CEL is on, but no noticeable probs

Friday night I'm driving to a friend's house and the Check Engine Light comes on. I check the usual suspects: plenty of coolant, belt is good, oil is good. Nothing changed about how the car runs...it's still very smooth, no stalling or anything. The owner's manual seems to hint that it's probably something emissions related. I'm going to put it up on stands tonight and see if there's anything underneath that's loose (at least something that shouldn't be loose). Took it to Advance Auto. The only code he could get was P0128: Coolant Temp Below Thermostat Regulating Temp. That doesn't sound like a problem to me since I believe that it's good if your coolant temp is low (but, obviously, not frozen). The guy at Advance thought that this message meant that the thermostat is bad.

Any ideas on what might make this light come on.

I would just spend a few bucks and change out the thermostat, and see what that does. Well it's good to not get your coolant too hot, at the same time, you don't want your coolant to be to cool. The engine's runs best at a certain temp range (normally between 180 and 210 F I believe) and if the engine can't reach the proper operating temp, it could effect your mileage etc. If that doesn't do it, then maybe your coolant's losing it's effectivness, and it's time to change it out...

jusst sounds liek an over cooling thing, if this be the case tacke small bit of card board at put it in front of the radiator to stop some air, only do this is the temp gase seems to low,.

nose-gobblin, no offense but that is a bad suggestion. Never interfere with the air flow to the engine (unless it's an aftermarket intake) I dont think cardboard falls in to aftermarket category. LOL. One thing that I had a problem with on my mustang was faulty wiring. I'm not saying this is the issue, but don't put the idea out of your head all together. Do you live in a cold climate? if so, then it probably is what Mustangman25 said, your coolant. Also check the oil viscocity. You don't want to put thick oil in the engine, use lightweight oil. Especially Royal Purple. If the engine isn't getting good oil, it can cause it to overheat from friction. but check your thermostat to begin with. take it off of the engine and put it in a glass of water. If bubbles rise then the thermostat is opening properly and most likely it's ok. I think thats how you can check it without taking it to a shop and having them do it.

if your car is over cooling this is a simple but "SHORT TERM techniquie. we use it on our trucks all the time, plus i didnt real whole article about cold climate chit

ok, i see