05 Magnum sxt 3.5l, CEL came on/ code P0128. I changed the thermostat,coolant sensor,new coolant reset the code. I went about 100 miles and the CEL came on with the same Code P0128.Bleed the system reset the code and about 175 mile later the CEL came on with the same code. No other code but P0128 shows. Anyone have any idea what the issue is.

p0016
P0016 OBD II Trouble Code Camshaft Position Camshaft Position Correlation...
p0128
P0128 OBD II Trouble Code Coolant Thermostat Coolant Temperature Below...
p0206
P0206 OBD II Trouble Code Injector Circuit Malfunction Cylinder 6...
p0306

P0306 OBD II Trouble Code Cylinder 6 Misfire Detected

p0436

P0463 OBD II Trouble Code Fuel Level Sensor Circuit High Input...

P0128 OBD II Trouble Code Coolant Thermostat Coolant Temperature Below...

code p 128 refers to coolant thermostat---coolant temp below thermostat regulating temperature ====causes --mechanical fault
you say that you have replaced the thermostat and it is just under 210 F which is 99 c or almost boiling point
you will have to investigate why it is running so hot.
check that you put the thermostat in correctly ( bulb section to block/head water
Next point --the codes are originated by the coolant temperature sensor ( puts the fans on and sets the ECM fuel/air ratio for the engine ) so to get it off you will need to reset with a scan tool

A code P0128 may mean that one or more of the following has happened:

• Low engine coolant level
• Leaking or stuck open thermostat
• Faulty cooling fan (running too much)
• Faulty ECT Sensor
  
• Faulty intake air temperature (IAT) sensor

Possible Solutions Past experience indicates that the most likely solution is to replace the thermostat. However here are some suggestions on troubleshooting and repairing a P0128 OBD-II code:

• Verify coolant strength & level
• Verify proper cooling fan operation (check if it's running more than it should). Replace if necessary.
• Verify proper engine coolant temperature (ECT) sensor operation, replace if necessary.
• Verify proper intake air temperature (IAT) sensor operation, replace if necessary.
• If the above items check out good, replace the thermostat

P0128 is coolant temp below threshold, and P0303 is a misfire at #3 cylinder-check your connection at the engine temperature control sensor and consider replacing the thermostat (is your temp gauge running unusually low?-might have a stuck open thermostat, it does happen)-this might be causing your engine computer to keep the engine in open loop, thinking it is not warmed up-if you have a bad plug wire at #3, or it is firing the injector rich, it might have tripped the P0303-no matter what, it does not sound real expensive, worst case you replace the thermostat, the ETC, plugs, cap, rotor and plug wires-less than $100 total, and it should run like new. While you are at it, clean the throttle body, it will help everything since it will run smoothly too.

First off who ever said $400 to change a thermostat is trying to rip you off. The actual part is like $20 bucks max and its so easy to change. I can guide you through it if you would like.

The P0128 code is Coolant Thermostat Malfunction.

These are the possible causes for this code:
- Insufficient warm up time.
- Low engine coolant level.
- Leaking or stuck open thermostat.
- Malfunctioning engine coolant temperature sensor.

These are the possible quick fixes:
- Top OFF Engine Coolant
- Replaced Thermostat

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

This means that the engine's PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. The intent of the P0128 code is to indicate a faulty thermostat. Similar codes: P0125

In determining the engine did not reach a "normal" temperature, it takes into account the length of time the vehicle has been running, the intake air temperature (IAT) sensor reading, the engine coolant temperature (ECT) sensor reading, and the speed of the vehicle.

Symptoms: You will likely not notice any drivability problems other than the MIL illumination.

Causes: A code P0128 may mean that one or more of the following has happened:
* Low engine coolant level
* Leaking or stuck open thermostat
* Faulty cooling fan (running too much)
* Faulty coolant temperature (ECT) sensor
* Faulty intake air temperature (IAT) sensor

Possible Solutions: Past experience indicates that the most likely solution is to replace the thermostat. However here are some suggestions on troubleshooting and repairing a P0128 OBD-II code:
* Verify coolant strength & level
* Verify proper cooling fan operation (check if it's running more than it should). Replace if necessary.
* Verify proper engine coolant temperature (ECT) sensor operation, replace if necessary.
* Verify proper intake air temperature (IAT) sensor operation, replace if necessary.
* If the above items check out good, replace the thermostat
* If Nissan vehicle, check for Technical Service Bulletins (TSBs), as the ECM may need to be reprogrammed to correct the P0128 code

Hope helps (remember to rate and comment this).

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.

It is very likely your thermostat is stuck open. Replace the thermostat and it should be fine.

P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

P0102 - Mass Air Flow (MAF) Sensor Circuit Low Frequency, clean inside the of MAF sensor with spray brake clean, if code returns replace the Mass Airflow sensor (MAF)
P0440 - Evaporative Emission (EVAP) System,
poss loose or leaking gas cap
P0128 - Coolant Thermostat Below Thermostat Regulating Temperature,
replace the thermostat, engine running cold
P0716 - Input Speed Sensor Circuit Intermittent, this is a problem with the VSS, vehicle speed sensor, replace it.
Clear all the codes after above repairs and drive a few days, then retest system for codes.