P0128 – Meaning, Causes, Symptoms, & Fixes
Code P0128 Definition
Coolant thermostat temperature below regulating temperature
What Does Code P0128 Mean?
The engine control module (ECM) tracks how long it takes for the engine of a vehicle 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 a “closed-loop” where readings from the oxygen sensors are used to maintain the efficient air to fuel ratio of 14.7:1. Most ECMs 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 will record if either of these two criteria is not met. If either fault is recorded again on the next engine startup, code P0128 is triggered.
What Are The Symptoms Of P0128?
- Check Engine Light is on
- Higher than normal idle
- Decreased fuel economy
- The temperature gauge is unusually low
What Is The Cause Of Code P0128?
- Stuck open thermostat (most common)
- Missing Thermostat
- Faulty coolant temperature sensor
- Faulty wiring for coolant temperature circuit
How Serious Is Code P0128? – Low
A stuck open thermostat is most likely the problem; however, thermostats are usually designed so that 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 vehicle. Overheating your engine will result in engine failure
Code P0128 Common Diagnosis Mistakes
Many just replace the thermostat without checking all possible causes. Check the condition of the cooling system for rust deposits or a mixture of different coolants. Make sure to flush the engine block and radiator when replacing the thermostat to prevent deposits from getting stuck in the new thermostat.
Tools Needed to Diagnose:
How To Diagnose And Repair Code P0128?
Difficulty of Diagnosis and Repair (1 out of 5)
- Scan your vehicle to verify P0128 is the only code present. If other codes are present, they must be addressed first.
- Check the 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.
- 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 or repair the wiring for the sensor if it is damaged.
- The most common cause for P0128 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. Nevertheless, 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, the 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 along with new gaskets and coolant
- Bleed air out of coolant system
- Test drive and monitor coolant temperature
- Check for leaks
Estimated Cost of Repair
For error code P0128, one or more of the below repairs may be needed to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to make the repair.
- Thermostat $220-$250
- Coolant temperature sensor $140-$190
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