P1151 Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 2.
Means the front bank appears to be lean.
O2 sensors read lean when they get old, this causes that bank to run rich. If you have 60,000 miles or more, you might just want to replace the upstream sensors. You could also pull the plugs and compare front and back. If the front plugs have more deposits on them the front O2 is probably bad.
Otherwise, something is actually causing the front bank to be lean. This is often a vacuum leak at the front of the intake manifold.
