I have a 2005 Gen II Prius with 105.000 miles. It has been trouble free until now and only been to dealer service for the recalls. I have always done my own maintenance following the manual intervals. About 3 weeks ago, driving home on the freeway, I suddenly get the red triangle, ASC, and yellow "brake" light on. Not knowing what they meant at the time, I carefully continued to drive home (about 20 miles). I couldn't detect any difference in the way the car ran or handled during this drive. When I got home, I got out the manual to see what the lights meant, and then shut it off to see how/if it would re-start. When I turned it on, I got all the dash trouble lights and couldn't shift into gear. The following morning (Saturday), after confirming I still had the same problem, I had it towed to the closest dealer. They contacted me late on Saturday and said things were pointing toward the transaxle, but they weren't sure yet and because that was so expensive, they wanted to check some other things Monday. I got a call midday on Monday with good news. The transaxle was OK after all. They updated the firmware, and "re-calibrated" the transaxle. They charged me about $225 for the labor spent. I drove the car about 2 weeks with no problems and then on Christmas Eve, while stopped at a stop light, the same lights came on. This time I was right near the dealer so I was able to take it directly to the service department without shutting it off. After about 45 minutes of diagnostics, they said it needs a new transaxle and quoted approx. $5700. They somehow reset the fault indicator and I drove off with the warning that it will probably happen again. Since I am a lifetime Toyota fan and have owned 8 between my wife and I, I am really dissappointed to need something this expensive at only 105,000 miles. I typically keep my cars 10 years and when I sell them to upgrade, they always have far higher mileage, and continue to run well. That is why I buy Toyotas. The DTC that was captured for the fault is "P0AA6 Hybrid Battery Voltage Isolation". The service manager said that when they did lower level testing of the transaxle, they got failures of 7 of the 8 parameters tested. He showed me a printout listing these items tested which each had a 4 letter acronym. He said it basically meant the transaxle is losing communication with the main ECU. Besides not wanting to spend this amount of money, I'm not really convinced they have diagnosed the problem correctly. Again, the car is running fine. Doesn't this seem more like an electronic communication problem rather than the transaxle which is largely mechanical except for the electric motors/generators? Isn't there a transaxle ECU that would be communicating over the serial bus to the main ECU and be more likely? Any thoughts or suggestions would be appreciated. Thanks in advance !!
I'm sure someone else will chime in, but if I'm not mistaken, the insulation breaks down on one of the electric motors in the transaxle, causing that code. Have you considered a used transaxle?
Hi GDH, Have the 12 V battery evaluated. It should be reliably above 12.0 V before startup. If its down 11.0 or lower, the ECUs might be not be working correctly. When is the last time you had the tranmission oil changed? Luckily, this is pretty uncommon occurence, and so there are plenty of servicable transaxles available from crashes. If you do need a transaxle give autobeyours.com a call to get a used one. My 2006 was hit from behind and totalled. Its transaxle should be great for you. Also, have the wiring evaluated, including underneath the covers in the battery. The Prius has a short detection scheme similar to bathroom ground fault. That ground fault can be anywhere, including underneath the battery covers (like if a rodent built a nest in there). Do you remember driving over any kinda road debris? The tires could have kicked it up and causing it to nick into the wires underneath the car.
I did put the car up on ramps over the weekend and checked that all the electrical plugs were tight into the transaxle (didn't mess with the high-voltage). The wiring harnesses that go to the transaxle are actually pretty high into the engine cavity from the bottom but I may have a closer look. When you talk about looking under the battery covers I assume you mean the traction battery instead of the 12 volt? Thanks for the suggestions and the tip on autobeyours.com .
Hi GDH, Yes, the covers for the traction battery. On a few occaisions, rodents have worked their way up through the air-ducts, and built nests inside the traction battery enclosure. If those bedding materials get moist, there will be a ground fault. To do this, one needs to pull the safety plug on the battery, and also, one needs to be a reasonably experienced power electronics technician, along with saftey equipment (line-mans insulating gloves...). Have you had the transaxle fluid changed on your car? Toyota's recomendation is quite long , we on Priuschat prefer something around 40 K miles, to get any wear-in fines (metal dust) out of the transmission fluid to avoid it wearing through the wire insulation.... Another maintenance organisation that specializes in hybrids is Luscious Garage up in the San Fransisco area.
At the very least get a 2nd opinion from a different Toyota dealer. Did you ever use anything other than Toyota's Prius-specific ATF?
Update To This Problem Same fault continued to happen at approx. 2 week intervals, usually when I was stopped at a traffic light. I bought a cheap code reader that couldn't read Prius trouble codes, but did allow me to reset the fault lights and continue driving without going to the dealer. I figured maybe the problem would eventually start happening more often and I could eventually nail down what the real problem was. One thing on my list to do during the process of elimination, was to change the transaxle fluid, since it had never been changed. Although dealer had recommended this be done, it was more just included in the list of $1400 worth of service they recommeded, not specifically related to this fault. They never even asked if it had been changed before, while diagnosing this problem. Following the instructions found here in the forums, I eventually changed the fluid. It has now been over 12 weeks with no more fault lights. From this I have to assume the insulation quality of the old transaxle fluid was breaking down and allowing a small hi-voltage leakage to ground. (At least I hope that was the answer). It certainly is good to know the Toyota dealers are the "experts" on these cars.
I hope he gets that fluid tested. I'd be curious to see how much the fluid actually broke down to allow current to flow from the generator to chassis.
Unfortunately, I didn't send the old fluid out for analysis and of course when I changed it, I didn't know that would be the solution. I'm still not positive. Only time will tell. I did look carefully at the old fluid but it just looked like used transmission fluid to me. I didn't find any metal bits on the drain plug or see anything as I poured it slowly into another container for disposal. I still have the old fluid in the garage as I haven't been to the re-cycling center since the change. But, I'm not sure if I put it in a clean container or mixed it with old oil. I will have a look and see if I can isolate it. I drive between 400 and 500 miles per week so I've gone between 5000 and 6000 miles since the fluid change.
If you've driven >5K miles without warning lights then it seems reasonable to declare victory. Congratulations to you for figuring that out. If you have the old fluid available as well as a digital ohmmeter, maybe you might measure the resistance of the fluid. Just dip the two ohmmeter probe tips into the fluid, one inch apart, and see what you measure. Of course, this is a low voltage resistance check but if you measure any resistance lower than infinity it would be interesting. (If 500V was available for testing the results might be quite different.) If you don't want to insert the probes into the ATF then maybe you could use a couple of pieces of wire attached to the probes.
Curiosity's got me now. I have access to a megger. If I set it to 500V, what should be my resistance reading for the transaxle fluid?
I'm shocked your trans axle is still good. 105K miles is just an insane amount of mileage to keep the original fluid. I thought I was pushing it at 60K. At 60K the original was burnt and brown. I'll never go more than 30K again without changing it. Fortunately it's way easier than a regular automatic transmission. Only takes about 10 minutes to do a full flush. To do that on a regular auto trans it requires an exchange pump which cycles all the old fluid out as a regular drain and fill only gets a small fraction. Why don't you find someone with a multimeter if you don't have one yourself and measure the resistance at say an inch between the two probes on the old fluid vs brand new fluid. That would tell you how the conductivity has changed without sending it out to a lab.
I bought my car used and it had 120K miles on it. I changed the transaxle fluid for the first time at that mileage. I know other Prius owners that changed their transaxle fluid for the first time at higher mileages and they've had no problems.
I think it should be close to infinite. I suggest you measure resistance for virgin and used fluid and let us know what you find.
if the problem returns...the dreaded trouble codes P0AA6 526 and 613 info codes could be set. look for them on priuschat
