System Too Lean (Bank 2)


The PCM has detected a lean exhaust condition for engine bank 2. The term “lean” is defined as insufficient fuel.

Code Set Parameters

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.

Common Causes

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.

Common Misdiagnosis

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.


  • Begin your diagnosis by visually inspecting all wiring and connectors
  • Look for shorted or burned wiring and replace circuitry and connectors as required
  • If the system wiring, connectors, and components appear to be in normal working order, connect the scanner to the diagnostic connector and record all stored trouble codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • After the codes are cleared, operate the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made. Don’t forget to check the intake manifold gaskets, throttle body gasket, and the EGR valve and gaskets for vacuum leaks
  • If no vacuum leaks are found, then obtaining a fuel pressure reading should be your next step
  • If fuel pressure is below the manufacturer’s specified amount, suspect a clogged fuel filter
  • If the fuel filter is free from debris, then check for proper fuel pump and fuel injector operation
  • If fuel pressure is within acceptable levels, then inspect the mass air flow for the presence of debris or other contaminants on the hot wire
  • You may clean the mass air flow sensor hot wire by carefully using a suitable cleaner (electronics cleaner or brake cleaner) and a cotton swab, then blowing it dry using compressed air
  • Be very careful when handling and cleaning the mass air flow and allow it to dry completely before reinstallation