RX 450h AWD 2010 Check hybrid system error (code P0A7A-122)

[Peter Popov]

This is a bit of a long story, but I'll try to make it as short as possible.

I started getting this P0A7A-122 code - generator inverter fail signal detection (overcurrent due to system malfunction) - about a year ago. Took it to an authorized service and was told to replace the HV inverter-converter and if that doesn't work, replace the transaxle assembly next. The parts alone cost more than I paid for the car (got it used in 2020) so that was definitely a no-no. I've been living with this error with no issues for over a year with no issues, and it would disappear after a while (from the dashboard, not from memory.) Then one day, I got it again, and then "Check AWD" and "Check braking system" followed in a matter of seconds. The vehicle then throttled down to about 10-20 kmph - just enough to pull over. I turned it off and on again and it lit up, but that's when I decided to take her to an independent mechanic. They took apart all the power cabling, cleaned up all contacts and applied contact spray, checked both MGs for shorts, and checked the inverter triacs (are they triacs?) It didn't help. Since then I've replaced the HV inverter-converter and coolant, the HV control module (ECM/ECU), the rear motor (thinking it was the only connection between the HV, AWD and braking systems,) and the 12V Battery for good measure as the old one was acting up. Neither helped.

The last few times that it happened, I finally noticed a pattern. The car would stall for a split second, not hard enough to engage the seatbelt but definitely causing me to noticeably lurch forward, and then the error would immediately follow. The last time it happened, I managed to pull the freeze frame data from it (see attached Excel file.) I've highlighted a few things in the freeze frame data that I hope can, collectively, give some pointers:

1. I was driving at 25 kmph and trying to accelerate slightly when the engine RPM dropped from 1280 to 32 within a second (two 0.5s freeze frames) as I was lightly depressing on the accelerator. This was the stall I felt.
2. Manifold Air Pressure dropped from 67 to 18 in the same period. It is significantly different from atmospheric pressure. Are either of these indications expected?
3. Engine rev (real and target) drop to zero right before the error occurs, which makes sense (I think) given the stall, but I don't know if the difference between the two values is indicative of anything.
4. Power resource IB is all over the place, going from -18V to +12V in just a second.
5. MG1 carrier frequency drops to 3.5 kHz for a single frame, as the car stalled right before the error code is triggered. Not sure if this is significant in any way, or is it normal for lower RPM?
6. Calculate Load, Throttle Position, and Generate Torque all trend down throughout the freeze frame. Is this expected given the car "stalled"?

Having ruled out the inverter-converter and ECM/ECU, I find myself obsessing over the MAP value. I don't know much about cars so this could be complete nonsense, but I'm thinking - if there's an issue with airflow or the sensor causing the air-fuel mixture to momentarily and unexpectedly stall the ICE, wouldn't that result in MG2 suddenly turning all the excess braking torque in an abnormally high current that the ECM/ECU doesn't expect to see? At this point I'm shooting in the dark, so I did a test to see how the MAP sensor behaves with respect to RPM (also attached.) I did this with the vehicle stationary, the shift stick in the D position and the brake fully depressed while I was slowly revving up with the gas pedal.

Is it normal for manifold pressure to drop as the RPM increases? Also, is it normal for air flow (MAF) to be at almost zero while revving up, only kicking in for a few seconds? Note this was done on a stationary vehicle. Should I be suspicious of the MAP/MAF sensors and/or the actual intake?

Any ideas? I'm grasping for straws here, so any help will be much appreciated!

Thanks,

Peter

Freeze frame data.xlsx MAP Idle Test 1.pdf MAP Idle Test 2.pdf

[steve2006]

Hi Peter and welcome to the LOC.

”I don’t know much about cars” seems to be an understatement given your knowledge of diagnostics and live data 😀

From what I remember from manifold pressures the wider throttle opening will reduce the air pressure, the Idle Air Control Valve controls the idle speed and bypasses the Throttle Body

[Spock66]

I am no expert, but possibly a vacuum leak on the intake somewhere?

[Peter Popov]

21 hours ago, Spock66 said:

I am no expert, but possibly a vacuum leak on the intake somewhere?

Sounds plausible, though it would have to be very intermittent and still somehow cause a complete engine stall down to 0 rpm. Not sure where to look for it under the circumstances.

The service diagnostics manual recommends replacing the HV inverter-converter (which I did) followed by the transaxle assembly, but I don't see any indications of this being a transaxle issue. If it were a belt slip, for example, I could see how it could result in a transient current spike coming from MG2, but an engine stall, how could that come from the transaxle? I'm not about to dish out 600 EUR or so on a hunch, especially given that all certified Lexus service hsops I've gone to have proven to be nothing but useless.

21 hours ago, steve2006 said:

”I don’t know much about cars” seems to be an understatement given your knowledge of diagnostics and live data 😀

I truly don't, but I am an engineer which means I know how to use Google 🙂 Now I must learn about idle air control valves and throttle bodies, whatever those are. Off to Google again... and yes, here it is: Working Principle Of A Car Air Intake System (carfromjapan.com) So, I'm guessing what's happening is that while idling, the idle valve is open, then as I rev up (in D, with the brake depressed) the throttle body opens to allow enough air. What I don't still understand is why MAF drops as I rev up even higher - is that MG1 kicking in to help the ICE to manage the load demand?

Either way, I'm still at a loss. Perhaps it's worth looking at the air intake indeed. The filter is relatively new, less than 1000km since the last change, so - ducts?

[Peter Popov]

I've been thinking about this a lot and it dawned on me - is it even possible for a 3500cc engine to go from 1300 RPM down to 0 in just a second? That's a lot of angular momentum to lose in a split second. Given that the car does physically slow down, could it indeed be the transaxle? How would I even diagnose that?