P0128 Maintenance Code : Causes, Symptoms, Severity, Fix and Cost

When the MIL (Check Engine) light comes on without any obvious symptoms, many drivers choose to ignore it. As a result, the MIL light can only tell you that there is an issue, and not how many problems the ECM/PCM has detected. It is always better to look into any detections as it ensures a safe drive. This blog is about P0128, a maintenance code that you might neglect but can backfire big time if you choose to do it. 

What is P0128? 

P0128 – Coolant Temperature Below Thermostat Regulating Temperature means that the engine does not reach the expected temperature after a sufficient amount of time, or that it takes too long to warm up. The engine control module (ECM) keeps track of how long it takes for a vehicle’s engine to reach and maintains the proper operating temperature. As soon as the proper operating temperature of the engine is reached, the powertrain control module does its work. It instructs the fuel system to enter a “closed-loop” where oxygen sensor data is used to maintain the efficient air to fuel proportion (14.7:1).  

Within 15 minutes of the engine beginning, most ECMs require the engine coolant temperature sensor to register temperatures above 160º F. Additionally, once the engine temperature reaches 160º F, it must not drop below that level while running. If neither of these two conditions is met, the PCM will record. Code P0128 is set off if either of these problems occurs again during the next start-up of the engine. In cold temperatures, this code frequently causes the Check Engine light to flash on and off. 

What are the possible causes of the P0128 code? 

• Faulty or sticky thermostat 
• Thermostat seal not sealing properly, allowing coolant to bypass a closed thermostat 
• Faulty engine coolant temperature (ECT) sensor 
• Engine computer (PCM) software needs to be updated 
• Low coolant level 
• Radiator fan does not turn off 
• Faulty intake air temperature (IAT) sensor 

What are the common symptoms of the P0128 code? 

• Check Engine Light is on 
• Higher than normal idle 
• Decreased fuel economy 
• The temperature gauge is unusually low 

How to fix error code P0128? 

The most common reason for error code P0128 is a faulty engine coolant thermostat, so the first step in troubleshooting it is to see if it is open or leaking. 

1. Start the vehicle and place your hand on the radiator hose after it has been totally cooled down. If everything goes according to plan, the hose will get quite hot. The thermostat may be stuck or leaking, in which case it has to be cleaned or replaced. 
2. Check your coolant levels and coolant system condition as well if that’s the case. The thermostat might become stuck if the coolant is dirty. 
3. Next, check your coolant levels and system to see if it’s getting hot fast. You should also check the radiator fan to determine if it is stuck on or if a hose section has collapsed.  
4. A defective coolant temperature sensor could cause your car to overheat, even though the coolant system appears to be in working order. Using an infrared thermometer, have a look at the readings your scan tool is providing you. Around 200° F (93° C) is considered normal. 
5. Make sure the sensor’s wiring and connectors are clean or replaced if they don’t match the results. Clear the code and start the car. If the error code recurs, a new sensor may be required. 

If the problem persists after you replace the sensor, you may have to get a new PCM, however, this is an extremely rare cause of code P0128. 

Is Code P0128 serious?  

It is not necessary to fix your car right away when you detect the error number P0128. As a result of its potential impact on fuel economy and emissions, this problem requires immediate attention. Engine damage might occur if the coolant temperature is too high for an extended period of time. Depending on what set off the code, damage may or may not be done soon. If the problem persists, it’s best to get it checked out as soon as possible rather than parking the vehicle or heading straight to the shop.  

How long can you drive with a P0128 code? 

Despite the fact that you may be able to drive for short distances with code P0128 set, you should have the problem rectified immediately by a professional. As a result, you’ll be able to keep your car in top condition and avoid any more damage. 

How much does it cost to fix code P0128? 

You’ll get an hour of “diag time” if you bring your automobile to a shop for a diagnosis (the time spent in labor diagnosing your specific issue). This can cost anywhere from $75 to $150, depending on the shop’s hourly charge. In many cases, if not all, businesses will waive the diagnostic price if you have them handle any subsequent repairs. Repairs for your P0128 code can be estimated from this point onward by a shop. 

One or more of the following fixes may be required to resolve the underlying issue when dealing with the P0128 error code. The estimated cost of repair includes the cost of needed parts and the cost of labor for each conceivable repair. 

• Loose/damaged gas cap 
• EVAP hoses 
• Fuel tank 
• Charcoal canister 
• Faulty vent control valve 
• Faulty purge control valve 

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