How to Troubleshoot and Fix P1128 Jeep DTC Code? (Detailed Guide)

If your Jeep has a P1128 code, the legal authorities might keep you from driving it. Because one of the many implications of the code is that your vehicle has become a driving hazard. 

So, you should detect and correct the reason for the code as soon as possible. Not only for your safety and the environment’s sake but also for the Jeep’s engine components. 

What Does P1128 Jeep Code Mean?

The P1128 jeep code means the vehicle could not achieve closed-loop fueling after starting the engine. 

If your Jeep runs on a six or eight-cylinder engine then the site of the error would be in Bank-1. 

Your Jeep’s fuel management system comprises the powertrain control module, a variety of sensors, pressure regulators, pumps, filters, and fuel injectors. 

All these components enter a closed loop shortly after starting the engine for precise fuel management and optimal performance. 

An error or dysfunction in any of these components will trigger the code P1128. Let’s break it down

  • P = Diagnostic Trouble Code for Powertrain issues
  • 1 = The number for manufacturer-specific codes. 
  • 1 = Fuel and metering problems 
  • 28 = Specific fault index (in this case, it indicates the fuel management system’s inability to reach a closed loop.)

Read Also: How to Troubleshoot & Fix P1D73 Jeep Cherokee Code? (Detailed Guide)

P1128 Code: What Causes the Closed Loop Fueling Error on Jeep?

When you first start the engine of your Jeep it uses an open-loop system to manage fuel distribution. 

In other words, the PCM waits for a short and specific amount of time for the sensors and other components to warm up so that it can enter the closed loop. 

If this time threshold is crossed and still the closed loop isn’t achieved, the PCM will store the code. This can occur for a variety of reasons, making P1128 one of the most ambiguous OBD codes. 

  1. A faulty Oxygen sensor
  2. The intake manifold absolute pressure sensor is malfunctioning 
  3. The engine coolant temperature sensor is at fault. 
  4. Any of the numerous wirings that connect these sensors to the PCM is compromised
  5. Fuel pressure is not within the normal range (for a variety of reasons) 
  6. Mechanical defects in the engine 
  7. The fuel injectors are out of order
  8. A leakage or defect in the engine’s intake or exhaust system.
  9. Defect in the powertrain control module.

How to Troubleshoot & Fix P1128 Jeep DTC Code?

To fix the P1128 code you have to first reach a correct diagnosis. It’s quite tricky as one or multiple components of the fuel management system might be failing. 

You should start with the upstream O2 sensor, as it’s the most common culprit. The rarest diagnosis is a faulty PCM. 

1. Fix Your Jeep’s Oxygen Sensor

The oxygen sensor, especially the upstream oxygen sensor, is the major reason for the appearance of a P1128 code. This sensor measures the oxygen level in your engine’s emissions. 

It then sends the signal to the PCM, which in turn adjusts the duration of the fuel injectors being open. 

When the oxygen sensor is destroyed or its connection is corroded, it can’t send the signal. 

Thus the engine is unable to achieve a closed loop fueling and triggers the code. Usually, time, vibration, heat, and stress factors wear down the oxygen sensor. 

Sometimes the heating elements of the oxygen sensor suffer damage. In this case, the sensor is fine but it can’t heat up to the required temperature for its activation. This also results in an open loop system.

A partially damaged sensor would send wrong information about the oxygen levels in emission to the PCM. The PCM will think that the air-fuel mixture is too lean and try to correct the situation.

But the malfunctioning oxygen sensor will keep telling the PCM that the oxygen level in the emissions is high. which, to the PCM, means the air-fuel mixture is still lean.

After multiple attempts at correction, the PCM will diagnose the lean mixture situation as a failure to achieve closed-loop fueling and trigger the code.

You will find the upstream Oxygen Sensor on the front of your vehicle’s catalytic convertor. The position of the sensor is slightly to the left.

If cleaning or repairing the wires doesn’t solve the code you have to replace the sensor. That would cost you around 250$ in parts and labor.

Read Also: How to Troubleshoot & Fix P0306 Jeep DTC Code? (Detailed Guide)

2. Repair or Replace Your MAP Sensor

If you find that the oxygen sensor in your Jeep is not the culprit check the MAP sensor

The intake manifold absolute pressure sensor measures the pressure of the air entering your engine’s cylinder and generates a signal for the PCM. 

The PCM analyzes the signal it receives from the MAP sensor. This helps the system to get an idea about the engine’s load and driving conditions. Based on that, it signals the fuel injectors to be ‘ON’ or ‘Off’. 

A faulty MAP sensor will convey the wrong information. As a result, the PCM might keep the injectors OFF for too long. This will reduce the amount of fuel in the mixture. 

Consequently, the Oxygen sensor will tell the PCM multiple times that the mixture is too long. 

When the PCM fails to correct the lean mixture after several attempts, it will detect an open loop system. And so, it will store the P1128 code. 

You can find the MAP sensor mounted on the engine’s intake manifold. Sometimes welding and replacing the faulty connections will solve the problem. 

If not, you have to replace the sensor. Putting in a new MAP sensor in your Jeep costs around 124-161$. 

3. Check Your ECT Sensor

When you find the oxygen and MAP sensors in acceptable conditions, check the ECT sensor. This component measures the temperature of the engine coolant and generates an appropriate signal for the PCM. 

This helps the PCM to ensure the engine is operating at proper temperatures. Based on what it learns about the engine’s operating temperature, the PCM adjusts the fuel injectors. 

For instance, if the ECT sensors tell the PCM that the temperature is too low, the engine shall try to manage the situation by keeping the injectors open for longer. And vice versa.

Now, when the ECT sensor is damaged by various stress factors, it sends the wrong information about the engine’s temperature to the PCM. 

This can also happen when there is a thermostat malfunction or coolant leakage. Compromisation of the harness that connects the ECT to PCM can also be the cause. 

As a result, there is an incorrect triggering of the injector. Which makes the O2 sensor detect a rich or lean condition. 

And subsequently, the PCM records a closed-loop achievement error after failing to correct the rich or lean condition several times. 

You will find the ECT sensor on the engine block. If the sensor seems fine, check for coolant leakage and connection damage. Installing a new engine coolant temperature sensor will cost you around 350$.

Read Also: How to Troubleshoot and Fix P0430 Jeep DTC Code? (Detailed Guide)

4. Correct Abnormal Fuel Pressure in Your Engine

If you still haven’t found the cause, check the fuel pressure situation in your Jeep. An abnormally high or low fuel pressure confuses the PCM. 

The module ends up messing up the air-fuel ratio, detects an open loop, and subsequently stores the code. 

The fuel pressure regulation system consists of a lot of working parts. Such as the fuel pressure regulator, fuel pressure sensor, fuel pump, and fuel filter. 

An issue with any of these components can cause abnormal fuel pressure in your jeep. The fuel pressure regulator is a spring and diaphragm system. 

When the fuel has abnormally high pressure the diaphragm responds by restricting the entry of fuel into the system. In the case of low pressure, it responds the other way. 

Dirt, debris, and mechanical wear and tear can damage the regulator and cause it to malfunction. 

Apart from the regulator, a clogged fuel filter, a bad pump, and a destroyed fuel-pressure sensor can disturb fuel pressure. You should also exclude the connection problems. 

To correct the abnormality you must clean the filter and replace the damaged parts. 

Putting in a new fuel pressure regulator will cost you 250-300$. You should also use good quality fuel to prevent these issues in the future.

5. Repair Fuel Injectors, PCM, and Leakages

If you find nothing wrong with the sensors and fuel pressure regulation system, then all that’s left is the PCM and Injectors. 

A faulty injector wouldn’t be able to open and close according to the PCM’s instructions. As a result, it can create a lean mixture and trigger the code. 

Low-quality fuel is usually the main cause of fuel injector damage. Your OBD scanner might show a P0200 alongside the P1128 when the injector is at fault. 

A leakage in the intake or exhaust system can also trigger the code. Leakages alter the air-fuel ratio and can result in the PCM detecting a closed-loop fueling error. Repairing the leaks shall solve the code in this case. 

The rarest cause for the code is a faulty PCM. You can look into it after excluding the other causes. (  

Power spikes and contamination can scramble the PCM’s programming and make it send faulty signals to the injectors. 

It will also make wrong analyses of the data it receives from the sensors. Replacing the damaged fuel injectors in your jeep can cost around 350$. 

Meanwhile, installing a new PCM can put you down for 700$. For professional inspection and repairs of leaks, you may have to spend around 200$.

Read Also: How to Troubleshoot and Fix P2017 Jeep Patriot DTC Code?


We hope that the above discussion was able to provide answers on fixing the P1128 code in your Jeep. There might still be some queries on your mind. Feel free to ask here.

Can you drive a Jeep with the P1128 code?

The P1128 code is accompanied by the vehicle shaking, slowing down, or coming to a complete stop. You will also start losing fuel. Plus, the faulty emissions will make your Jeep unfit for the environment. So, no, you cannot drive a Jeep with the P1128 code. 

How much does it cost to fix the Jeep P1128 code?

Unless it’s the PCM which costs around 700$, fixing the other causes of P1128 stays within the 200-330$ range. But if multiple parts need repairing or replacement, the costs might add up.

What can I do to prevent a P1128 code in my Jeep?

Use high-quality fuel to prevent clogging in the fuel pressure management system. Good fuel also keeps the fuel injectors in their best shape for a long time. Lastly, regular servicing of the engine and inspection and correction of leaks in the system will also help.


The jeep trouble code P1128 can be quite challenging. There are an overwhelming number of connections and parts to deal with. 

One way to reduce time and effort is to check for other accompanying codes in your OBD scanner alongside the P1128. Following the serial as mentioned above also helps.

Read Also: How to Troubleshoot & Fix P0123 Jeep DTC Code? (Detailed Guide)

Scroll to Top