Monitor fuel trims while in closed loop at idle, and on a test drive.
Remember, long and short-term fuel trims combined should be within plus or minus 10%.

This is our best indicator of a fuel-air ratio problem.

Remember, if fuel trims are high or low, it does not mean the exhaust is rich or lean.
If the feedback system (O2 sensors) is functioning properly, this means the exhaust would have been rich or lean had a fuel trim correction not been made.
Regardless, the cause of the fuel trim correction still needs to be determined and repaired.

Verify the front and rear O2 sensor operation.

This can be quickly verified by causing a rich & lean condition while monitoring that the O2 sensor responds.

Another method is to monitor LAMBDA using a gas analyzer in the tailpipe. LAMBDA should be very close to 1.00.

A LAMBDA reading higher than 1.0 indicates excess oxygen or a lean condition.

A LAMBDA reading less than 1.0 indicates a lack of oxygen or rich condition.

You can determine a faulty oxygen sensor if Lambda is not near 1.0, and the percentage of fuel trim correction roughly matches the LAMBDA difference.

For example,
if total fuel trim is 20% and LAMBDA measures near .80, this would indicate the oxygen sensor reported roughly 20% lean. The ECM added 20% fuel trim to compensate, yet the resulting exhaust is 20% rich.

Verify there are no exhaust leaks after the repair.

With the engine running, hold a rag against the tailpipe and listen for exhaust leaks.
This is generally best tested when the exhaust system is cold.
Check when spark plugs are due for replacement and recommend replacement if due.
If no records are available, remove a spark plug for inspection
.
Monitor Mode 6 data for any counted misfires. Perform further testing if found.