Code P1135, O2 sensor bank 1 sensor 1, which one is bank 1 is it in front or back of motor?

Hello,

The engine has 2-cylinder banks. The bank closest to the firewall is Bank 1 and the bank closest to the radiator is Bank 2.

Bank 1 sensor 1 is the upstream O2 sensor of the rear cylinder bank.

Hope this helps.

Cheers, Boris