Help diagnosing ODB-II Codes

New poster who is seeking the wisdom of the folks on this forum. My friend purchased a 2005 2nd generation Prius in June of 2014. She really likes the car...especially the gas mileage. With that said she recently encountered a problem with it.

First the history...it's a 2005 generation 2 purchase in June of 2014 with approximately 160K miles. Everything was in great operational condition when purchased. In November of 2014 the dashboard lit up like a Christmas tree and a friend of hers, who is a Toyota mechanic, diagnosed the problem as a failure in the HV battery. He wanted to replace it at a cost of $2,500 but that was too much for her. A little research and we discovered one can replaced the individual modules. We purchased a module from E-Bay and proceeded to replace the failed module (I have some electronics experience and was able to identify the module with a traditional multimeter). After the module replacement everything was back to normal.

Fast forward a couple of months and the car wouldn't start. Long story short the 12V battery was almost completely discharged (she thinks she left some lights on). We pulled it, took it to the auto parts store where they tested and charged it. It appeared to be the original but it was holding a charge so we opted to re-install it. Again everything was back to normal.

Fast forward to two weeks ago. She informed me the vehicle was experiencing the same dashboard lights as she observed in November 2014. However this time the car would not exceed 30 MPH. We suspected another module had fallen outside of specification and the car was operating in a degraded, protection type of mode. So we ordered a couple of additional modules from E-Bay. Over the weekend we pulled the HV battery, located the module, and replaced it. Put it all back together and the dashboard lights remained. No problem we thought...we just need to drive it so it can reset everything. We took it for a drive and it still refused to exceed 30 MPH. However when we stopped at a stop sign the vehicle refused to move when traffic cleared. We ended up having it towed back to her place (just a 1/2 block). Thinking we must have assembled something incorrectly we pulled it all apart and checked everything. Finding nothing incorrectly assembled I started investigating other possibilities.

First on the list was the 12V battery. Measured it and unloaded it was approximately 12.2V. I thought this a little low so I pulled it and we took it to the auto parts store for testing. The determined it was bad and so we purchased a new one (from them). Knowing a weak 12V battery can cause all kinds of problems I was certain this was going to solve our problem. Installed it and...same conditions. Nothing changed.

It was at this point we obtained an OBD-II reader and found the following codes:

P0AA4 - Hybrid Battery Negative Contractor Circuit Stuck Closed
U0100 - Lost Communication with ECM/PCM "A"

I was told the U0100 code was typical and nothing to worry about. Doing some research I came across the following thread here on this forum:

"My 05 prius has the P0AA4 and P0AA1 Codes, what can be the problem"

I pulled the HV battery again, pulled out the two relays (those which connect to the orange HV feedlines), measure the resistance of the relay solenoids which read approximately 35 ohms and also measure the resistance across the HV terminals which, as expected, measured infinite resistance. I didn't have a 12V power source to try and engage the relays so, as done in the above thread, I shook them. Reinstalled, put it together, and tested the vehicle. Almost the same problem however after a few cycles of powering on the vehicle the gasoline engine, for the first time, started. Progress! We took it for a test drive and all appeared to be working. After a couple more cycles the dashboard lights went off. Ah, problem solved! Given it was late we decided to reassemble the car the following day.

The next day we started off by first testing the car. Unfortunately the warning lights had come back on. However everything else seemed to work fine. So I took it for a test drive and all appeared to be in order. We decided to go to the auto parts store and have the codes read / cleared. They were the same code. Thinking they were stale code I had them cleared. Took it for another test drive and all appeared to be in order. We decided to let it sit for a little while (it seemed the codes only reappeared after it had been sitting awhile) and we proceeded to perform a detailing of the car. To include pulling the seats, center console, and trim panels to clean it up really nice. During that cleaning, which took a few hours, I powered on the car to see if any warning lights appeared. To my pleasure none did. Great, problem solved. Put it all back together and took it for a 30 mile test drive. No lights, no problems, life is good!

Fast forward to yesterday and the warning lights popped on again. Car still operates as expected. No discernible operational problems. Save for the warning lights. Took it back to the auto parts store to read the codes:

B1653 - Seat Position Airbag Sensor Circuit Malfunction
B1655 - Driver Side Seat Belt Buckle Switch Circuit Malfunction
B1821 - Open in Front Driver Side - Side Squib Circuit
B1826 - Open in Front Passenger Side - Side Squib Circuit
C1259 - HV System Regenerative Malfunction
C1310 - Malfunction in HV system
U0100 - Lost Communication with ECM/PCM "A"

I suspect the B codes were the result of my having powered on the car while the seats were removed during the clearning we performed. I'm not concerned about them. It's the C and U codes which are of interest. Does anyone have any suggestions of what could be the possible cause. As an FYI I was never really happy with the relay shake "repair" so we ended up ordering a used relay off of E-Bay. I also decided to order a replacement Hybrid Battery Control Module. Figured I may as well have it available if I go into the HV battery again.

If you've read this far I appreciate it. One thing I find frustrating on forums is when people say "Help, my car broke...what's wrong with it?" and no additional information is provided. I hope I've provided enough to give you an idea of what we've done and where we're going. Any help is appreciated. I've also uploaded a couple of pictures of the warning lights we're experiencing. The triangle is the standard warning alert. I believe the car icon, which appears on the MFD, is the indicator as to what the problem may be (it's in the owners manual but I forgot what it was...though I think it had something to do with the HV system...which matches the codes). We're not worried about the maintenance light (she needs to get the oil changed).

 

Thanks for the information Patrick. I am suspecting the ECU or the connections to it. Where is the ECU located? I'd like to check the connections.

 

I believe I've got the software however I need the cable. A search shows I can buy the software / cable together but estimated delivery is near the end of the month / mid-next month. I suspect because it's coming from overseas. Where can I obtain a cable locally which won't take a month and a half to arrive?

 

I think I found the supplier. Ordered it earlier today for delivery on Friday. This is definitely going to be a much more useful tool than a generic OBD-II reader. Plus she'll have it for the future.

 

I think I'm just going to use XP. The software I downloaded has a video which shows me what to do and I'll just take the path of least resistance (I need to focus on fixing the car...might build it into a VM for the future).

 

Wanted to provide an update on this situation. Some more history: On Tuesday my friend ended up having the car stop running on her while she was out running errands. I went over to pick her up and, on the way, picked up an OBD-II reader from the auto parts store. Reading the codes we only had U0100...the other codes listed above were not present (as expected). I used the OBD reader to clear the codes and, for some reason, the car started up and ran fine. We immediately took it home and that's when I ordered the replacement relay, HV battery monitor module, and Mini VCI. I loaned her my car and the Prius sat in the garage...

The parts arrived on Thursday but we decided to hold off on any repairs until we had a chance to read the codes with the Mini VCI (which was expected to arrive the next day). On Friday my friends Toyota mechanic stopped by to read the codes. To our surprise not a single code was found (FYI...the check engine light was not on nor were any other warning lights...however we thought some codes might be stored). She took the car for a quick ride and all worked well. They checked the codes again and none were present. So they took if for a longer drive. Still no problems and no codes.

On Saturday she brought the car over to my place and we ran some errands. No problems all day. We then checked for codes and none were found. Today she drove the car and no problems were experienced. The car is driving as expected with no issues. We're puzzled as to why...we didn't do anything Tuesday afternoon but clear the codes with a generic OBD reader. She's pleased as the car is working fine but a little nervous as to the reason why. At this time we cannot perform a diagnosis on a car that is operating as expected. I'll check back in should we encounter a problem or after a few weeks if not (just to let people know).

 

We wouldn't be surprised if the generic OBD-II reader cleared all the codes, even the manufacturer specific ones. However what is puzzling is after clearing the codes the car started up and has run fine ever since. As of the date of this posting the car is running fine and there are no codes (as read with the Mini-VCI). We're puzzled as we didn't replace anything, didn't change anything, didn't do anything except clear the codes. I don't believe clearing the codes resulted in a working car (wouldn't that be great if that's all it took?) however whatever the cause "fixed" itself and we don't know what. It's been almost a week and things are looking positive. If something changes I'll update this thread. Otherwise I'm going to consider this issue resolved.

To anyone who may stumble across this thread in the future. The only thing we did, after replacing the failing module, was remove the two relays feeding the two orange high voltage wires and shake them as I mentioned in the original post. Save for a couple of code clearings with a generic OBD-II reader we literally did nothing else (we haven't installed the new parts we ordered as there isn't a reason to).

 

I wouldn't be surprised if it does recur. I don't like things which "fix" themselves. If it does we hope we have the right tools to help diagnose the problem...thanks to those here on this forum.