My Autometer 3579 air-fuel gauge was poorly designed (probably by an offshore firm to be as cheap as possible). Replacements behaved exactly the same: if you don't wait with the key on until the O2 sensor warms up and then start the car, it will stay in warmup mode forever.
I experimented on the bench and found that it can get confused by the drop in voltage while cranking. So today I put a toggle switch in the dash (in the hole left by my recently removed choke pull cable). Now I start the car, then flip the switch to power-up the gauge. Again, bad design but I wanted it to match all my other Sport-Comp gauges. Some of which are over 30 years old and still working fine!
Driver side took 45 mins. Yikes! 
