Possible Causes

WARNING: This page is about a different car, the 2003 Toyota Camry. However, it is still accessible from the selected car via links, so may be relevant.

Possible causes are:

If the following conditions: after engine is warmed up, engine speed is 1400 RPM or more when driving vehicle and vehicle speed is 38 mph (60 km/h) or more, when the A/F sensor output voltage change value is below the regular value against the fuel trim change value, the ECM judges that A/F sensor circuit has slow response (2 trip logic).
Open or short in A/F sensor circuit.
Defective A/F sensor.
Defective A/F sensor heater.
Defective EFI. ECU relay.
Defective A/F sensor and EFI. ECU relay circuit.
Defective air induction system.
Defective fuel pressure.
Defective injector.
Defective ECM.