HELP! Nissan Altima Hybrid 2007, engine stalling at high spe

[HansZ]

Hello,

I'm battling the following issue for over a week: vehicle is 07' NAH, high mileage on it. Car functions normally, however at speeds over 80km/h engine shuts off and car switches to hybrid mode! Following codes come up:
P0AC4
P0A94

I went twice to local Nissan dealership. Immediately I was told by service rep that they don't see to much of NAH... I was told my catalytics are gone and I need to replace them both, as back pressure is causing engine to stall... Bill was over $1200, so I've decided not to jump on in right away and to try some troubleshooting on my own.

I Have removed both manifold cat. And second one located on exhaust. We have welded some temporary pipe, and after road test, same thing happened, vehicle stalled on motorway... I went back to Nissan the second time, this time I was charged for 2.5 hours of diagnostics as tech. Line had to be contacted, only to their conclusion that again catalytics have to be replaced.
I was told unless I follow tech line recommendations they won't be able to help me any further. Does anyone had any clue what might be wrong here? Should I good ahead and spent moneyn on cat's or no?

Thanks for any help!

[RicerX]

Welcome to NICO!

The first code (AC4) is hybrid speak for the electic motor requesting the ECU to fire a MIL (malfunction indicator light).

The second code (A94) is a DC to DC converter failure.

In my (very basic, not very familiar) opinion, there is something electrical causing this. I would almost say the battery needs replacement. I highly doubt the catalytic converter is the cause of it. My bet is something electrical, maybe even a sensor. These are difficult to troubleshoot, however.