How to Troubleshoot & Fix P0128 Jeep Grand Cherokee DTC Code?

OBD II codes or Diagnostic Trouble Codes (DTCs) are one of the most common ways for mechanics to identify and diagnose car issues quickly. One such code is P0128, which has been reported to be a common issue in Jeep Grand Cherokees. 

In this post, we will look at what P0128 is, its causes, and how to troubleshoot and fix the issue. 

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What Does P0128 Jeep Grand Cherokee Code Mean?

P0128 is a generic trouble code that indicates your Grand Cherokee’s engine coolant temperature hasn’t reached the required temperature within a specific time after starting the engine. This denotes a faulty thermostat with an open circuit.

This code is stored when the Engine Control Module (ECM) detects the coolant temperature is below the specified limit, usually after the engine has been running for a few minutes. 

Here’s an outline of how the code relates to this issue.

• P = Powertrain, the unit that propels your vehicle.
• 0 = A generic, or “global,” code that applies to all OBD-II-equipped vehicles.
• 1 = Type of issue, with “1” indicating a manufacturer-specific code.
• 28 = Specific faults within fuel and air metering (Injector circuit malfunctions). In our case, the problem is with the engine coolant temperature sensor circuit, such as a low coolant level, a faulty ECT sensor, or a problem with the thermostat.

P0128 Code: What Causes Below Thermostat-Regulating Coolant Temperature on Jeep Grand Cherokee?

P0128 code has several potential causes, which you should know to specify the exact issue with your Grand Cherokee’s thermostat-Regulating coolant temperature and troubleshooting it accordingly.

A stuck open thermostat is the most common cause of this code. It prevents the coolant from reaching its normal operating temperature fast enough and triggers a low-temperature warning.

This can happen for several reasons, such as:

• Low or weakened coolant levels
• A faulty cooling fan
• Clogged or damaged radiator
• A faulty ECT (engine coolant temperature) sensor

Read Also: How to Troubleshoot & Fix P0113 Jeep Code? (Detailed Guide)

How to Troubleshoot & Fix P0128 Jeep Grand Cherokee Code?

Since an issue with your thermostat is the main cause of this code, you might be tempted to go at it right away. 

However, you should always go through the step-by-step diagnostic process and verify the root cause of the issue before replacing any parts. To do this, you should:

1. Check Coolant Levels

Low or weaker coolant levels can trigger the P0128 code in your Jeep Grand Cherokee because it can alter the engine running temperature, causing it to deviate from its ideal operating temperature. 

This can be detected by the engine coolant temperature sensor, which sends signals to the engine computer (PCM) about the engine’s temperature. 

When the engine coolant level is low, air pockets can form around the engine coolant sensor, leading to incorrect readings that may result in the P0128 code.

So, if your Jeep’s coolant levels are low, top it up and test drive the vehicle to see if the issue has been fixed. Also, check for leaks in the hoses since they can often lead to low coolant levels.

2. Inspect the Cooling Fan

A malfunctioning or stuck-on cooling fan in your Grand Cherokee can cool your engine excessively. And if the fan does not turn off, the engine temperature may drop below the expected operating temperature, leading to the P0128 code.

For example, if the engine expects around 200 degrees Fahrenheit to ignite, but the cooling fan keeps turning to keep it below 160 degrees Fahrenheit, this will result in a lower thermostat-regulating coolant temperature. 

Hence, you should check the coolant fan’s behavior before going about the thermostat. If it keeps running, it might be due to mass short circuits in wires, faulty relays, or ECU issues, which should be addressed.

Read Also: How to Troubleshoot & Fix U1110 Jeep Code? (Detailed Guide)

3. Inspect the Radiator and Flush It If Necessary

If your Jeep’s cooling system has been neglected, debris and dirt in the radiator may build up. This can interfere with the normal flow of coolant and reduce its ability to dissipate heat.

Also, the radiator may contain small air pockets, which can alter engine temperature readings and lead to the P0128 code.

So, you must inspect the radiator to check for debris, dirt, and air pockets. If you find it clogged, it’s best to consult a professional auto shop to flush the entire radiator system.

However, if the radiator is damaged, you may have to repair or replace it. This should be done with the help of a certified mechanic.

Read Also: How to Troubleshoot & Fix P0320 Jeep Grand Cherokee Code?

4. Inspect & Replace the ECT Sensor If Necessary

If the above steps fail to resolve the P0128 code, you should inspect and replace the ECT sensor in your Grand Cherokee.

The ECT sensor sends accurate engine temperature readings to your vehicle’s computer. When faulty, it may send incorrect signals, leading to a mixture of air and fuel that’s off-balance.

For example, a faulty ECT sensor can signal that the engine is cold when it is not, leading to more fuel added to the engine and decreasing fuel economy and engine performance. As a result, this incorrect temperature reading can cause the P0128 code to be triggered.

You can replace your Jeep Grand Cherokee’s ECT using a crescent wrench and unplugging the electrical connectors. However, depending on the year, you may need to remove the intake manifold and other components.

5. Check for Thermostat Issues

Finally, if all the above steps fail to resolve the P0128 code in your Jeep Grand Cherokee, you should inspect and replace the thermostat.

A faulty or sticking thermostat can allow the coolant to flow through independently, preventing your engine from reaching its ideal operating temperature. It leads to an incorrect temperature reading, resulting in the P0128 code.

You should remove the thermostat housing, inspect it for any damage, and replace it if necessary. It is an intricate and tricky task, so it would be advisable to have an expert mechanic’s assistance. Yet, the following video should give you an idea of how it’s done.

Read Also: How to Troubleshoot & Fix P0038 Jeep Code? (In-Depth Guide)

Frequently Asked Questions

If you’ve been troubleshooting the P0128 code in your Jeep Grand Cherokee and have further queries, here are some frequently asked questions and answers that may help.

Conclusion

Your Jeep Grand Cherokee displaying the P0128 code can signify many underlying issues. And you now understand what causes this error code and how to troubleshoot and resolve it.

However, If you feel it’s beyond your technical knowledge or skill set, don’t hesitate to hire a professional mechanic or certified technician.

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