P1128 Code: Closed Loop Fuel Not Achieved Bank 1

Your ride’s fuel system delivers fuel to the engine as needed. It has critical components like the fuel pump, fuel tank, fuel filter, and injectors. If a problem develops in the fuel management system, your vehicle might log a P1128 code.

What Does the P1128 Code Mean?

Diagnostic trouble code (DTC) P1128 stands for “Closed Loop Fuel Not Achieved Bank 1.”

This code is logged when the powertrain control module (PCM) detects an issue in the fuel system that’s causing it to stay in open-loop operation within the timeframe set by your manufacturer.

Code P1128 can point to an issue with the sensor, the wiring or the PCM. In some cases, other factors affecting the air-fuel mixture can cause the entire system to remain in open loop but there will typically be noticeable symptoms in a case like this.

“Bank 1” means that the issue is in the first bank of cylinders in a V-type engine. A P1128 code can also indicate that the heated oxygen (O2) sensors in your engine are malfunctioning.

A Quick Note About Closed Loop and Open Loop Control

Closed loop fuel control is achieved very soon after the engine starts due to oxygen sensor heaters bringing the sensors online as quickly as possible. But wide open throttle (WOT) is an open loop mode by default due to the engine’s need for maximum fuel delivery at that point.

Closed loop fuel control is achieved very soon after the engine starts due to oxygen sensor heaters bringing the sensors online as quickly as possible. But wide open throttle (WOT) is an open loop mode by default due to the engine’s need for maximum fuel delivery at that point.

–Richard McCuistian, ASE Certified Master Automobile Technician

Note: The definition of code P1128 may be different depending on the vehicle manufacturer. Consult the appropriate repair manual or repair database for the exact code definition.

What Are the Common Causes of the P1128 Code?

• Clogged fuel pump internal strainer
• Defective upstream O2 sensor
• Contaminated O2 sensor element
• Abnormally high or low fuel pressure
• Problems with wirings
• Bad MAP or MAF sensors
• Malfunctioning engine coolant temperature sensor
• Excessive oil consumption
• Intake air temperature sensor issues
• Malfunctioning PCM

What Are the Common Symptoms of the P1128 Code?

• Illuminated check engine light
• Decreased fuel efficiency
• Poor engine performance
• Rough idling
• Engine misfire
• Failed emission test

How to Diagnose the P1128 Code

The P1128 is a manufacturer-specific code, so there’s no universal diagnostic procedure that would work for all vehicles. After all, vehicles can be structured differently depending on their manufacturer.

If you’re not sure how to diagnose the issue, it’s best to take your vehicle to an auto repair shop for a proper diagnosis. Otherwise, you can do it yourself.

How to Fix the P1128 Code

Resolving a P1128 isn’t easy, especially if you don’t have in-depth knowledge of the fuel management system. Your best option is to let a licensed mechanic fix the problem for you.

But if you’re confident that you can resolve the trouble code on your own, make sure to read up about this trouble code before you do anything with your vehicle. Resources like Chilton can give you vehicle-specific information that can help you fix code P1128.