Starting up an hybrid (when the 12V auxiliary battery is depleted) is a well documented procedure in the NX300h user manual. If done correctly, it shouldn't cause any harm to your car electronics.

Nevertheless, in my specific case, the 12V battery wasn't down completely, there was enough energy left to activate door opening and to display a couple of lights on the dashboard after pushing the POWER button. No action was able to deactivate the car completely! In this intermediate status, "jump-starting" started the engine, … but subsequently, the ECU (Engine Control Unit) was dead. Are there any recommendations on what should have been done differently?

 

"Can a jump start damage your car? The simple answer is yes!" concludes this excellent article: http://ricksfreeautorepairadvice.com...-damage-a-car/

The danger seems to be higher if the receiver battery is low on voltage...

My car was started with a professional jumper pack connected directly on the jump-start terminal under the hood of the NX300h; charging the disconnected 12V battery in the rear of the car could have saved my ECU (repair > 5500 US$) ...

 

Most common mistake is not hybrid specific, incorrect polarity of the jump cables will cause harm to computers in any car.

 

I agree, but this mistake wasn't done by the mechanic called in by Lexus Assistance 24h, at least as far as I could see ...

 

Did they jump off 12V in the trunk or the way it should, off + bus in the fuse box and any good engine ground?

 

Just to add something. 12V in hybrids is a non cranking battery. All you need is proper 12V supply for all the computers and relays to kick in ICE with hybrid battery. If they habitually floored donor engine and sent high power jolt down the system.....

 

The jump-start was done by connecting the jumper pack to the "+ bus" in the fuse box and to a bolt on the combustion engine (ground). Subsequently, the Power Button immediately started the ICE (Internal Combustion Engine).

All was fine, I was starting the trip as recommended: the only systems which didn't work were:

• the ACC (Adaptive Cruise Control) --> message displayed: "Go to your dealer"
• the AWD (All Wheel Drive) --> message displayed: "Go to your dealer"

I went to my dealer, who connected the OBD2 scanner, and concluded on a "terminal disease of the ECU", as the scanner "couldn't detect the ECU, so its 12V power supply must be interrupted".

My BURNING QUESTION is: with a dead ECU, will you still be able to perfectly drive your car for about 40 miles (as I did), or isn't the ECU as dead as I was told? (cost of repair: 5500 US$...).

 

No. You can NOT drive a vehicle with dead ECU as it's the "brain" of power train.
Toyota warns that after say battery replacement, an array of warning signs may be displayed in the dash. Toyota recommends, for such situation, to do 3 consecutive complete system start up/shut down procedures, and it should clear the warnings.

 

Hey, if I were you, I'd have located every fuse in the fuse box that has words like "ECU", "ECM", "control module" and pull those fuses out for about 20-30 min. You have several ECUs. Maybe that willreset them.

 

Thanks ukrkoz, your recommendations are extremely valuable for anyone running into similar issues. Fortunately, I found very, very few stories of presumed ECU deaths on this forum for NEW Lexus cars (one in Canada / RX 4th gen., and my own). This demonstrates how reliable Lexus cars are inherently, but it also raises serious questions on how Lexus repair shops are trained to handle ECU issues. If I summarize:

• the Lexus mechanic jump-starting my car was right in saying that subsequent displayed error messages were kind of "normal",
• he was not right in saying that they would disappear by themselves after driving for about half an hour,
• if the ECU would have been dead, I wouldn't have been able to drive for about 40 miles, the engine (power, torque, etc.) behaving absolutely normal,
• my repair shop didn't follow the Toyota/Lexus procedure on correctly resetting the system,
• my repair shop prematurely (and most likely wrongly) concluded on an ECU terminal illness,
• the stated 5000 Euros repair cost estimate was either a self-protection to cover any unforeseen issues, or an "incentive for me" to buy a new car.
• As "real ECU failures" are so uncommon on new Lexus cars, I'm confused on why Lexus doesn't replace them on a goodwill basis. I assume they are so convinced about their reliability, that if they fail, there must be an external cause.

I will confront my repair shop & dealer with these findings. As in the meantime, I traded in my old car against a new NX300h, I might never know the truth ("which is out there"). If I were two weeks younger, I would have followed your recommendations to the letter.

 

Epilogue: Apparently the culprit was the EBCM (Electronic Brake Control Module), not the ECU (Engine Control Unit)… At least this explains why I was perfectly able to drive with all the error lights on. (The repair shop calls "ECU" whatever module has built in electronics hardware & software).

The supervisor showed me the failed system having caused the trouble: he replaced the complete "ABS actuator" (ABS Antilock Brake Master Cylinder Pump with its embarked Electronic Brake Control Module). See picture below (photo not the actual one, but similar one on ebay). It seems the electronics module (box on the top), not the mechanical system, was flawed, but Lexus only replaces both in one set (explaining the $$$$), at a total cost of about 10% of a new NX300h !

As the failure occurred just after jump-starting the car (12V battery depleted), the repair shop claims voltage spikes (transients) as the most likely cause of this unit's death. So, whatever the reason, think twice before deciding on any jump-start.