So I got these error lights with a jerk while accelerating. I ignored them, and removed the codes for the first time. Then, they reappeared, to my horror. I didn't know much about the hybrids, except that I learnt how to check battery health and bought a bunch of scanners before buying my car, which is, a self imported car, with genuine mileage (now at 65k, imported at 16k kms). I knew I was on my own if it gave any issues, but went to a guy someone recommended, who works only on hybrids. He looked at the error code P0A78, drive motor A inverter performance, told me to change the inverter, that he would only charge if the change helped. Well, it didn't. The issue was still there. He put my inverter back in the car and removed his and then said, it must be the transmission. I didn't believe him, and he didn't believe me when I told him to check all the connections first. I was told this is such a high voltage system that it won't work at all if there was some faulty connection. Well, I have a degree in Electrical Engineering, but I kept quiet and came back. Then started my journey towards troubleshooting.
I found out that the normal scanners will only tell part of the diagnostic codes. There are some other info codes, 3 digits, which pinpoint the issue exactly. These codes can only be read using Toyota techstream. Well, luckily, I had ordered one of their Chinese copies too. Checked it, after multiple tries, and found the information code, 506. Looking up the docs Toyota provides, I found that it is related to some kind of short circuit, or high current in the inverter. I pulled out the freeze frame data, and found that yes, the inverter was working overtime boosting voltage, and when it reaches a threshold, the ecu shut it down, causing the jerks. But the ecu was reading the error code as something wrong with the drive motor A, which is the MG1, to which this voltage was to be provided. Hence the conclusion came to be, that the MG1 is not getting sufficient power, which causes the inverter to produce high voltage, and get shutdown by the ecu, causing jerks and the ecu deciding that the MG1 performance is not up to par. I looked at the procedure for fixing the issue, and found out that the first step is to....... Well, check the wiring connections. Now, I knew the connections to MG1 must be the problem area, because that is what is giving the error. MG1 is in the transmission, so went to an electrician, and told him to check all the grips connected to the transmission, and voila, this little connection over there, a grip was loose, not properly locked (the one u can see here in the pics). I fixed that, and took the car for a spin. Tested it a lot, and no issues at all.
Moral of the story is, if you get a hybrid system malfunction error, get the car scanned with Toyota techstream, get the error code and info codes and diagnose it properly. If you go to run of the mill mechanics, they will make you spend hundreds of thousands without actually fixing the issue. And no, not all hybrid system malfunction errors are related to the traction battery. Don't misguide people, let them diagnose the issue first, and then tell them how to fix it.
