The temperature where I live is in the fifties Fahrenheit and it seems like its taking a very long time for the engine to warm up and exit open loop mode. I have a 97 Mazda Lantis 323 and I'm thinking that maybe the thermostat is stuck open. Is it normal for it to take 15 minutes for the engine to warm up at that kind of starting temperature ?
I think you are on the right track with the thermostat. No it should only take a few minutes max, even in colder temperatures for it to warm up. The thermostat is a cheap part to change and usually pretty easy to get to, so I don't think you are going to lose much by changing it out.