategies.
2004-2009 Prius (additional information for newer model Prius further down)
I always like to begin with the most common scenarios that our customers experience. In this case its pretty simple- the Prius will turn on, have warning lights on the dash, but will not go into drive or reverse. In this situation, if you were to clear the warning lights via diagnostic code reader or by resetting the 12v battery the car will go into drive until it’s turned off and back on again. That’s one trick some Prius owners have used to limp the car to a mechanic. If driving the car in this condition isn’t something you’re comfortable doing, I recommend towing or calling a mobile mechanic.
Part of the trouble with diagnosing inverter failure is that the most common diagnostic code to come up (P0AA6 voltage isolation fault) is also the same code for a voltage leak on the hybrid battery, transaxle and AC compressor. Thanks Toyota…
If you’ve found code P0AA6 and are unsure where this voltage leak is coming from, you have to look for the sub codes (also called detail codes) to be sure. To get the sub codes you’ll need to be using a high quality code reader. Most of the cheaper ones will not find these codes. There will always be sub code 526 present, but that’s not what we are looking for. There should be a second sub code and it will be either 611, 612, 613 or 614.
611: The voltage leak is in the AC compressor
612: The leak is coming from the hybrid battery cells, relays or ECU
613: The leak is in the transaxle
614: The leak is in the hybrid inverter/converter
*If you’re able to get code 526, but not the second necessary sub code you may have to repeatedly turn the car on and off and re-check codes.
Most mechanic shops and even the Toyota dealerships we’ve encountered do not even look at the sub codes. You have to advocate for yourself and ask them to recheck the codes. Dealerships will almost always suggest replacing the hybrid battery or ask permission for unnecessary further diagnostics. I will say that the hybrid battery is usually the culprit when P0AA6 is logged, but when you’re talking about replacements that cost into the thousands of dollars its best to be sure.
Ok, now on to another scenario…
Inverter failure with code P0A08/P0A09
The Prius has warning lights on the dash and may or may not drive, but diagnostic code P0A08 (not to be confused with hybrid battery code P0A80) and/or P0A09 code are logged. These codes can mean that the hybrid inverter has failed OR the 12v battery is weak/improperly connected. I recommend checking that the connections to the 12v are properly secured and not horribly corroded either. Next, check the health of the 12v using this method. Link 12v video. If you replace the 12v and it is still showing weak voltage, this means that the inverter is not properly supplying power to it and must be replaced.
*Improperly jumping the 12v battery (putting jumper cables on backwards) can damage the inverter and cause P0A08 and P0A09.
Lastly, if your Prius stalls while driving, but letting it cool down allows the car to drive again, the inverter may be overheating. Check for inverter pump turbulence using this video. Code P0A93 suggests that the pump needs to be replaced. Codes P0A94 and P0A78 are much trickier (show QCLT), but typically require full inverter replacement.
2010 and Up Prius
Toyota has issued a warranty enhancement for certain model 2010-2014 year Prius vehicles. If you’ve experienced stalling or have diagnostic codes P0A94, P324E, P3004 or P0A1A your Prius may be covered under this warranty. Best thing to do is call your local dealership with the codes and provide your VIN number. More information on this warranty enhancement can be found here.