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P0128 Code: Meaning, Symptoms, Causes, and Fixes

When it comes to the car’s engine temperature, most drivers are more concerned about it being too hot than too cold. Overheating is a more serious problem, certainly, but your engine can overcool itself. The P0128 OBD2 code triggers when this happens. While this isn’t a catastrophic problem, it is an indication of a fault you should fix.

You’re not likely to see P0128 in the summer. When it’s hot outside, your engine will heat up just fine even if it’s receiving too much coolant. In the winter it’s a bit harder for the engine to warm itself up. That’s when issues with your thermostat will become most apparent.

P0128 code is related to the Coolant Thermostat
P0128 code is triggered when the Coolant Temperature is Below Thermostat Regulating Temperature

P0128 OBD2 Code Definition

Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature).

What Does P0128 Mean?

This code tells you that the engine is not maintaining the correct temperature. The car tracks how long it takes the engine to reach the right operating temperature, and how effectively it maintains it. Within 15 minutes of starting the car, the engine temperature should read above 160°F, and should stay there until the car is turned off.

The fuel system should use readings from the oxygen sensors to maintain the right air to fuel ratio while the car runs. This process is impeded if the engine won’t reach the correct temperature, or if the temperature falls too low during operation. 

What Are The Symptoms of The P0128 Code?

There could be no noticeable symptoms of the P0128 code aside from the trouble code itself. If there are symptoms, they’re relatively mild, and including the following:

  • Activation of check engine light
  • Engine takes a long time to warm up
  • Temperature of engine drops while driving at highway speeds
  • Air conditioner or temperature gauge stop working

What Are The Causes of P0128?

This code is almost always triggered by a faulty thermostat. Typically you’ll find the thermostat is either stuck open or leaking. There are other issues that could trigger the P0128 trouble code, though, including:

How Serious Is P0128?

This is a low-priority code, especially if it is caused by the thermostat. Car thermostats are designed to stick open when they fail, letting coolant still flow freely into your engine. 

Check your coolant level to ensure that wasn’t the cause of the error code. Running out of coolant could overheat your engine, causing damage. If the P0128 code was triggered by anything else, though, you can drive safely even while this trouble code is active.

How To Diagnose The P0128 Code

You can detect the p0128 code by using OBD2 scanners.
OBD2 scanners can help you detect the p0128 code.

Tools you’ll need:

  1. Scan your vehicle to make sure there aren’t any other codes present. If you find any, fix those first.
  2. Evaluate the coolant system. Check for rust, clogs, and leaks, and repair as necessary. Look at the coolant itself, as well. It should be a vibrant, bright color—usually green, though it may be red or orange. If it’s dark in color or contains visible debris, flush the coolant and re-fill the system with fresh fluid.
  3. Determine if the problem is the engine coolant thermostat by assessing the temperature of the radiator hose. If it’s functioning correctly, the hose will barely feel warm before the thermostat opens. Once it does, the hose will quickly get very hot as coolant flows through it. If it heats up gradually, the thermostat is stuck. Be careful conducting this test. If the thermostat is functioning correctly, the hose can get hot enough to burn you.
  4. Check the radiator fan to see if it’s running correctly. It should remain off until the car reaches optimal temperature. If the fan starts too early, check it with a multimeter to ensure it’s receiving 12 volts of power. If not, the fan or wiring may need to be replaced.  
  5. Use a multimeter to check the coolant temperature sensor (you’ll find it on the front of the engine block). The ohm reading on the multimeter should change with the temperature. If it doesn’t, replace the sensor. 
How To Diagnose The P0128.

Common Mistakes To Avoid When Diagnosing Code P0128

Many people assume they need a new thermostat and immediately replace it before checking for other issues. Don’t forget to check your system to find out why the thermostat failed. If your coolant is bad or there are rust deposits in the coolant system, your new thermostat is likely to fail just like the last one.

Thoroughly check your entire system before replacing any components. It’s also a good idea to flush your radiator and engine block before replacing the thermostat. This will clear any lingering deposits and prevent future failures.

What Should You Do To Fix The Code P0128?

Diagnosing the trouble code tells you whether you need to replace the thermostat or the coolant temperature sensor. Always make sure your car is turned off and completely cooled down first. The coolant system gets very hot and could cause serious burns if opened too soon after operating the car.

To replace the thermostat:

  1. Drain the coolant out of the radiator tap until it’s lower than the thermostat housing.
  2. Disconnect the hose from the top of the thermostat housing and remove the nuts securing it. If it won’t come off, don’t pry it. This could damage it and cause leaks. Instead, tap it loose.
  3. Remove all traces of the old gasket with a scraper.
  4. Insert the new thermostat and fit the housing into place. 
  5. Re-attach the hose and re-fill the coolant.

To replace the coolant temperature sensor:

  1. Remove the lead wire from the top of the terminal. 
  2. Loosen the sensor with a ratchet wrench and deep socket. Do this gradually so you don’t damage the threads.
  3. Clean the threads and surrounding area with a dry rag.
  4. Set the new sensor in place and twist it partially in by hand. Use a torque wrench to tighten the sensor to its required torque.
  5. Check the lead wire for damage or dirt and clean or replace if necessary. If the wire is good, reconnect it to the sensor.

After every repair, clear the trouble codes and take your car for a test drive. Scan it again after to make sure you’ve actually fixed the problem.

Tips To Avoid P0128 In The Future

Proper maintenance of your car’s coolant system is the best way to maintain the long-term health of your thermostat. Check your coolant regularly, replacing it when it’s dark or dirty. Flush the entire system to avoid the build up of rust and other debris.

Read more: OBD2 Codes: Full List Meaning & Fix Guide

Tim MillerFounderOBD Advisor

I’m Tim Miller from Denver, Colorado. I’m the founder of obdadvisor.com, an automotive blog about "Diagnostic Tools and Auto Repair". My fan page is facebook.com/autozikcom. I've been working as an automotive mechanic and blogger for over 10 years writing articles to share my experiences and expertise.

Web: https://www.obdadvisor.comEmail: [email protected]
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