Car threw a check engine light yesterday on a long drive.. scanned it and got p0128. Noticed the engine temp gauge would not fully go to normal operating temperature. Do I need to change my thermostat?

 

It couldn't hurt to change the thermostat. If you are still using a plastic housing, you might want to take this opportunity to replace it with a metal one.

Edit: There's also the possibility that the cooling fan is running when the coolant temperature is not hot enough to warrant that.

 

changed the thermostat today.. very easy took me all of 20 minutes and the car is running at normal temperature again. my only concern was before i changed the thermostat this morning my coolant reservoir tank was dead empty with the engine cold. I filled it up before the thermostat change and it didn't solve the problem so i then changed the thermostat and everything seems alright. Just wondering where my old coolant went, i have zero leaks but did drive the car 300 miles yesterday with this thermostat issue, did the coolant get evaporated due to the engine being in fail safe mode??

 

I don't think the coolant would have evaporated due to a faulty thermostat that was causing a low temperature. Keep an eye on the level in the reservoir (degas bottle).

 

p0128: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

Check out http://www.obd-codes.com/p0128 for more info.

 

If your bottle is empty then you are losing coolant somewhere, it doesn't just evaporate.

Take another look at the thermostat housing - and look for signs of leakage toward the back. The split usually happens on the back of the housing and the coolant goes into the valley under your intake manifold.

 

Just looked under the hood again today when the engine was cold, tank was empty again no coolant.. looked around the thermostat housing seems like it must be cracked as i can see some coolant underneath by the intake manifold- looks like i need to change the housing, wish i could wait for a metal one as i see a group buy thread but i need my car running asap so I'm headed to the dealership parts dept tomorrow.