Problem: The vehicle was brought to the shop because the MIL was on and the gas mileage was down. There were no noticeable drivability issues.
Details: The technician pulled two DTCs: P2196 (Lack of H02S-11 Switch, Sensor Indicates Rich) and P2198 (Lack of H02S-21 Switch, Sensor Indicates Rich). First he checked the fuel pressure – it was at specifications. The fuel pressure regulator was not leaking fuel into the vacuum line. The air filter was new and the oil was not overfilled. With the scan tool connected, he monitored the PIDs for the oxygen sensors. They respond to vacuum leaks, but stayed around 900 millivolts.
Next, he connected an injector tester and performed a flow test. He found several injectors that were flowing too much fuel.
Confirmed Repair: The technician replaced the full set of injectors, cleared the codes and test drove the vehicle. The oxygen sensor responded normally and the DTCs did not return. Vehicle fixed!