P0128 Code: Meaning, Causes, Symptoms, And How To Fix It

The engine is one of the most important parts of your vehicle. If there is something wrong with the engine, your vehicle won’t be able to run efficiently. The powertrain control module (PCM) constantly monitors the performance of your vehicle. Your engine needs to operate at normal operating temperature for optimum performance. When the PCM observes that the coolant temperature is lower than the temperature set by the thermostat, it triggers the P0128 code. This article explains the P0128 code meaning, causes, symptoms, and how to fix it.

P0128 Definition

The P0128 code stands for “Coolant Thermostat Temperature Below Regulating Temperature.”

What Does the P0128 Code Mean?

The P0128 code indicates that your powertrain control module (PCM) monitors that the coolant temperature is lower than the thermostat regulating temperature.

P0128 Code

For your engine to work efficiently, it’s essential that the engine reaches its designated normal operating temperature. When the engine temperature reaches normal operating temperature, the PCM sends a command to the fuel system to operate in a closed-loop mode, where the oxygen sensors perform an important role in maintaining the ideal air-to-fuel ratio (i.e., 14.7:1) to improve the engine performance.

Some PCMs have a requirement that the vehicle’s coolant temperature sensor must register coolant temperatures more than 160º F within the first 15 minutes of engine startup. When this threshold of 160º F is reached, the recorded engine temperature shouldn’t dip lower than 160º F while the engine is running.

In simple words, when your engine is started, it’s expected that the coolant will reach the temperature set by the thermostat within a specific timeframe. If your engine coolant is unable to achieve this specified temperature limit, the PCM triggers a P0128 engine code.

Causes of P0128 Code

A faulty thermostat or coolant temperature sensor is one of the major causes of the P0128 code. However, insufficient coolant, damaged wires, faulty radiator fan, faulty IAT sensor, coolant flow restrictions, or contaminated coolant may also cause the P0128 code.

Let’s discuss the causes of P0128 in detail:

1) Faulty Thermostat

Bad Thermostat

A faulty thermostat is one of the major causes of the P0128 code. When your thermostat goes bad or stuck open, the engine coolant can’t achieve the optimum operating temperature.

Read More: Bad Thermostat Symptoms and Causes

2) Insufficient Coolant

Your engine requires a sufficient level of coolant to work efficiently. It helps to maintain the normal operating temperature of your engine. Insufficient coolant levels may lead to the failure of the thermostat and cause the P0128 code.

3) Bad Coolant Temperature Sensor

The coolant temperature sensor constantly monitors the temperature of the engine coolant. When it goes bad, it sends the wrong information to the PCM.

When PCM gets wrong information about coolant temperature, it may open or close the thermostat valve at the wrong time, preventing the coolant from properly regulating the engine temperature.

4) Damaged Wires

The electrical wires ensure the proper communication between different engine sensors and PCM. Damaged wires may disrupt the communication between the PCM and coolant temperature sensor, causing poor performance of the thermostat. When the thermostat performance is affected, it also affects the performance of the coolant.

5) Faulty Radiator Fan

The radiator fan plays a vital role to properly cool down the hot coolant. When the radiator fan goes bad, it doesn’t properly cool down the coolant. When the coolant doesn’t cool down properly, the thermostat will not send it back to the engine.  

6) Bad Intake Air Temperature Sensor

The IAT sensor may also prevent coolant from reaching the normal operating temperature, causing the P0128 trouble code.

7) Coolant Flow Restriction

Restrictions in the coolant flow, such as clogged coolant passages or radiator, may also prevent it from achieving the normal operating temperature.

8) Contaminated Coolant

Different contaminants present in the coolant may badly affect its ability to properly regulate the engine temperature. A dirty coolant may also cause the thermostat to stuck open.

Symptoms of P0128 Code

A vehicle with DTC P0128 usually produces one or more of the following symptoms:

  • Illuminated check engine light
  • Temperature gauge showing the low temperature of the engine
  • Higher idle speed than usual
  • Poor fuel economy
  • The engine takes too long to warm up  
  • Poor performance of the heater
  • Fluctuating engine temperature 
  • Increased emissions (very rare)

How to Diagnose the P0128 Trouble Code?

Follow the following steps to diagnose the P0128 code:

  • Firstly, use an OBD2 scanner to ensure the existence of the P0128 code and other codes in the PCM. Write all the stored trouble codes on a notebook and clear the PCM memory. If the codes return, perform a deep inspection.
  • Turn off the engine and allow it to cool down completely before performing an inspection.
  • When the engine is cold properly, examine the coolant level and refill it if needed.
  • Check the quality and condition of the coolant. If your coolant is dirty, it may cause the thermostat to stick. Typically, it’s recommended to change the coolant every 30,000 miles.
  • Inspect all the electrical wires and connectors for damage.
  • Properly inspect the coolant passages for damage or leak.
  • Turn on the engine and cautiously place your hand on the radiator hose. A properly functioning thermostat will cause the radiator hose to heat up rapidly. If this hose doesn’t warm up or takes a long time to heat up, it means your thermostat is faulty, and you need to replace it.
  • Check the operation of the radiator fan. If it is not operating, you should replace it. If it remains continuously on, it might be stuck and require replacement.
  • Check the performance of the coolant temperature sensor by using a scan tool. Compare the readings shown on the scan tool with the readings written in the owner’s manual.
  • Test the IAT sensor performance using a scan tool. Always visually inspect the physical condition of the sensor.
  • Once you’ve addressed the issue, reset the error codes and take your vehicle for a test drive.

How Serious is the P0128 Code?

The severity level of the P0128 code is medium. Despite this, it is crucial to take the problem seriously because your engine may be damaged when the engine hasn’t reached the optimal operating temperature.

Driving with this code for a long time may also lead to the failure of costly engine parts. Therefore, it is recommended to fix the P0128 code as soon as possible.

What repairs can fix the P0128 Code?

To fix the diagnostic trouble code (DTC) P0128, you need to repair or replace one or more of the below-given parts:

  • Clear the codes and take your car for a test drive
  • Replace the bad thermostat
  • Replace the faulty IAT sensor
  • Add more coolant if needed
  • Flush the coolant if it looks dirty
  • Replace the faulty radiator fan
  • Repair or replace the damaged wires or connectors
  • Replace the bad coolant temperature sensor

How Much Does It Cost To Fix Code P0128?

After finding the main issue, you need to know the cost to fix it. The fixing cost of the P0128 code depends on different factors such as labor cost, the area where you live, and your vehicle mode.  To fix the P0128 code, you need to replace one or more of the following parts:

Thermostat replacement$140 to $780
Cooling fan replacement$530 to $850
Engine coolant$30 to $100
Intake air temperature sensor replacement$70 to $250
Coolant temperature sensor replacement$170 to $360

How To Prevent P0128 Code?

Follow the following preventive tips to prevent the P0128 engine code:

  • Regular maintenance of the thermostat
  • Ensure sufficient coolant level in your vehicle
  • Drain the old coolant and add fresh coolant after the manufacturer’s recommended interval
  • Regular inspection of the ECT sensor
  • During colder weather, let your engine warm up properly before driving
  • It is recommended to drive consistently. Frequent short trips or stop-and-go driving may prevent your engine from warming up efficiently
  • Ensure regular maintenance of your engine
  • Don’t drive your car in extreme cold weather conditions

FAQ Section

Is code P0128 serious?

The P0128 code is not a serious trouble code. It doesn’t pose an immediate threat like engine overheating. However, you shouldn’t ignore this code and address it promptly. Driving with this code for a long time may lead to potential damage. It’s worth noting that running your engine below the optimal temperature can also result in increased fuel consumption.

Can P0128 be caused by low coolant?

Yes, insufficient coolant is a major cause of the P0128 code. This code is primarily triggered by a faulty thermostat, but low coolant levels may also lead to it.

Can I drive with a P0128 code?

Yes, you can drive with the P0128 code for a short time, but it’s recommended not to drive for long distances with this code. This issue won’t cause sudden vehicle shutdowns, but it will be clear through your temperature gauge not reaching the normal operating temperature. However, neglecting this issue for a long time may lead to the failure of expensive engine parts.

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