You can diagnose Oxygen Sensor problems by using the on-board diagnostics system built into the Buick Reatta. Instructions for accessing the onboard diagnostics system can be found here:
How To Access Trouble Codes - '88 & '89 Models
How To Access Trouble Codes - '90 & '91 Models

Reading the On-board Diagnostics:
When you enter on-board diagnostics, select the ECM service mode, There is a triangle along the bottom marked "OXY" that should be flashing. Once the engine has warmed up, and is operating in "Closed Loop", "OXY" should be flashing slow at idle and faster when driving speeds. On indicates a rich fuel mixture and off is lean.

Next select "ECM Data" from the on-board diagnostics screen.

The actual reading of the oxygen sensor is reflected in two ECM data points. ECM data point ED07 (sensor voltage) which should be constantly changing from something around .2 to around .7. The second data point, ED18 (cross counts), is the number of transitions from rich to lean per second. Cross counts of 4-6 at idle and greater than 10 when driving is normal. If the data point readings are out of range replace the oxygen sensor and run this test again.