O2 open circuit should be code 13 - is that the one you're getting?
According to the service manual:
Code 13 = Engine has been running for at least 2 minutes, throttle position is above 6%, and the O2 sensor voltage has stayed between 0.35-0.55v for more than one minute. In other words, the signal voltage is not swinging as it should. This suggests an open circuit, because the unmodified voltage provided by the ECM to the sensor is 0.45v vs the ground reference. (I don't know where this ground wire is located, sorry.)
It might be helpful to short the A-B pins on the ALDL connector, just as you would do when setting ignition timing. Do this while the car is turned all the way off. If you then start it, the check engine light will blink according to the O2 signal. If I'm not mistaken, it will blink rapidly when in open loop, then the blinking will slow down when it enters closed loop. At that point, the light will be on when it's reading high voltage (rich) and off when it's reading low voltage (lean). I don't know what the light would do at midrange though.
To test the sensor, the manual suggests doing the above, then disconnecting the O2 sensor and grounding it's connector to simulate a lean condition. Then start the car and watch whether the check engine light is reacting to the lean signal.
This is how they describe what to expect:
|1986 manual pg 6E2-A-30|
This step simulates a lean exhaust. If the ECM and wiring are OK the ECM will see the lean condition and turn the "SERVICE ENGINE SOON" light off for at least 30 seconds after engine start, and then flash "open loop". It should be considered normal if the light remains off for a longer period of time before flashing open loop.
If this check passes, then the sensor and it's connector are suspect.
Since your problem is intermittent, I'm not sure how well you can test it. I'd be inclined to blindly replace the O2 sensor - they are cheap for these cars and they degrade gradually with age, so it might do some good to get a new one regardless.
[This message has been edited by armos (edited 12-31-2014).]