Some faults that trigger a CEL are immediate, most oxygen sensor codes are not - with the exception of a heater circuit fault. The CEL will go on and off depending on operating conditions. It will come on more often as the O2 continues to degrade.
Even though some buildup may occur on the O2 sensor, this is not what causes failure. Oxygen sensors do not respond to cleaning.
Oxygen sensors generate electricity in the presence of oxygen due to the ionic conductivity of zirconia electrolyte at high temperatures. Chemical degridation due to the heat and chemical reactions with contaminates require eventual replacement. Contaminates like tetraethyl lead and ethylene glycol antifreeze cause rapid deteriorization.
