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Nov 02, 2024

P0128 Code: What It Is And How To Fix It

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There can be many reasons why a Check Engine Light (CEL) illuminates. The reason, cost, and time to repair can vary widely. If your vehicle is displaying the P0128 fault code (Coolant Temperature Below Thermostat Regulating Temperature), your engine may be suffering from a loss of power or fuel efficiency. A defective sensor, malfunctioning thermostat, improper fan operation, or poor coolant circulation could all cause the P0128 code to be set. In this article, we'll delve into greater detail about what this error code is and how you can fix the issue causing it.

Code Number

Code Description

Related Codes

P0128

Coolant Thermostat (Below Regulating Temperature)

P0125 - Insufficient Coolant Temperature P0127 - Intake Air Temperature (IAT) Too High

The illuminated check engine light is designed to alert the driver of a malfunction or fault with the vehicle's powertrain. The severity of these faults varies, and you should have your car assessed as soon as possible to identify the problem area. The Powertrain Control Module (PCM) can set the P0128 code if the computer identifies the engine did not reach the specified operating temperature despite the sufficient warm-up time elapsing. Essentially, your engine is running cooler than it should.

The operating temperature specified by the manufacturer varies by year/make/model. Commonly, the temperature ranges from 170 to 210 degrees Fahrenheit, but always consult the proper service manual for your vehicle's specified operating temperature. The engine's temperature is regulated by the thermostat; the thermostat commonly uses a wax pellet design that controls the flow of engine coolant based on its temperature. The thermostat on some European vehicles is electronically controlled. These can suffer from their own set of failures, including the wiring, PCM, or the component itself.

If the P0128 code has been logged by your PCM, your vehicle can display multiple symptoms or faults. It's advisable to ensure that no other Diagnostic Trouble Codes (DTCs) have been set.

The P0128 code can be set while the PCM is monitoring engine warm-up. The PCM draws from a few key pieces of data during this time to maximize engine efficiency, including the Engine Coolant Sensor (ECT), air-conditioning settings, Intake Air Temperature Sensor (IAT), engine load, and engine run-time. After all this data is observed, the PCM will calculate when the vehicle will reach the desired temperature. If the temperature is not reached within the calculated time, the PCM will set the P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) DTC. The most common repair reported to fix the P0128 DTC is a worn thermostat (which opens too early). The following components can result in this error code:

*Will most likely set other DTCs

The symptoms of a P0128 error code are not always completely noticeable, and in most cases, the only issue reported by customers is an illuminated check engine light. The following are potential symptoms your vehicle may exhibit with the P0128 fault:

This code indicates the engine is most likely running cooler than it should, but it shouldn't affect the ability to drive your car. However, a coolant leak can potentially cause this code to be set. If this is the case and you continue driving, the vehicle will eventually no longer have enough coolant to prevent the engine from reaching dangerous temperatures. Therefore, we recommended addressing the P0128 fault code sooner rather than later.

You can begin by confirming the DTC is "active." To do this, have your vehicle's On-Board Diagnostic (OBD) system scanned for fault codes. This service can be performed by a local auto parts store, a repair facility, or you can even perform the scan yourself if you possess an OBD-II scan tool. You can pick these up relatively cheaply today, and you even have the option of purchasing an adapter to use with a smartphone app.

Once you've confirmed your vehicle has stored the P0128 fault code, you can begin by performing a few simple visual cooling system checks.

Start by checking your coolant level and type. If your vehicle's coolant was mixed with the incorrect type, the system will need to be flushed as this can be a cause for the fault code. Today's vehicles specify coolant changes around the 110,000-150,000-mile mark and your vehicle may be due for a coolant change. It's helpful to know that manufacturers use different colors to identify the type of coolant used in the engine. For example, a 2022 Toyota Corolla specifies Toyota Super Long Life Coolant - this coolant is pink in color and should not be mixed with any other color coolant. Another example is the 2022 Nissan Versa, which requires Genuine Nissan Long Life Antifreeze/Coolant. This coolant's color is blue.

Most coolant overflow tanks have easily identifiable markings, such as "Min" and "Max" lines or Full (F) and Low (L) lines. It is important not to overfill a cooling system as this can cause additional problems.

CAUTION: Never open a "Hot" pressurized overflow tank as this can result in severe burns.

Once you've verified the correct coolant level and type, you'll want to check the system for any visible signs of leaks. Carefully inspect all cooling hoses and their condition. A cooling hose should feel firm. If you feel a soft spot in the hose, it should be replaced, as the interior structure can be compromised.

If no visible leaks are detected and you have access to a cooling system pressure tester, you'll want to perform a cooling system pressure test. The testing spec is listed on the radiator pressure cap or the pressurized overflow tank cap. In the case of this Toyota Camry's radiator pressure cap, the system pressure specification is 108 kPa (15.6 psi).

Important Pressure Testing Tips:

If the system pressure drops, a leak is present in the system and further inspection is required. Common areas reported for leaks are the radiator, thermostat housing, cooling hoses, and at the heater core.

To replace the thermostat, you will need the following tools:

Some of the steps to replacing a thermostat include the following:

Like most other jobs and services, the cost to repair a P0128 fault code will vary depending on the specific fault, year, make, and model of the vehicle.

Using the MotoLogic software, we priced out a thermostat replacement service for a 2020 Jeep Wrangler. This is a common issue seen on the 3.6-liter V6 engine fitted to these Jeeps. The cost of a thermostat replacement comes in between $250 to $400 but can be higher.

If you have a cooling system leak, you can expect a low repair cost of $100 for simple hoses or a leaking pressure cap, with a high of $1,400 or more for more serious repairs such as a head gasket.

Cooling system flushes are moderately priced between $150 and $250. A coolant temp sensor for a 2019 BMW 330i costs about $50 for the part, with reported labor times exceeding five hours. You could end up with an unexpected repair bill exceeding $600. However, if we look at a 2019 Acura RDX for comparison, a similar service will run around the expected average of $250.

Ultimately, if your vehicle is showing a P0128 fault code, you may have some time until you need to address it, as long as your cooling system is not leaking. Most thermostat replacements can be performed relatively quickly at a cheap cost, especially if you manage the repair yourself.

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