The O2 sensor is in closed loop while in startup and warmup, then it goes 
to open loop after the sensor is at operating temperature.

Erik Peterson
Vehicle Research Institute

On Fri, 6 Jun 1997, kenneth d lewis wrote:

> Malcom:
> I beg to differ. I have a dash mounted LED (O2 sensor) monitor. I have
> NEVER seen the system go into open loop once it reaches operating
> temperature. In fact the higher the RPMs the faster the lean/rich cycles
> occur.
> >From the outer banks of North Carolina to the Smokies.
> Ken Lewis
> 86 VW Transporter Three Door Pickup
> 
>  
> On Fri, 6 Jun 1997 02:32:51 -0500 mholser@Adobe.COM (Malcolm Holser)
> writes:
> >since the O2 sensor is usually not in use when driving up high (^;
> >You see, the system goes into an open-loop mode. bypassing the sensor 
> >at  something like 3000 rpm and above. Then engines make so little power
> 
> >at altitude that you usually downshift and keep the rev's high.  The 
> >airflow meter, which kinda measures air mass, kinda compensates.  I
> understand 
> >the hot-wire versions do a better job.
> >
> >In closed-loop operation (normal driving) the O2 sensor will do a good 
> >job of controlling for optimum burn, including compensating for
> altitude.
> >
> >malcolm
> >
>