Open Loop to Closed Loop

'95 Grand AM 4cyl has a problem at start up from cold. After two to three minutes the idle will stumble and then go high several times. Some times it
will die some times it will recover. It will run fine until the next cold start. This is worse in colder whether. According to the scan tool it happens just after the computer goes from open loop to closed loop.
I am thinking that this happens too soon for the O2 sensor to be properly heated.
Does any one know what tells the computer to switch to closed loop?
What should the voltage be for a cold O2 sensor?
Thanks, Scott
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I think its controlled by time.
When I had a faulty 02 sensor, I was able to reach the garage by stopping when the problem starts, disconnecting the battery cable for a while and then reconnecting battery back. the computer would go to default settings for a while enough for me to travel some distance before the engine stops again.
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