GM P0128 – Meaning, Causes, Symptoms, & Fixes

GM Code P0128 Definition

Coolant thermostat temperature below regulating temperature

GM  Code P0128 Meaning

The powertrain control module (PCM) tracks how long it takes for the engine of your GM to reach and maintain the correct operating temperature. When the correct engine operating temperature is reached, the powertrain control module orders the fuel system to enter “closed loop” where readings from the oxygen sensors are used to maintain the efficient air to fuel ratio of 14.7:1. Most PCMs mandate that the engine coolant temperature sensor record temperatures above 160º F within 15 minutes of the engine starting. Additionally, once the 160º F threshold is achieved, the recorded engine temperature must not fall below 160º F during operation. The PCM of your GM will record if either of these two criteria are not met. If either fault is recorded again on the next engine startup, code P0128 is triggered.

GM P0128 Symptoms

  • Check Engine Light is on
  • Higher than normal idle
  • Decreased fuel economy
  • Temperature gauge is unusually low

GM P0128 Causes (* indicates most common)

  • Stuck open thermostat*
  • Faulty coolant temperature sensor
  • Faulty wiring for coolant temperature circuit
  • Radiator fan constantly running

GM Code P0128 Severity – Low    

A stuck open thermostat is most likely the problem, however thermostats are usually designed so when they fail they are stuck open allowing full flow of the coolant protecting your engine. Check your coolant level first to make sure it is full before continuing to operate the GM. Overheating your engine will result in engine failure

GM Code P0128 Common Diagnosis Mistakes

Many just replace the thermostat of their GM without checking all possible causes. Check the condition of the cooling system for rust deposits or mixture of different coolants. Make sure to flush the engine block and radiator when replacing thermostat to prevent deposits from getting stuck in the new thermostat.

GM Code P0128 Diagnosis Steps

  • Tools Needed to Diagnose:

How To Diagnose P0128 For Your GM

  1. Scan your GM to verify P0128 is the only code present. If other codes are present, they must be addressed first.
  2. Check coolant level and condition. If there is excessive rust and  poor coolant condition this can clog up the cooling system or cause the thermostat to stick. If your coolant condition is poor, flush the coolant system and replace the coolant. If the coolant level is low, fill the coolant system and check for leaks.
  3. The coolant temperature sensor can be checked with a multimeter. The ohm reading will change with the temperature. If the ohm reading is not changing with the temperature, replace the coolant temperature sensor in your GM or repair the wiring for the sensor if it is damaged.
  4. The most common cause for P0128 in a GM is the engine coolant thermostat is stuck open. A simple way to diagnose this is to feel the radiator hose and monitor how hot the temperature of the coolant is when it starts flowing through the radiator hose, however, you should be extremely careful when doing this as you could be burned. The hose should be barely warm until the thermostat opens. When the thermostat does open, hot coolant should start to flow and quickly warm up the radiator hose. If the radiator hose heats up slowly, the thermostat is stuck open or opening prematurely and needs to be replaced.
  • Replace thermostat with new gaskets and coolant
  • Test drive and monitor coolant temperature
  • Check for leaks

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