P1128 Trouble Code – Meaning, Causes & Solution

If you see the P1128 code pop up on your dashboard, you may be wondering what’s wrong.

You may notice symptoms such as reduced engine power or decreased fuel economy. The overall driving experience would become poorer.

This article will provide information about this code, what it means, and how to fix it.

What Does Code P1128 Mean?

To put it as simple as possible, a P1128 code indicates a problem with the information the engine receives from engine sensors, preventing it from running properly.

Here is a more detailed explanation of how the fuel management system works.

The fuel management of the vehicle maintains the rate of fuel consumption by the engine. These systems are so designed as to ensure effective regulation of the fuel.

This system operates in a closed cycle. The feedback is provided through the oxygen sensor. All these sensors transfer the information to the Powertrain Control Module (PCM). The PCM, acting as fuel feedback, has the responsibility of regulating the fuel mixture.

Thus, it depends on the input received from these sensors. This whole cycle goes from a rich mixture to a poor mixture. This helps in achieving the maximum efficiency of the catalytic converter.

However, an open loop situation is sometimes received. This happens when the O2 sensor provides no signal. One common situation is igniting the vehicle due to cold weather.

In such cases, the powertrain control module will forward the command of a comparatively rich furl mixture. When the open-loop conditions continue to exist for more than a defined period of time, the code will be set.

P1128 is an OBD II generic code. It has multiple symptoms and causes. Read on.

Understanding P1128- Terms and Meaning

Let us have an understanding of the following terms. These will assist you in getting what this code means.

PCM: The powertrain control module (PCM) is a key part of the modern automobile. It is responsible for controlling the engine and transmission, among other things. The PCM receives input from a variety of sensors in the car and uses that information to make decisions about how to best power the vehicle.

This can include things like fuel economy, emissions, and performance. The PCM is also responsible for monitoring critical systems like the engine and transmission, and it will take action if it detects a problem.

Catalytic Converter: A catalytic converter is a device used to convert harmful emissions from a car into less harmful emissions. The catalytic converter is present in the exhaust system of the car.

The catalytic converter uses a catalyst, usually platinum or palladium, to help convert the emissions. The catalyst causes a chemical reaction that breaks down the harmful emissions into harmless gases.

Is P1128 A Major Issue?

P1128 can result in the malfunctioning of the engine. From increasing the rate of fuel consumption to causing the vehicle to stall, it can be quite dangerous.

Thus, you should definitely diagnose the issue as soon as possible. If it is not possible to do it on your own, it is strongly recommended to visit a mechanic.

Common Symptoms of P1128 OBDII Code

This generic code comes with multiple symptoms, along with illuminated light. Plunge into the following to have a deep understanding of what is happening with your vehicle.

Rough Engine Idling

One common symptom of the P1128 OBDII code is poor idling quality. This can result in the engine idling roughly, the car shaking, or even stalling altogether. In severe cases, it may be difficult to start the engine at all.

If your engine is idling poorly, it can cause a lot of problems, including excessive wear and tear of the engine, decreased fuel efficiency, and problems with the emission control system.

Increased Fuel Consumption

If you are having to refuel more often than usual or your gas tank is emptying more quickly, you may be experiencing symptoms of the P1128 OBDII code.

When the air-to-fuel ratio is not correct, the engine may not be able to operate as efficiently as it should, leading to decreased fuel economy.

Stalling Engine

OBDII code P1128 can cause a car to stall. The incorrect air-fuel ratio can lead to this issue. Many drivers have experienced a car stall while driving. This can be a dangerous scenario, as it can cause a car to lose power and control.

Foul Spark Plugs

Spark plugs are a few of the most vital components in your car. They provide the spark that ignites the air-fuel mixture in the engine, and this process continues while the engine is running.

If your car isn’t getting as good mileage as it used to, the reason might be foul spark plugs. Poor acceleration or lack of power also is caused by foul spark plugs.

P1128 Code – Potential Causes and Diagnosis

There are several causes why the P1128 code pulls up. It is crucial to be aware of all of them to perform a diagnosis.

Faulty Oxygen Sensor

The oxygen sensor reads the concentration of unburned oxygen in the exhaust and sends that information to the car’s computer. When this sensor is starting to fail, it can cause the P1128 OBDII code to be thrown.


To ensure if the problem lies with the oxygen sensor, you can do the following.

  • Measuring how much voltage is being output by the sensor
  • Measure how much air is flowing through the sensor.

Abnormal Fuel Pressure

Fuel pressure is an important part of the engine management system. The fuel pressure regulator maintains specific fuel pressure in the fuel lines. The powertrain module then uses this information to calculate the correct amount of fuel to inject into the cylinders.

When the fuel pressure is too high or low, it can cause a variety of problems, including the P1128 OBDII code.


If you experience any of these symptoms, it may be indicative of abnormal fuel pressure:

  • The engine fails to start
  • Engine stalls frequently
  • Hesitation while accelerating
  • Rough idle
  • Black smoke coming from the exhaust pipe

If your vehicle is exhibiting abnormal fuel pressure, the first step is to check the engine’s vacuum. A low engine vacuum can be an indication of a clogged fuel filter or a leak in the vacuum line. The next step is to check if there is a spark knock or excessive black smoke.

Shorted Wirings or Damaged Connections

One common cause of the P1128 OBDII code is a short in the wiring or a damaged connection. This can be caused by a variety of things, such as corrosion, wear and tear, or accidental damage.


  • To diagnose if damaged connections are the issue.
  • Start by checking the fuses.
  • If any of them are blown, that’s your first clue that there’s a problem.
  • Then check all the connectors and wires for damage.
  • If you find any damage, replace the connector or wire.

Malfunctioning MAP Sensors

If your car’s computer is displaying a P1128 OBDII code, it means that the vehicle’s MAP (manifold absolute pressure) sensor is not working properly. The sensor may be dirty or blocked, or the wiring may be damaged. It’s also possible that the sensor itself has failed.


  • To check the malfunctioning MAP sensors, the first step is to check the sensor’s wiring and connectors.
  • If everything looks okay, then the next step is to test the sensor’s output.
  • This can be done with a voltmeter or an oscilloscope.
  • If the sensor is found to be defective, then it will need to be replaced.

Failed PCM

Failed PCM is another major cause of this error code. A PCM failure can be caused by several factors, including a faulty power supply, corrosion on the circuit board, or a bad chip.

If your vehicle has been experiencing problems with starting, stalling, or other electrical issues, it may be worth checking the PCM for signs of failure.

PCM plays a crucial role in controlling the engine and transmission, and when it fails, the vehicle will not operate correctly.


You can diagnose failed PCM with a voltmeter or an electronic scan tool. If you find that the PCM is not getting power, or if you find a short or an open circuit, then the PCM is likely bad and needs to be replaced.

How Can I Fix P1128 – Complete Insight?

After a thorough diagnosis of the issue, now is the time to fix P1128. This code should be dealt with as soon as possible. However, you should not directly replace the PCM. It is wide to dig into the problem deeper. Adopt the following measures to fix the code.

  • Checking the connections
  • Inspecting the wire harness
  • Changing the corroded pins
  • Replacing the faulty oxygen sensor
  • Setting the right fuel pressure
  • Changing the MAP sensors
  • Replacing PCM

Possible Measures to Avoid P1128 From Showing Up Again

You do not want to go through all the fuss of diagnosing and fixing the issue, right? Not only does it cost you a lot but also demands physical effort. Thus, you should take the following measures to prevent it from happening again.

  • Take your vehicle for a regular sensor check
  • Ensure tight wire connections
  • Make sure that the fuel pressure is right