Engine Alarm on 996

[Michael 996]

Hi there, i hope that somebody could help me with this problem that i have with my car.
it is a 2001 996 carrea and my engine alarm light in the instrument cluster.
I took my car to Porsche garage and had a diagnostic . There was a failure on the oxygen sensors after catalyst.

I orders 2 new sensor es and installed them, 1 new on on each bank after the cataly. I removed the negative connection on the car battery after the job was done to reset the "brain" for some minutes and reconnected it .

I then started the car and everything seems nice, the cars idle was nice and the Engine alarm was gone. I was happy to eliminated the problem and took my car for a long ride and it felt good to drive it. It had it's power back.

Then the day after when i was going to start the car,,the same alarm came up again,,,so my question is what could be wrong again,,?????

Anybody out there who had the same problem ?

Regards

Michael 996