Note On Intermittents:
WARNING: This page is about a different car, the 1986 Oldsmobile Toronado and 1986 Buick Riviera. However, it is still accessible from the selected car via links, so may be relevant.
If an intermittent Code EO13 is being set, manipulate the related wiring while observing engine data parameter ED07 at part throttle with the engine warm. System should be in "closed loop". If the failure is induced, the oxygen sensor reading will change from its normal fluctuating voltage (above .60 and below .30) to a fixed value around .45 volts. This will help to isolate the location of the malfunction.