My wife drives a 2K LS400 which I almost never drive. Last week I drove it and noticed the temp gauge only goes up about 1/4. I inquired and although it runs very well was told it could be the coolant sensor which I was gonna replace soon. I drove it again today and the temp gauge seems to be up to almost 1/2 BUT when on auto the heat cuts out when going slow or idling and it seems like the AC goes on. Other than that it still runs fine. Any ideas? My first thought is the T stat. Please help! Also, if it matters. I went looking for the coolant temp sensor, found it and unplugged it just to make sure it came off easily before doing the replacement. Put it back on and now the problem described above.

 

I don't mean to confuse you any more then you already are but which coolant sensor did you pull the plug off? There are 3, You have the ECT which is the main sensor for the fuel system, the single wire coolant sensor that is for the gauge, and the coolant sensor on the radiator which works the fans. It sounds like you might have an air bubble in you coolant b/c your gauge fluctuates and you're losing heat at lower RPMs.

 

I must have pulled off the ECT because it was not the one on the radiator and it had 2 wires. I found the problem. By luck, after pulling the cover over the radiator I saw a little water and barely touching it the plastic pc of the radiator broke off where the small line goes on. Just replaced the radiator and it's now good to go.