GM: The light's on, but it runs fine
Owners of various GM vehicles may comment about an SES light with no engine performance concerns. Upon inspection, DTCs P0171 and/or P0174 will be found with a Tech 2.
This may be the result of unmetered air entering the PCV system by way of a valve rocker arm cover gasket or filler neck/tube leak.
Applicable vehicles include 2009 Buick Lacrosse and Allure, Cadillac Escalade, Chevy Corvette, Impala SS, Avalanche, Express, Silverado, Suburban, Tahoe and Trailblazer, GMC Envoy, Savana, Sierra and Yukon, Hummer H2 and H3, Pontiac G8 and Saab 9-7X, equipped with the 4.8, 5.3, 6.0 or 6.2L V8 engines.
If SI diagnosis does not isolate the cause of this concern, inspect the valve rocker arm covers for an obvious oil leak and/or loose oil fill neck/tube and repair as needed.