Some owners of a 2004-2013 Cadillac CTS or SRX may complain about a SES light on with no drivability issues (gee, imagine a Caddy owner who complains). The technician may find DTC P2097 or P2099 set.
Perform a fuel injector balance or AFIT test prior to going further. If normal diagnostics does not lead to a resolution for this code, 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 using soapy water and compressed air. Apply air to the rear of the downstream converter or to the tailpipe on single systems at 5 psi to 8 psi. Apply a soapy solution to the gasket areas and the O2 boss. Look for large bubbles. “Foam leaks are considered normal.”
If a leak is found in these areas, along with gasket replacement, filing the mating surface of the pipe is suggested. If no leaks are found, at this point you should look for a mechanical valvetrain concern such as a sticking valve, or a lifter follower out of place.