Owners of 2004-2013 Cadillac CTS models may comment about an SES light on, with no drivability concerns. The technician may find DTC P2097 or P2099. The most common concerns related to this condition involve the fuel injectors.
Perform an injector balance or AFIT test prior to going further. If normal diagnostics does not lead to a resolution, look for a small leak in the exhaust, usually at the outlet to the catalytic converter. This leak will allow air to be pulled into the exhaust and cause a false lean concern at the post O2 sensor.
To diagnose the leak, GM recommends applying air to the rear of the downstream converter or to the tail pipe on single systems at about 5 psi to 8 psi. Apply a soapy solution to the gaskets and the O2 sensor boss. Leaks will cause the code, and will be visible as large bubbles. Do not apply full shop air, as too much pressure can damage the converter.
If no leaks are found, look for a mechanical valve train concern such as a sticking valve or a lifter follower off the camshaft out of place.