Possible Causes

The value of the signal measured by the engine control module is not plausible given the operating conditions
Exhaust system leakage
Post catalyst heated oxygen sensor incorrectly installed
Pre catalyst heated oxygen sensor circuit short circuit to ground, short circuit to power, open circuit, high resistance
Post catalyst heated oxygen sensor circuit short circuit to ground, short circuit to power, open circuit, high resistance
Pre catalyst heated oxygen sensor contamination or failure
Post catalyst heated oxygen sensor contamination or failure