The engine light came on and a code reader states "Coolant thermostat - coolant temperature below thermostat regulating temperature". I assume the thermostat is bad. Can you provide details on changing the thermostat on the 2005 Chrysler 300c with 5.7 L hemi engine. Thank you very much.

code p0128 refers to coolant thermostat--coolant below thermostat regulating temperature==causes mechanical fault. ( thermostat stuck open --replace it) If you are experiencing cold weather then there is a problem with the ecm settings as the ecm should be compensating for the cold ambient temperature and setting the fuel mixture to richer than normal ( same as using a choke). Check the fault codes again and check for a coolant sensor fault.

Trouble Code: P0128

Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

Possible Causes:


Insufficient warm-up time.Low engine coolant level.Leaking or stuck open thermostat.Inoperative engine coolant temperature sensor.Inoperative cylinder head temperature sensor.Here are the reasons.

Diagnostic Test Code (DTC) P0128 has a generic description of "Coolant Temperature Below Thermostat Regulating Temperature".

The Poertrain Control Module (PCM) checks the coolant temperature at startup. It then calculates the period of time that it should take for the engine to reach operating temperature (or thermostat regulated temperature). After the calculated period of time, the PCM again checks the coolant temperature. If it has not reached operating temperature, a code P0128 flag is set in mode 6.

If the engine still has not reached operating temperature after 5 more minutes of continuous engine operation, code P0128 is set in continuous memory and the Check Engine (or Malfunction Indicator Light "MIL" is turned on.

Possible causes:

> A stuck-open engine coolant thermostat (MOST common)
> A skewed Engine Coolant Temperature (ECT) or Cylinder Head Temperature(CHT) sensor (working, but not reading correct temperature)
> Low engine coolant level (causing the sensor to not read correctly)

First, the engine coolant (antifreeze) level should be checked. If it is low, it is recommended to perform a cooling system pressure test to find out WHY.

Then, the actual engine temperature should be measured with an infrared thermometer near the ECT or CHT. The reading from the thermometer should be checked against the live sensor data with a scan tool. (PLease note that MOST generic OBD code readers CANNOT perform this function.)

If the reading from the thermometer and the sensor data is within 10 degrees F of each other, replace the thermostat. Otherwise, the sensor circuit should be checked for high resistance (loose or corroded connector, damagd wire, etc.). If no high resistance is measured in the rest of the circuit, the sensor should be replaced.

Well, I can't give you any specific information regarding your vehicle because you forgot to mention what kind of vehicle you have. (This is kinda important if you really want some help figuring out what is wrong with your car)

The "Generic OBD" of code P0128 is "Coolant Temperature Below Thermostat Regulating Temperature"

The most likely cause is a stuck-open thermostat, however, I have also seen defective Engine Coolant Temperature (ECT) sensors also cause this code.

your thermostat is designed to open at a specific temp, for example a 195 thermostat is supposed to start opening at 195 but will not fully open until it reached about 210. Your ECM has determined that your thermostat is not 'closed', but is hung open, not allowing the engine to reach a temp that the computer is set to see. So the thermostat is either malfunctioning, or your temperature sender unit is reporting the wrong thing to the ECU. Probably a good time to replace the thermostat, if the condition persists, replace the sender unit-both items together will cost less than $30 total.

CODE P0128- THERMOSTAT MALFUNCTION. POSSIBLE CAUSES 1. ECT SENSOR IS OUT OF CALIBRATION. 2. CHECK THE OPERATION OF THE THERMOSTAT IT MAY BE STUCK OPEN.3.INSPECT FOR LOW COOLANT LEVEL OR FOR AN INCORRECT COOLANT MIXTURE.

Trouble Code: P0128

Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Possible Causes:


Insufficient warm up time Low engine coolant level Leaking or stuck open thermostat Inoperative engine coolant temperature (ECT) sensor Inoperative Cylinder Head Temperature (CHT) sensor

P0128 - Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

P0410 - Secondary Air Injection System Malfunction
(has to do with exhaust system)

Note: There is a technical service bulletin (TSB) regarding code P0410 for a number of GM models. The TSB calls for the replacement of the AIR pump and the installation of a new secondary injection hose.

I checked generic code #128-- coolant thermostat (coolant temperature below thermostat regulating temperature) hope this helps. Check for code after car totally warned up. If it isn't running at operating temp. thermostat may be stuck open.