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Check engine light stays on. diagnostic code is P0128. what is this and how hard is it to fix.

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HI, P0128: P0128 OBD-II Trouble Code Technical Description: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

What does that mean? 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.

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
You have to check one after the other to isolate the problem before proceeding to fix it....
  • Faulty intake air temperature (IAT) sensor

Posted on Oct 15, 2010

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P0171 is a lean code indicating more air than fuel being burned in the engine. P0128 is refering to the coolant temperature being below operating temperature. Usually this is caused by a defective thermostat. There are many reasons for a lean code to develop. This code may be due to the thermostat not operating correctly. Have the thermostat replaced and clear the codes to see if P0171 returns. If it does, further diagnostics will be required

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The check engine light comes on when there is a failure. If the failure is not fixed before clearing the codes than the computer turns the light back on because there is still a failure! In order for the light to stay off what ever is not working needs to be repaired. Then the codes can be cleared and the light will stay off. What code specifically do you have. It should be a P followed by four digits (P0128).

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2004 trailblazer code po128


P0128:
P0128 OBD-II Trouble Code
Technical DescriptionCoolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

What does that mean?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.

CausesA 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 .Here's a link on replacing your thermostat.

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P0128 code on 2006 ford escape, solutions?


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.

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Code P0128 indicates a faulty thermostat, more than likely it is always in the open position causing the coolant to come up to operating temperature too slowly.
My suggestion is to find a different repair shop.

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Leave 02 sensor alone ,it is doing it's job . It is heated up because of lean fuel mixture due to incorrect engine temp.,replace thermostat before looking further.

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What can keep causing Check engine code P0128


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.

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My engine light is on and the faulty code is P0128, what is this mean?


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

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Check engine light on, diagnostic code 0128 and 0410 keep coming up


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.

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