P0A94, 553 inf (Inverter?)

Hello everyone-
Tomorrow I will be working on diagnosing my Prius with a P0A94 code, 553 inf sub code.
"Boost converter fail (FCV) signal detection (boost converter overheating)"

07 Prius, 367,000 miles

I would love any insight from anyone who's dealt with it- (can't find much on that specific code and subcode).

The code has popped up twice- first on a road trip when varying speed getting ready to pass on a highway. I cleared the code and continued with the rest of the roadtrip with no issues.

2nd time was driving around town, during a quick change in speed.

I also have a P1121 code for the coolant control valve- but my understanding is that is completely unrelated to the inverter cooling. This code has been on for quite a while... I've been procrastinating repairing it.

Following the diagnostic chart, I have done the following so far:
- I checked the inverter pump electrical plug, and the retainer is cracked on it- which means it comes off pretty easy. The coolant was bubbling in the reservoir, so I assume it is functioning correctly. There's a small possibility that the plug connection was loose, ie partially removed. But that seems like an unlikely scenario since the issue seems tied to acceleration rather than a bump or something.
-Cooling fans are operating, connectors appear to be in good shape.

-I haven't checked for a clog, per se, but I did replace the inverter coolant when I got the car under 2 years ago. Everything looks super clean.

-Next steps are to check hybrid vehicle control ECU wiring and inverter wiring plugs. I don't anticipate finding any issues with those.

-The final step is "Confirm information- exclusive info 4"- which apparently is done by removing the HV service plug and the inverter cover then clicking on the 553 inf code in techstream. It outputs some numbers which should tell you what part to replace or what diagnostic tree to follow from there. Parts to replace are ECU, Inverter, or Transaxle. So I'm keeping my fingers crossed its one of the first two!!


I'd love any help you can offer! If not, I'll at least document my troubleshooting so anyone who comes after me has a resource.

 

Yes, I have techstream and mini vci. That's how I pulled the inf code. I'll see if I can post the freeze frame data later. Would love some help interpreting that.

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Here is the freeze frame data.

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Ok, I misread the instructions for "exclusive info 4". It has nothing to do with the service plug and inverter cover, there was just a warning in the instructions that if you turned on the car without reconnecting the plug and closing the inverter cover that a DTC code would be set.

Now I'm not sure how to access exclusive info 4. Nothing appears in the freeze frame data I saved when I click on the 553 sub code. Maybe that portion isn't saved? I reset the code so I guess I may just need to wait til the DTC appears again to do further diagnosis? Anyone have any information on "Exclusive Info 4"? Attached is the page from the manual.

 

Ok, I made some progress.
I completed the troubleshooting up to the point where "Exclusive information 4" is to be read and confirmed.
The "Exclusive Info 4" is accessed through the dropdown tab at the bottom that says "all data" by default (See the screenshot in previous post). I clicked on that, then clicked on "information 1" then the following list comes up (see attached screenshot). Under the heading "Exclusive Information 4", the value of -127 is indicated.

The chart in the diagnostic tree indicates that a value of -127 means to continue down the tree, with "Inspect Hybrid Vehicle Motor Assembly".

I'll be working on that now.

 

Ok I have an update. I walked through some more steps in the diagnostic tree. The first part involved measuring resistances between the AC cables to the generator and motor. I was unable to perform the first part of the test measuring the resistance between the cables themselves, because I don't quite have a way to measure milliohms. I started working on a DIY way to do that, but decided to move on with further testing instead of wasting time on that right now.

So I moved on and performed the other part of the test, which measures from the generator and motor cables to ground, because those specifications are in mega ohms. It passed no problem.

I then moved on to the active tests, the final two steps of the diagnostic tree. This involves back probing the multi-pin connector inside of the inverter and checking voltages between six pins. four of the pins checked out just fine, registering about 14 volts. However, two of the pins read just about 0 volts. And they appear to be related pins, as they have similar nomenclature (see attached picture).

I did double check my back probing and a pin numbering to make sure this was a real result, and not just a mistake on my part.

I thought that if this was not good, the indication was to replace the inverter, but I just realized that It's actually the opposite. If these are not good, you continue with the diagnostic tree. So here we go



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Ok, I just finished replacing the inverter. Got the coolant bled. Fingers crossed the issue is resolved. I'll update of the issue recurs.

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Well. I made a mistake. I'm quite frustrated. I looked again at the I10 plug I tested to confirm the inverter needed to be replaced. I discovered that on the far side of the connector (towards the firewall) is one unused pin- no wire in it. From my vantage point at the front of the car, I could not see that extra pin. So when I was counting pin numbers in order to do my testing, I only counted wires, and did not count the first blank pin. Therefore my pin numbers were offset by 1, and that almost certainly explains why my tests came back the way they did (fail).

Luckily, due to the nature of the diagnostic tree, in step 1, the indication was to replace the inverter if the voltages tested CORRECTLY with I10 connected. (My tests most likely incorrectly indicated that the voltages were bad during the first test, and therefore to proceed to the next step). The next step is then to unplug I10 and retest- If voltages in this test are BAD (which mine most likely incorrectly were), then the inverter is also to be replaced). Confusing, I know. Basically, once the Motor/Generator tests are completed, the next step was to replace the inverter as long as it was getting voltage.

So in reality, replacing the inverter was probably a good roll of the dice for $150. The other possibility is that the Hybrid Vehicle Motor Assembly needs to be replaced... which I imagine is a more expensive and more labor intensive job.


If the P0A94 recurs, I will go back and do the milliohm test for the motor and generator assemblies that were prescribed prior to the test on I10 I did. I just learned through this process that a milliohm meter is a much more specialized and sensitive piece of equipment than a standard multimeter. Apparently there is a way to measure milliohms indirectly by creating a simple voltage divider circuit with the component with unknown resistance included in one side of the circuit, and a resistor of a known resistance on the other side of the circuit. You then run 5 volts through the divider and measure the voltage output of each side of the split and then do some simple calculations to find the resistance of the unknown component. That will be my plan if I get the red triangle of death again.

The very end of the diagnostic tree is to replace the hybrid vehicle control ECU if EVERYTHING else tests good, which would be a very simple and cheap fix.

So I think I've resolved most of my frustration. While I'm disappointed that I made that mistake, I would have come to the same conclusion either way since I skipped half of the motor/generator test. I probably would have built the voltage divider circuit yesterday and done the motor/generator testing if I had thought it through and realized my mistake. But with the work being done now, I'm OK with a 50/50 gamble on a new inverter for $150.

Worst case is the code returns and I have to replace the hybrid motor... which will make the inverter replacement seem like a drop in the bucket anyways!!

 

I just wanted to update this thread. Now, a couple months later, I've had no further issues with the inverter. The problem seems to be resolved. Only problem I may still run into is if the issue was heat related- in that case, the problem may reappear when the weather heats back up, but hopefully not!

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Update 2: I filmed a video of the process. I hope this can help out anyone else working through this process. It was a challenge to figure out with very little user-created documentation on this specific code.

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I opened up the failed inverter to prep it for scrap and found some... interesting stuff in the cooling fins. Looks like some sort of mineral/corrosion growth. I filmed a little video about it.