Jeep P0128 – Engine Code Solution – Coolant Temperature is Below Regulating Temperature

What does the Jeep code P0128 mean?

If you used your scan tool on your Jeep and saw the engine code P0128, that means the coolant temperature is below regulating temperature. You might assume you can fix that problem by adding more coolant – but there’s more to this problem than that.

More often than not, you will not trigger a coolant related engine code because of low coolant. It sounds crazy, I know!

The thing is, responsible car owners (and I know you’re one; otherwise, you wouldn’t be here) keep the coolant levels in check the same way they deal with oil and other stuff.

So, what caused the code P0128? It could be a faulty sensor or a stuck thermostat. The latter is usually the case, but don’t rule out any sensor malfunction just yet.

Now that you know what’s wrong, you’re probably wondering if you can still drive your car. Well, you can – but you shouldn’t delay repairs more than you have to.

Temperatures are a delicate subject when it comes to cars, and appropriate levels are set for a reason.

And, you know, low coolant temperatures will have your car using more fuel than usual – and that will put a dent in your wallet!

Jeep Code P0128

Causes of code P0128

  • Stuck thermostat
  • Low coolant temperature
  • Faulty sensors

Symptoms of code P0128

  • Engine light is on
  • Poor fuel economy
  • Higher idle than usual

Fixes for code P0128

  • Repairing or replacing the thermostat
  • Replacing the coolant
  • Repairing or replacing the coolant sensor

Is it safe to drive with a code P0128?

It may or may not be safe to drive with a code P0128 depending on what caused it. Driving with a faulty sensor is far from a problem. At the same time, low coolant temperatures could damage your engine if you overwork your car.

Driving your Jeep with a code P0128 is going to feel like a bet – unless you know what’s wrong, that is.

If you don’t know what’s wrong, you’re betting something harmless is going on. What if you have something you should worry about, though?

What I’m trying to say is, you can drive your car like this. Nothing will happen on a short term basis.

You have to consider you may be overworking your engine – and that could lead to trouble.

It’d be better if you fix this problem first and drive your car second. Of course, that’s not always possible. You may have to drive to the repair shop, and it’s unlikely something bad will happen on such a short drive.

Then again, you shouldn’t postpone this for next month or even longer than that. At that point, if something bad could happen – it will happen.

So, sure, you can drive around with a code P0128, but I wouldn’t do it myself!

Why is my coolant temperature low?

There are a lot of reasons behind a low coolant temperature. You may have poor quality coolant or low levels of it. You could also have issues with a faulty sensor giving you a bad reading.

The only way to know what’s going on is to roll up your sleeves and check the different parts that could be malfunctioning.

If you’re worried about code P0128, you should know low coolant temperature is not the only thing that can trigger it. Don’t worry. I’ll explain everything down below.

How do I fix the code P0128?

Most Jeeps who are having trouble with a code P0128 have a stuck or faulty thermostat; replacing it will fix the issue. Sometimes, a faulty sensor will cause this code – and you will have to change that instead.

On rare occasions, low coolant may be the root cause of it all, but it’s far from likely.

As usual, I’m going to recommend you start troubleshooting from easiest to hardest issue to solve.

Check the coolant levels

You should start checking the coolant level. I know I said it’s probably something else, but you should consider two things: one, it’ll never hurt to replace your coolant; two, you may solve this issue by doing that.

Check the coolant level and its condition. Poor quality coolant may clog your thermostat and can cause plenty of damage elsewhere – make sure you get the good stuff.

Run your scan tool. If the code persists, it’s something else.

Check the sensors

There are two possible issues left. It’s either the thermostat or a faulty sensor. The best way to pinpoint the issue is to use a multimeter and check the coolant temperature.

If the multimeter shows a good reading (i.e., the coolant temperature is actually okay), that means the sensor is malfunctioning (and showing you the wrong numbers).

Repair or replace your coolant temperature sensor, then run the scan tool.

Check the thermostat

If the problem persists, you have a stuck thermostat. You can verify that yourself by checking the radiator hose.

When you start the car, the radiator hose is going to be cool. Once the car starts to rev up, hot coolant will flow out of the thermostat, warming everything up quickly.

If the radiator hose is lukewarm or cold five minutes after you’ve started your car, you have a stuck thermostat.

How do I fix a stuck thermostat?

Replacing a thermostat is easy enough that anyone can do it – but somewhat dangerous enough for most to consider asking a mechanic to do it. You should troubleshoot the thermostat before replacing it, though.

I’ll say two things before you begin working: one, if you never worked on a car before, go see a mechanic (replacing a thermostat isn’t beginner friendly); two, make sure you get a bucket nearby – because fluids will spill.

Okay, I’ll say three things. Number three, you should check if your thermostat isn’t clogged or something. Other than that, let’s get to work.

How to replace a thermostat

  1. Locate your thermostat. If you don’t know where it is, follow the radiator hose all the way to the engine. It’s usually there.
  2. Remove the hose clamp and pull off the hose (here’s where you’ll need the bucket!). Then, remove the bolts from your thermostat.
  3. Remove the old thermostat. Take out the old gasket too.
  4. Make sure both the old thermostat and gasket look like the new thermostat and gasket. Otherwise, you bought the wrong ones.
  5. Put the new gasket in place. Do the same with the thermostat.
  6. Secure everything, including the gasket, thermostat, hose, and hose clamp.
  7. Add new coolant and call it a day!

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