The control unit detects an air/fuel mixture on bank 2 with too much air relative to fuel. The fuel trims reach their rich limit while trying to compensate.
Common causes
Intake or vacuum line air leak
Dirty mass air flow (MAF) sensor under-reading the airflow
Low fuel pressure or weak pump
Clogged fuel filter
Partially clogged bank 2 injectors
Exhaust leak upstream of the oxygen sensor
Symptoms
Rough, unstable idle
Hesitation and stumbling on acceleration
Possible misfires and loss of power
Higher fuel consumption
Check-engine light on
Severity
Moderate: the car is still drivable, but a prolonged lean mixture can raise combustion temperatures and damage the engine.
What to do
Read the fuel trims and compare them with bank 1 to isolate the issue on bank 2
Look for air leaks in the intake and vacuum hoses (including a smoke test)
Check fuel pressure, filter condition and bank 2 injector flow
Verify the MAF is clean and rule out an exhaust leak upstream of the sensor