P0128 - What it means (and How to fix it)
The check engine light can be a headache for many vehicle owners. Having to go to a professional mechanic or auto parts store for them to read the codes can be time-consuming and not provide all the information you may need.
Especially for the P0128 code.
Coolant temperature problems are nothing to ignore and P0128 code is a serious problem that should be resolved quickly.
In this article, we are going to talk about the P0128 code, the common causes, and how to repair it.
What causes the P0128 trouble code?
The most common cause for the P0128 code is a faulty thermostat. Other issues can be low coolant, bad radiator fan or water pump, inaccurate engine coolant temperature sensor (ECT sensor), or bad engine control module.
What does the P0128 error code mean?
The P0128 code indicates a problem with the cooling system (coolant system). The powertrain control module notices that the engine temperature is below where it should be operating.
The powertrain control module uses the engine coolant temperature sensor to monitor the engine's temperature. An engine coolant thermostat will regulate the amount of flow of coolant through the engine to obtain the optimal temperature.
A thermostat will stay closed until the engine reaches operating temperature. After the operating temperature is obtained, the thermostat and radiator fans will keep the engine's temperature in a specific range.
What does a bad thermostat look like?
A failed thermostat will generally be what's called an "open thermostat."
As you can see from the picture below, the thermostat has a small gap in it.
This small gap allows coolant to freely flow all the time.
In a vehicle, this is not good. The thermostat needs to open and close to regulate the amount of coolant flow in order to keep the engine in the correct temperature range.
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What are some symptoms of the P0128 code?
Though each vehicle may vary, some common symptoms of the P0128 code are a lack of heat in the vehicle, the needle on the temperature gauge not moving up to operating temperature, and/or the cooling fans running constantly.
Irregular fuel consumption or bad fuel economy may occur due to the P0128 code.
How do you fix a P0128 engine code?
The best way to fix P0128 is to find the root cause of the problem and repair it.
A quick diagnostic process for a p0128 is:
Define the code.
Check your coolant (with a cold engine. Never open the cooling system when the engine is hot).
Use a scan tool to monitor the temperature of the coolant and the length of time to reach operating temperature.
Determine if it is the thermostat staying open.
Replace the thermostat.
Can I still drive my car with a P0128 code?
Generally, you can drive with a P0128 code. You may notice that with this code, the temperature gauge will not reach normal operating temperature.
It is best to resolve the P0128 code, so the vehicle can reach normal operating temperature to run efficiently.
In some cases, not reaching normal operating temperature will cause your vehicle to stay in "open-loop". Open loop is a mode of the engine's computer. Open loop controls the fuel system without "listening" to all of the sensors (like oxygen sensors) on the engine. Generally, open loop happens when the engine is cold and will operate in this mode until it reaches a certain temperature. Once the correct temperature is reached, the engine’s computer will switch to "closed loop."
When a vehicle is not operating in "closed loop,” monitors and tests that the engine’s computer performs will not run. This could mean that different issues will reveal themselves after the P0128 code is resolved.
Unresolved issues can be serious and cause engine damage.
Can a coolant sensor cause a P0128 code?
A coolant temperature sensor reading that is incorrect can cause a P0128 code.
The coolant temperature sensor (coolant temp sensor) is what monitors and reports the engine temperature to the engine computer.
To determine if the coolant temperature sensor is a fault, an ohm reading of the sensor would have to be in the correct range.
Common problems with the Cooling System
Air pocket
Low coolant level
Cracked hoses - Like a radiator hose
Coolant leak
Incorrect coolant temperature sensor reading
How much does it cost to repair a P0128 code?
For an accurate estimate, a professional mechanic may need to inspect your vehicle to understand the cause of the P0128 code.
What do I need to fix a P0128 code?
An important thing is to check technical service bulletins (TSBs) before any repairs are done.
A simple software update may fix your issue.
To check for TSBs, check Google or go to AlldataDIY.
AlldataDIY has all of the TSBs for your specific vehicle.
After you have verified that it is your thermostat that is faulty, here is a list of what you'll need to replace it.
Note: the vehicle should be cool and not hot when repairing any component on the cooling system, or serious burns may occur.
Appropriate tools
A good socket set, pliers set, and screwdriver set should work in most applications.
Coolant
It’s always recommended to use the manufacturer’s recommended coolant in your vehicle.
Thermostat
The thermostat can be purchased from your local auto parts store. In most cases, they will have it ready the same day, but it may be worth the effort to call ahead to check. Some thermostats come with a thermostat housing also.
Thermostat gasket
Double-check your thermostat box for a gasket. In most cases, they will come with one in it. If it does not, then you will need to purchase one.
Case Study
Below is a case study from a 2018 Chevrolet Colorado with a dtc P0128.
The customer came into my auto shop with a complaint of:
Check engine light (malfunction indicator lamp)
The temperature gauge not going up
The cooling fans run all the time
A/C not working
With the customer’s vehicle, the cooling fans would run constantly, even for several minutes after the truck had shut off.
The first thing I did was check the engine codes with a diagnostic scanner.
The diagnostic scanner I'm using is a Snap-On Zeus. It is a powerful scanner that has many functions, but you can do the same basic functions with any scanner.
The scanner revealed the Chevy Colorado has the code P0128 in it.
The next step was to check the to see if coolant was at the required level. It was full.
After that, I looked at the "live data", especially the coolant temperature and engine run-time data PIDs.
After reading in Alldata, the P0128 code sets when this truck's engine temperature does not reach 160 degrees (F) within 24 minutes of first starting.
The results of the P0128 code setting are:
The engine coolant temperature sensor defaults to cold.
The A/C is commanded “off” and inoperative.
The cooling fans are commanded “on.”
I wanted to see for myself if the engine would reach normal temperature within the correct amount of time.
During the test drive, the engine did not reach the desired temperature in the proper amount of time.
With this information, I decided to condemn the thermostat.
The thermostat was not operating correctly and either open all the time or is opening up too soon.
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I used Alldata to read step-by-step procures on how to replace this thermostat!
The thermostat on this truck is located under the engine cover at the end of the radiator hose (upper).
Since this thermostat was in the engine valley, I needed to drain the cooling system so coolant would not be trapped.
The easiest way to drain this cooling system was to remove the lower radiator hose.
Be sure to catch and dispose of your coolant in a proper way. Most auto parts stores will dispose of coolant for you.
If you are reusing your coolant, be sure to check the coolant condition. Coolants can become corrosive over time and not distribute heat effectively.
After replacing the bad thermostat and installing the new one, I cleared the P0128 code.
Then I started the Chevy Colorado and monitored the coolant temperature and the engine run time.
The engine temperature began to rise as it should.
Note that some vehicles may experience air blockages when filling the cooling system. Air blockages can cause a car’s engine to overheat. Make sure to purge all air out of the entire system.
Just like that, the Chevrolet Colorado was fixed!
If you don't have spare time to replace your thermostat, schedule a trusted mechanic to do it for you. The repair and test drive could vary from car to car but it's best to have a professional perform the task in some cases.
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