Hi. I bought an 2003 Xterra model car back in 2005 (long story). It currently has 76,000 miles on it and has regular maintenance.
In March 2016, I went up a long hill and got stuck trying to turn around. I don't have 4-wheel drive and I may have pushed the engine a little too hard. I didn't notice it at first but a mile down the road the air-conditioning got warm. I looked at the temperature gauge and noticed it getting close to the top. I turned it off and a friend told me I was out of coolant. He went and poured some in. I wasn't sure what to make of it.
A couple weeks later I noticed a change in the level and a friend installed a new thermostat. There was no visible leakage under my car and the temperature gauge remained in its normal range. The A/C wasn't as cold as it should be (according to my friends) and someone decided to try recharging the Freon. That had no effect.
Without a low coolant sensor, I made sure to check regularly. In June 2016, I noticed it lower than the minimum bar. I went to store to buy coolant and refilled the container to a decent level.
It is now October 2016. People still complain my A/C isn't as cold as it should be and I just spotted my temperature gauge bouncing around as I drove up-and-down some hills. Yep, coolant was low again.
I have been told to check the thermistor (I guess the thermostat sensor), use dye, check coolant pressure, and check for air pockets. One guy told me to just buy a gallon of coolant every year since I don't drive that much. I'm hesitant to hand it off to some mechanic. I don't want to pay a lot of money but I do think I should fix it before taking a long trip somewhere. The only thing I can add is that the oil looks fine and I still get 16 mpg.
What do you think I should do?