The PCM has detected a lean exhaust condition for engine bank 2. The term “lean” is defined as insufficient fuel.
The upstream oxygen sensor measures oxygen/fuel levels leaving the combustion chambers, before they reach the catalytic converter. The PCM compares signals received from the upstream exhaust oxygen sensor, the throttle position sensor, the mass air flow sensor, and various other engine drivability sensors to calculate an acceptable degree of fuel and air found in spent engine exhaust gases. If this amount differs by greater than twenty-five percent from the manufacturer’s referenced value or if the sensor maintains an abnormal reading for an extended period of time (typically 20-seconds), a code will be stored and a malfunction indicator lamp will be illuminated.
Symptoms include poor acceleration, engine stall under acceleration or at idle, a no start condition, rough engine idle, an engine misfire, or a surge upon acceleration.
This code is typically caused by an engine vacuum leak of lack of sufficient fuel pressure. It is frequently accompanied by a P0171. Probable causes include a clogged fuel filter, faulty fuel pump, an EGR malfunction, or engine vacuum leak/s. Other causes include a fouled or dirty mass air flow sensor, the use of high-performance cold air intake systems that require an oil charged filter element, positive crankcase ventilation system components that are drawing excessive vacuum or leaking, a bad oxygen sensor, an exhaust leak between the combustion chamber and the oxygen sensor, leaking, sticking, or inoperative fuel injector/s.
Oxygen sensors are often replaced in error when this code is exhibited. Many vehicles are successfully repaired by replacing a fuel filter that is long overdue.