Engine Code p0128 Meaning, Causes, Fixes (5 Steps)

You hooked up a scan tool to solve the mystery of the check engine light, and it came up with OBD-II trouble code p0128 coolant thermostat is running below the thermostat regulating temperature

So what does the code p0128 mean?

Based on most online feedback, it's looking like the large majority of the time that when this code appears, the vehicle's thermostat needs to be replaced; but that's not always the case. Scroll down to learn more.

Engine Code p0128 Meaning, Causes, Fixes (3 Steps)

This is a common problem in vehicles makes like:

  • Dodge
  • Toyota
  • ..and others

Stick with this Proper Mechanic on how to diagnose and fix the p0128 code and turn off the check engine light that has appeared on the dash.

What Does The p0128  Code Mean?

This code means that the vehicle failed to reach running temperature and that the engine temperature is excessively low.

That's right, the engine can't get too hot, and it also shouldn't be excessively cooled.

This may not be much of a problem in summer conditions, however, in the wintertime, the engine will run rough if this error code is stored in the vehicle's ECM.

Not to mention the fact that the check engine light should be immediately taken care of always when it is illuminated.

What Caused The p0128 code (Possible Symptoms)?

When you start the engine, the vehicle is constantly trying to heat to its ideal engine operating temperature, so that it can burn fuel efficiently.

When the engine gets hot, the thermostat opens letting coolant flow through the radiator.

But, the engine doesn't always need to be cooled; sometimes it gets too cold, like when the p0128 OBD-II code shows up stored in the engine control module:

Here are some possible causes of the p0128 code

  • Stuck open thermostat/faulty thermostat
  • Cooling fan stuck on
  • Bad coolant temperature sensor 
  • Bad intake air temperature sensor

And when your vehicle is running colder than it normally should, its symptoms are:

  • Temperature gauge abnormally low
  • Decreased fuel economy 
  • Heater not blowing hot air

Read More >> What Do You Do About The Service Engine Soon Light?

How To Diagnose A p0128 Code?

Don't worry, it's not the end of the world if your engine is running excessively cold; but this is a problem that should be fixed immediately to maximize engine performance and assure the driver of the vehicle's safety down the road. 

This repair focuses most on the vehicle's coolant system and components. 

With a little bit of studying and patience, you should be able to pin this problem down yourself. 

Step 1: Check The Coolant

The first natural procedure when a cooling system code is set off is to check the coolant level and quality. 

Shining a flashlight onto the coolant reservoir helps to accurately inspect the coolant. 

If the coolant is low, fill it back up and reset the check engine light. Drive the vehicle to see if the check engine light comes back on. Low engine coolant can cause all sorts of problems in the cooling system. a

Step 2: Electrical Problems: Is The Fan Stuck On?

Sometimes cars have weird electrical problems.

For example, the cooling fans can malfunction causing them to stay on when they're not supposed to be. This cools the engine excessively at times and causes the p0128 code. So before you replace the thermostat, pay attention to how the coolant fan is behaving. 

Step 3: Put In A New Engine Coolant Thermostat

If all the wiring visually checks out, most technicians just assume that the thermostat is stuck open. 

The thermostat is a part that may need to be replaced multiple times in the life of a vehicle. Using the vehicle's service manual will helps to successfully replace the thermostat.

Did you know: Coolant cools down your car much like sweat cools down your body. When your car reaches a certain temperature, a thermostat opens up and allows coolant to flow through the engine to prevent overheating!

How Do I Fix An p0300 Code?

For all of these repairs, it is ideal if you have the specific vehicle's service manual, but there are also systems like ALLDATA that have almost any vehicle's specs and guidelines. 

What you should also take in

Potential Fix 1: Replacing The Spark Plugs

Spark plugs need to be serviced every 20-100 thousand miles depending on their quality and correct installation. 

They are also one of the main causes of this engine code. 

You can replace the spark plugs in your vehicle by removing the engine cover(if there is one), removing the ignition coils, and using a spark plug socket to carefully remove each spark plug. 

Remember to always double-check the spark plug gap before reinstalling, and always use a torque wrench to torque the spark plugs to spec. 

Potential Fix  2: Repair Vacuum Leaks

The procedure for fixing a vacuum leak varies depending on where the problem is; 

For example, if you have found that the air intake or its gasket needs to be replaced, you'll have to unbolt the fuel rail, unmount/unclip and remove the fuel injectors and replace the intake or the intake gasket. 

Always torque down the air intake in the right order upon reinstallation.

The vehicle-specific service manual will help aid in a successful repair of this p0300 code. 

Other vacuum hoses and other air-related valves may simply be broken, unplugged or improperly routed.

Follow the vacuum system like a map, and trace everything to the correct spot. 

Potential Fix 3: Fixing A Fuel System Problem

Get your hands on a fuel pressure tester and test the pressure of the fuel in the fuel rail. 

  • Fuel filter

Replacing fuel filters is a pretty simple task in most vehicles, but accessing them is another story because sometimes they are in hard to reach locations.

Nonetheless, the fuel filter is simply a filter between two fuel hoses.

  • Fuel pump

Fuel pump making strange whining noises? Use the vehicle's service manual to remove and replace the fuel pump.

  • Fuel pressure regulator

The fuel pressure regulator is easy to find on the fuel rail of most engines, and it's typically clipped in with a metal clip that you have to pull out. 

Potential Fix 4: Battery Check

It never hurts to check the battery. 

Visit your local auto parts store and have the attendant run a quick check with a battery tester to ensure that the engine is running with full power. 

Replacing the battery is simple, and it is typically mounted into where it sits with a bolt or two; disconnect the negative terminal first to avoid a short circuit and then the positive. 

Potential Fix 5: Timing Belt Replacement/Engine Rebuild

Servicing the timing belt is a more intermediate task because it requires removing some panels, timing belt tensioner, etc. 

Reference the service manual specific to the vehicle on how to service the timing belt on each specific vehicle.

Removing/installing parts in the right order is crucial when repairing the timing belt!

And let's face it, sometimes there is no easy or simple solution to the problem. Like if a cylinder fails a compression test; more wrenches come out of the bag. 

We encourage you to have the confidence to fix this p0300 code yourself! 

Ask us any questions and you'll quickly be answered with quality advice about your auto repair.

Read More >> What Does The P0420 Engine Code Mean?

How Do I Fix A p0128 Code?

Remember, let the vehicle cool off before draining the coolant or twisting open the radiator cap. 

You can find all of these parts either on websites like Amazon or at your local auto parts store. 

Keep in mind that servicing the cooling system on an engine should be a top priority, and keeping this system running properly is a must for preserving the life of the engine. 

Potential Fix 1: Replace The Thermostat 

To remove the thermostat, you'll need to drain the coolant, disconnect the radiator hose going to it by loosening it from its retaining clip, and there are usually two bolts that mount it to the engine that you'll need to remove.  

Replace the thermostat gasket(if necessary) and torque the bolts to spec before securing the coolant hose and retaining clip back on.

Reset the check engine light to verify you have made a successful repair.

Potential Fix 2: Replace Engine Coolant Temperature Sensor

The engine coolant temp sensor is a relatively easy part to replace because it simply threads into the engine. It's also a cheap part.

Some pro technicians don't even like to mess around with this code and they throw a thermostat and a temp sensor in just to be sure that the repair lasts a long time. 

Besides, these are cheap parts on sites like Amazon. 

You can replace the temp sensor with simply a crescent wrench.

Locate it by referring to the vehicle's service manual, unplug the electrical connector, unscrew the temp sensor, and replace it with a new one. 

Reset the check engine light to verify that you have made a successful repair.

Potential Fix 3: Electrical Repair

Who knows, maybe a rat chewed through the coolant temperature sensor wire(yes, crazy stuff like that happens). You never know, and that's why a close visual inspection of all the cables going to the coolant system is a good idea when dealing with this problem.

  • Radiator fan stuck on

In the case that the engine fan doesn't want to shut off, the correct procedure is to identify the electrical component of the engine fan.

The cooling fan could be stuck on because of failed relays/resistors that are associated with it, or it could be that the coolant temperature sensor is bad. 

You can also test the electrical connectors on parts like the coolant temp sensor/thermostat

Reset the check engine light to verify that you have made a successful repair.

Now you have a fighting chance for fixing this check engine light code.

Feel free to ask Proper Mechanic any questions you have about your engine repair!

About Your Mechanic

About Ryan Nichols

Ryan here! My pro mechanic career began as a technician for Mercedes-Benz. After two years of that great experience, I went out on my own in both the automotive and construction fields. I've since pulled my share of salvage yard parts and fixed more vehicles than I can count.

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