The temp gauge on our 91 Maxima SE sometimes dips well below the cold mark with the engine fully warmed up. It happens inconsistently and could drop during idle or cruising. In a 30-minute drive the needle could drop once or twice. I think the thermostat is going to go soon. How easy is it to replace this $20 part?

TIA

Check your coolant level first. sounds like it's low and you've got air in the system.

The coolant level is fine. How can I find out if there is air in the system and how do I resolve this?

TIA

There are also two temp sensors on the aluminum piece leading to the water pump. Follow the upper radiator hose from the radiator back tworad the firewall. When you come to the aluminum neck look on either side of it and you will see two harnnesses. these are temp sensors. Often times they need to be cleaned. You'll have to unbolt them and take them off the aluminum neck in order to do so. Be easy on them.

As for getting coolant out of the system, there's a bolt on the top of the intake manifold, just above the PCV valve. the head of the bolt points straight up.
start the car with the engine cold, then unscrew that bolt while the engine is warming up. this will bleed the air out of the cooling system, then simply tighten it back down when coolant starts to come out. (with no bubbles)
if only coolant comes out as soon as you open the plug, then there was no air in the system and you jeed to look at your temp sensors and possibly the thermostat.

btw..
this is a 5 minute job.
temp sensors are a 20 minute job.
replacing the thermostat is a 3 hour job.

Good bleed technique!

Finally, some more info. I couldnt get any more info than where the thermo was located. Thanks, Im gonna work on it this weekend...hope its as easy as the sensors.

Thanks much!