P0128 Code: Most Common Causes And Fixes

Understanding the P0128 Trouble Code: Causes and Fixes

If you’ve ever seen a P0128 code pop up on your OBD-II scanner, you’re dealing with a cooling system issue. This code is officially defined as “Coolant Temperature Below Thermostat Regulating Temperature,” which means your engine isn’t warming up to the proper operating temperature quickly enough. It can also mean that the engine is reaching operating temperature but can’t maintain that temperature for some reason.

This might not seem like a big deal at first, but trust me—it can lead to problems if left unresolved. The engine needs to reach its optimal temperature for fuel efficiency, emissions control, and overall performance. A persistently cool engine can cause poor fuel economy, excessive wear on components, and even carbon buildup inside the engine. So, let’s break down the possible causes and the best way to approach the fix.

How Does the ECM Set a P0128 Code?

As I said above, your engine needs to reach operating temperature as quickly as possible to produce and efficient burn and get maximum fuel economy. When you start your engine, the ECM looks at the following data:

• The outside air temperature: From the intake air temperature sensor.
• The Engine Temperature: From reading the rate-of-rise of the Engine Coolant Temperature Sensor
• The Engine RPM: It knows this by reading data from the crank and cam sensors.

The Possible Causes of a P0128 Code

1) A Quickly Opening or Stuck-Open Thermostat – This is by far the most common cause of a P0128 code. The thermostat controls the flow of coolant between the engine and the radiator. If it opens too quickly or is stuck open, the coolant circulates too quickly, preventing the engine from reaching its optimal temperature.
2) Faulty Engine Coolant Temperature (ECT) Sensor – If the ECT sensor is sending incorrect data to the ECM (Engine Control Module), the car might think the engine is running cooler than it actually is.
3) Low Coolant Level – If there isn’t enough coolant in the system, the engine may struggle to reach the right temperature. A leak in the radiator, hoses, or water pump could be the culprit.
4) Faulty Cooling Fan – If the cooling fan is running when it shouldn’t be, it can cool the engine too much, leading to a P0128 code.
5) Wiring or Electrical Issues – A damaged wiring harness, loose connector, or poor ground connection can cause inaccurate readings from the coolant temperature sensor.

The thermostat is supposed to stay closed at cold startup

At cold startup, the thermostat should be closed. Then, as the engine heats up, it should open slowly. If it opens too quickly, the engine won’t reach the expected temperature within the time window because cold coolant will be circulating through the engine.

The Two most common causes of a P0128 code

The two most common causes of a P0128 are bad thermostat or a bad engine coolant temperature sensor. The vast majority of the time, it’s a bad thermostat that opens too quickly.

How to Diagnose and Fix a P0128 Code

Step 1: Check the Coolant Level

Before diving into part replacements, I always check the coolant level. If it’s low, I inspect the system for leaks. A leaking radiator, hose, or water pump could be the real cause of the P0128 code.

Step 2: Inspect the Thermostat

I usually replace the thermostat rather than test

Engine Thermostat

it because it’s an inexpensive part and is the cause of a P0128 in over 90% of the cases. A properly functioning thermostat should remain closed until the engine reaches the right temperature.

However, here’s how to test it. Consult the shop manual to see what temperature the ECM expects to see and what time period from cold startup. Then, using a non-contact infrared thermometer, measure the temperature rise at the thermostat housing to see if the coolant temperature is reaching the expected temperature.

Click here to lean more about engine thermostats and how they work

Step 3: Test the Coolant Temperature Sensor

Compare the readings from the infrared

Engine coolant temperature sensor

thermometer to the ECT sensor reading on your scan tool to see if they correlate. If the sensor is giving unusually low readings, even when the engine feels warm, I replace it.
Step 4: Inspect the Cooling Fan Operation

If the fan is running constantly—even when the engine is cold—it could be overcooling the engine. I test the cooling fan relay and control circuit to make sure it’s not activating too soon.
Step 5: Check for Wiring or ECM Issues

If none of the above fixes resolve the P0128 code, I will inspect the wiring harness connected to the coolant temperature sensor. Damaged wiring or bad ground can cause inaccurate temperature readings.

Key Takeaways From This Article

A faulty thermostat is the most common cause of a P0128 code.
But check the coolant level first before replacing any parts.
Then check the radiator cooling fans to make sure they’re not running at cold startup.

© 2012 Rick Muscoplat