Driving my 00 explorer this weekend in temps best described as holy-freaking-crap-my-niblets-have-fallen-off levels of cold and got a CEL.
Scanned it with the torque app and got a low coolant temp code, which I expected, but also got a lean code on bank 1 and 2. Think the cold caused the lean code ? The low coolant temp code mentioned not being able to enter closed loop mode for fuel because of low coolant temp.
According to my app coolant temp was 127 f after idling in traffic for awhile 
. This car has always ran very cold, great when I'm climbing passes at 110degrees and never exceed 205 coolant temp, but not so good now. I may replace the thermostat anyway for winter
You could have an issue with the temp sensor too. Something causing extra resistance in the circuit will cause it to read cooler than it is, be it a faulty sensor, corrosion in the connector or damaged wiring up in the harness.
Check the actual running temp with a real thermometer before changing the T-stat to be sure the sensor is sane. Check for a sticking fan clutch too, if it has one.
Put a thermostat in it. Because its a Ford, ignore the lean codes until the IMRC leak gets bad enough to make a noticeable difference in idle quality.
I'll second the thermostat. My Town&Country did the same thing.
Fan clutch is working properly. I'll replace the thermostat because hey why not.
Idle is still smooth. It's thrown lean codes before but never when I run higher octane gas in it( tested it for a long period). I kinds wrote that up to an older engine needing a little extra love. Plus it gets better gas mileage that offsets the extra cost of high octane anyway
P0128 is ALWAYS a bad thermostat. It's one of the very few codes that you can diagnose just by hearing what the code is.
Fix that first, ignore the lean codes for now. Fords always seem to run lean (computer adding fuel, that is) anyway, between EGR leaks and IMRC shaft leaks and dirty MAF sensors and leaky PCV hoses...
Definitely a bad thermostat if it won't even warm up sitting still. Also throw a radiator cap at it as a preventative measure while you have cooling systems on your mind.
My older sister bought a 2001 Focus for DD duty for her husband and she dragged me along to check it out for her (Ha!). Plugging my code reader in showed a P0128 as the only code present so I suggested she get a new thermostat too. I didn't know that code could be
that common. I'll leave the thermostat install to the BIL to deal with though. :)
