The aforementioned idle issues can also be caused by a thermostat stuck open. Happened just as described to me. The stock gauge reads normal and doesn't change from about 143* to at least 215* so it is not a good indicator of engine temperature. Mine ran from the 140s to the 180s erratically when the thermostat was stuck. It should normally operate at least 192* or so.
If you don't have a PCM reader (I use a scangauge) you can get a good idea if your thermostat is stuck open this way: If your heater is not as potent as it should be and you have idle issues, replace your thermostat. They are about $26 and though fiddly to change, not technically difficult. Small hands would be helpful but not required. I realistically spent about 3 hours doing it, but I'm kinda slow.
For me, after changing out the thermostat it cleared up the idle issues and I picked up about 3 mpg consistently over 3 tanks.