2001 Toyota 4Runner Engine Light, VSC Lights On

My wifes vehicle's (2001 Toyota 4Runner 4x4) Engine Light is on along with the VSC Lights (2). I had a diagnostic check performed and it indicated that the O2 Sensors were the reason. I replaced both the front and rear sensors with new ones, had the codes cleared and the engine and vsc lights went off. She drove the vehicle for 3 days with no engine or vsc lights on, thought the problem was fixed. However, after the 3rd day the same lights came back on?? Note: The code that was in the computer when I replaced the O2 sensors was
P1135. Do you have any suggestions as to what my next step should be?

Thanks in advance for your time!

Respectfully,
Steve