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P0128: Coolant Thermostat Temperature Below Regulating Temperature

When your check engine light is illuminated due to trouble code P0128, you may have an issue with your engine thermostat. Here are some helpful steps to take to find out how do I fix trouble code P0128? Learn how to solve this trouble code and repair your thermostat.

Decode Your Check Engine Light

What Is Trouble Code P0128?

This generic trouble code can affect most makes and models of vehicles. The code is signaled when your powertrain control module detects an unusual temperature reading while your engine is running. Typically, it’s signaled when your thermostat isn’t reading any temperature or is reading an incorrect temperature. This conclusion is reached based on the intake air temperature sensor reading, the length of time your vehicle has been running and the engine coolant temperature sensor reading.

While there is a small chance that there is an issue with the running temperature of your engine, by far the most common issue with trouble code P0128 is a faulty thermostat. The exact location of the thermostat and issues related to a damaged thermostat may vary depending on your vehicle. For most vehicles, trouble code P0125 is related to this issue as well.

Common Symptoms

Your thermostat is an essential component for regulating coolant temperature and checking for more serious issues with your engine. However, a damaged thermostat won’t typically cause any performance issues.

So, what does the code P0128 mean? You’ll likely see your malfunction indicator lamp illuminate. You may also experience a longer amount of time for your engine to warm up, or the temperature dropping while you’re driving at high speeds. The most important reason to replace your thermostat is to keep your coolant system working properly and to catch any early signs of engine damage.

Likely Causes

The most likely cause is a leaking or stuck open thermostat. Related issues would typically signal another error code, so this should be one of the first areas to inspect. Consult your service manual to find the exact location of your thermostat.

However, before you replace your entire thermostat, inspect these other areas as well. Low engine coolant can alter the engine running temperature enough to signal trouble code P0128. Your intake air temperature sensor, coolant temperature sensor and coolant fan could also signal this trouble code, so these should be inspected after you look over your thermostat and coolant level.

How to Fix P0128

Start with the easiest repair solution, which is low coolant levels. Before you start, turn off your engine and wait until it’s sufficiently cool to open your coolant reservoir. Under normal operation, a coolant system is pressurized. Opening a hot coolant system could cause burns and other injuries if you aren’t careful.

Once your engine and coolant system are cool enough to inspect safely, check the levels and strength of your coolant and compare it with the recommended levels in your service manual.

While the thermostat is one of the most likely issues with this trouble code, it can be difficult to inspect your thermostat. A faulty thermostat may not have any visible signs of damage. Before you replace your thermostat, it’s a good idea to inspect these other systems:

  • Intake air temperature sensor
  • Engine coolant temperature sensor
  • Cooling fan

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