My service engine light is on and it reads
P0128 "coolant theromostat - coolant temp below thermostat regulating temperature".
It's an 04 Explorer with the original coolant, it's full (plan to change it this spring). Any suggestions on what to try ? Thanks

 

Usually, that means the thermostat isn't closing properly.

If you have adequate heat, your coolant level is correct, and you don't have a pending emissions test, you can probably wait until spring and replace the thermostat at the same time as your coolant.

Otherwise, do both jobs sooner.

 

Thanks, is the thermostat at the radiator where the large hose from the block connects ?

 

No, its where the UPPER hose comes out of the motor/intake.

 

i have the same code on my 03 and the temp never gets but a lil bit higher than the C on the gauge, i was told its the thermostat, makes sense, i have crap heat in it. its sooo damn cold out i dont wanna swap it out.

 

the thermostat is closed at startup and lets the motor warm up quick. After 4-5 minutes on the road, the thermostat opens as the temp goes up, and the gauge should stabalize at about 50% of needle travel on the gauge. If it takes forever to rise, or dont go up more than a little off the Cold mark, then the thermostat is probably bad-- and you have no heat.

 

Mine (a 97 4.0L) will warm up and throw plenty of heat, but the gauge bounces around a little within the operating zone - sometimes dropping way down toward "C", but rarely going up over halfway (no threat of overheat). It threw the check engine light recently, but I've yet to pull a code on it and see exactly what it is. That's priority for this week with a trip coming up to be sure it's ok and/or take care of whatever it might need..

 

Thanks guys and one more question, where is the best place to drain the coolant from ? I see a white cap on a black tee on the passenger side of the vehicle to the side of the radiator-- (not sure if that is even in the pipeline)--or is there a better place ?

 

There should be a drain plug on the bottom of one of the radiator tanks.

Other alternative is to pull the lower radiator hose and drain the system from it.

 

Mine (a 97 4.0L) will warm up and throw plenty of heat, but the gauge bounces around a little within the operating zone - sometimes dropping way down toward "C", but rarely going up over halfway (no threat of overheat). It threw the check engine light recently, but I've yet to pull a code on it and see exactly what it is. That's priority for this week with a trip coming up to be sure it's ok and/or take care of whatever it might need..

that kinda sounds like you might just have air in the coolant lines, did you try to burp the radiator

 

I haven't yet, no. I did hear some gurgling coming from somewhere too, which could confirm that suspicion. Wonder how I'd have managed that as I've not had any part of the system apart for any reason...?