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P0128 Code – Engine Coolant Temperature Below Thermostat Regulating Temperature

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Seeing the check engine light come up on the dashboard is something most drivers fear. Why? Because in most cases it means that your bank account is about to take a hit. Some of the issues that trigger check engine light codes will be followed by other noticeable symptoms, but a CEL won’t always have an obvious cause.

The biggest problem with the check engine light coming on is the fact that you can’t know how many things are wrong with your car. This is why it’s important to have an OBD II scanner nearby.

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Speaking of codes that can creep up on you, P0128 is a common one. It is also a code that you definitely don’t want to ignore as it is related to your engine’s cooling system. Here’s everything you need to know.

What is the P0128 Code, and What Does It Mean?

The generic P0128 code stands for “Engine Coolant Temperature Below Thermostat Regulating Temperature”. This basically means that the coolant in the engine isn’t reaching the ideal operating temperature and that the engine is essentially running cold. While it’s not as bad as having the engine run too hot (for obvious reasons), a cold engine will wear faster. Here’s how it works.

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When you start your engine cold, the coolant stays in the engine block until it reaches a certain temperature before it starts circulating through the cooling system. At this time, the car is sending more fuel to the engine, so it would warm up quicker.

Once the engine gets to a proper operating temperature, the ECU starts reading the stats from oxygen sensors and regulates the air to fuel ratio accordingly for the best fuel economy. If the coolant doesn’t reach a certain temperature, the check engine light will come up on the dash with the code P0128.

P0128 Code VS P0126 – What is The Difference?

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Some fault codes are very similar to each other, and telling them apart can become tricky. This is the case with the P0128 code and the P0126 code. Although they sound similar, these two codes point to two different issues.

The fault code P0126 stands for “Insufficient Coolant Temperature for Stable Operation”, which means that the coolant hasn’t reached the operating temperature within a certain time frame.

Both of these fault codes point to a possibly faulty thermostat, but the P0128 code can also mean there’s an issue with the temperature sensors as well.

The fault code P0126 is usually followed by other symptoms, which can include increased fuel consumption, “limp-mode”, faulty heater, and abnormal temperature gauge readings.

Known Causes

The P0128 fault code is a pretty common one, and there are a couple of things that cause it. If you’re dealing with a P0128 code, make sure to inspect the following things components.

The first and most common cause for the P0128 code is a faulty thermostat. The thermostat can get stuck in an open position, preventing the coolant from ever reaching the optimal operating temperature. However, you should check the coolant level first as it can also trigger the P0128 fault code. While you’re at it, make sure that your car is using the right kind of colant. Using a wrong type of coolant can cause all kinds of coolant system issues.

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Although it’s less likely to happen, faulty air intake temperature sensors, coolant temperature sensors, and faulty radiator fans can cause the P0128 code to come up. The thermostat and coolant level should be the first thing to take a look at, but these other components should also be on your inspection checklist.

Common Symptoms

The first symptom that comes with the P0128 fault code is the check engine light. If you read the codes as soon as it comes up, you’ll save yourself a lot of trouble, but you’ll also save your engine from unnecessary wear.

The second symptom you’ll notice is that your car will reach operating temperature much slower or not at all. You’ll notice the temperature gauge slowly moving up and once you hit the highway, the engine temperature will suddenly drop below optimal levels.

Since the engine will struggle to compensate for the sub optimal operating temperature, you’ll also experience a hit on your fuel economy. Although most cars increase fuel consumption during cold weather to warm up the engine faster, you’ll notice a hit on your gas weekly gas budget.

The last symptom you should look for is a non-working heater. The heater directly takes heat from the engine to blow hot air into the cabin, so if the engine doesn’t get warm enough, you’ll find your car to be a chilly place in the winter.

How to Easily Diagnose P0128 Code

Different mechanics will approach diagnosing the P0128 code in different ways, but there are a few common things that all of them do. The first and easiest step is to connect an OBD II scanner to the ECU and read fault codes directly from the car.

This will tell you what triggered the CEL, but it won’t tell you exactly what caused it. To properly diagnose this code and find the failing part, you’ll need to get your hands dirty and take a look under the hood.

Start by checking the coolant level. The coolant tank is visible in most cars as soon as you pop the hood, so it’s very easy to inspect. If the coolant level is below the indicated minimum, make sure to top off the coolant until it reaches the recommended levels. With this out of the way, you can proceed to the next step in diagnosing this issue.

The second thing you need to do is check all the electrical connections of the temperature sensors. Every car vibrates to some extent while working and the connections become loose after a while, especially if you have an older car.

If the connections are all good, the next step would be to check the radiator hose and how fast it’s warming up. If the thermostat is working correctly, it should get very warm, very fast. We’re talking 10 minutes or so after starting the car. If the hose gets warm slowly, it means that the thermostat is stuck open, and you’ve most likely found the culprit behind your P0128 code.

What Kind of OBD II Scanner Is Needed to Diagnose P0128 Code?

Most, if not all cars require the use of an OBD II scanner to read fault codes. It’s recommended to use a quality OBD II scanner when working on cars, but you can also get away with using universal scanners that have flooded the market in recent years. Especially if all you want to do is read generic codes such as the P0128.

There are two main types of OBD II scanners available – handheld and Bluetooth ones. The difference is that handheld ones come with pre-installed software that’s used to read the fault codes, while Bluetooth OBD II scanners use Bluetooth technology for reading the error codes.

The Bluetooth OBD II scanners can be connected to any smartphone, tablet, or laptop with a Bluetooth device. These run generic software that will usually do the job just fine when it comes to the P0128 and similar codes.

How to Fix This Code?

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After correctly diagnosing the P0128 fault code, it’s time to get down and fix the issue that’s causing the check engine light to come up on your dash.

The first thing you want to do is let your car cool down before trying to fix anything. When idling, an engine can get hot despite failing to reach operating temperatures on the highway. Open up the coolant reservoir cap and inspect the level of coolant. If the level is under the minimum requirement, add more coolant so the level comes up to the maximum level. Take the car for a short drive and see if the same P0128 fault code comes up.

If topping off the coolant doesn’t help, the thermostat should be the next thing to replace. It’s the most affordable part next to the coolant and relatively simple to replace on most cars. Before taking it out of the car, make sure you know how to replace coolant in your car, it’s important.

Air pockets can cause the engine to overheat, which leads to warped cylinder heads and other wonderful things you definitely don’t want to deal with. If you’re not sure how to properly fill-up the cooling system with coolant, take the car to your mechanic to replace the thermostat and coolant for you.

If replacing the thermostat doesn’t help with the P0128 code, the next thing to replace would be the coolant temperature sensor. To replace the sensor, you’ll need to drain a certain amount of coolant until the switch is exposed. Next, you’ll need to disconnect the sensor and remove it from the manifold. The new sensor should pop right back in. Reconnect the sensor to the wiring and replace the missing coolant to the required level.

After doing this, delete the fault code from the ECU using your OBD II scanner and take the car for a test drive. Replacing these parts will most likely fix the P0128 code issue on your car.

Can You Drive with P0128 Code, and For How Long?

The P0128 fault code is pretty common and isn’t a very serious one on the severity scale. The only issue you can experience is the engine running colder than it should and increased fuel consumption. This isn’t something that will instantly damage your engine, but it will if given enough time.

Because there’s only one check engine light that is triggered by all sensors in your car, it’s important to address every fault code as soon as possible. Avoiding fixing the P0128 code can lead to other hidden issues that you might not be aware of.

If you don’t address the P0128 fault code right away, you’ll need to be careful. Whenever there’s any kind of problem with the cooling system, it can quickly go in any direction.

How to Avoid Dealing With P0128 In the Future?

The best way to stay ahead of this fault code is to use quality parts in your cooling system. A cheap aftermarket thermostat is often a gamble, despite the inherent simplicity of these devices. On the other hand, when you purchase a Genuine or OEM part, you’re getting a guarantee of performance. We suggest you go for the latter.
Additionally, only use the recommended type and make of coolant in your vehicle. Some car manufacturers are very strict when it comes to what coolant can be used in their cars, and for a good reason.

If you’re looking for quality parts to sort out your P0128 code, you’re in the right place. Here at eEuroparts.com, we offer one of the largest catalogs of Genuine, OEM, and quality aftermarket parts for European vehicles. To find what you’re looking for, simply head over to our store and select your vehicle using our car selection feature.

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