What Does "System Too Lean" Mean?

"System too lean" means that an oxygen sensor in an automobile has detected a lean condition in the combustion process. It is a common diagnostic trouble code for automobiles. A lean condition means that too much oxygen is present in the automobile exhaust.

"System too lean" is a common trouble code for car computers. Lean conditions indicate too much air and too little fuel. Common causes for this condition include intake or vacuum leaks, a clogged or faulty mass air flow sensor, an improperly installed aftermarket cold intake, low fuel pressure or a clogged catalytic converter. A faulty oxygen sensor can also cause this troubleshooting code as a misdiagnosis.

Symptoms of a lean-running system include lack of power, rough idle, hissing noise from the engine, engine stalling or running hot.

In most cases, cleaning the mass air flow sensor and finding and fixing vacuum leaks solves this problem. Other solutions include inspecting all vacuum and positive crankcase ventilation hoses and replacing or repairing as required, checking for a dirty fuel filter and proper fuel pressure, and monitoring short- and long-term fuel trims using an advanced scan tool.

Combustion engines operate by burning an air/fuel mixture of about 14.7 parts air to 1 part fuel. If the air rises above 14.7 parts, the mixture is considered "lean."