|Trouble Code||Fault Location||Probable Cause|
|P1128||Manifold Absolute Pressure Sensor Signal Lower Than Expected (ACURA, HONDA)
Long Term Fuel Trim B1 System Too Lean (AUDI, VOLKSWAGEN)
Closed Loop Fueling Not Achieved Bank 1 (CHRYSLER, DODGE, JEEP)
Front O2 Sensor Connectors Swapped (FORD, LINCOLN, MERCURY)
Fuel System Lean Part Load (HYUNDAI)
Throttle Control Motor Circuit Short (INFINITI, NISSAN)
MAP Sensor Voltage Lower Than Expected (ISUZU)
Throttle Control Motor Lock Malfunction (LEXUS, TOYOTA)
HO2S Heater O2 Sensor Too Cool Before KOER Test (MAZDA)
We recommend Torque Pro
What Does Code P1128 Mean?
OBD II fault code P1128 is a manufacturer-specific trouble code that is defined by carmakers Chrysler, Dodge, and Jeep* as “Closed Loop Fueling Not Achieved- Bank 1”. On these applications, this code sets when the PCM (Powertrain Control Module) detects a condition in which the fuel management system remains in open-loop operation for a period that exceeds a maximum allowable threshold. Note that “Bank 1” on V-type engines refers to the bank of cylinders that contains cylinder #1.
* Be aware that not all online resources list this trouble code. Nonetheless, resources that do list this code use the definition as given above.
One of the principal strategies that make it possible for modern engine management systems to exert precise control over fuel consumption and exhaust emissions involves the use of input data from various sensors in real-time to make appropriate adaptations to injector pulse widths. In simple terms, an injectors’ pulse width is its “ON” time, or put differently, the amount of time it is opened to inject fuel into a cylinder during an injection event.
However, the PCM cannot monitor the ratio of air to fuel directly, and therefore, it obtains input data from the oxygen sensors that measure the level of oxygen in the exhaust stream. In practice, and in a fully functional fuel management system that is in closed-loop operation, the level of oxygen in the exhaust stream is directly proportional to the quality of the air/fuel mixture. Thus, the PCM monitors this parameter continuously via the upstream* (of the catalytic converter) oxygen sensor to maintain the air/fuel mixture in a stoichiometric ratio.
*Note that while the downstream (of the catalytic converter) oxygen sensor on some GM applications are also implicated in fuel control, this implication is limited, and the upstream sensor remains the primary source of input data for short-term fuel control.
As a practical matter, however, oxygen sensors do not start to work directly after a cold engine is started, because their sensing elements need to be at a temperature of about 6000F to generate a signal voltage. While older unheated oxygen sensors relied on the exhaust stream to heat their sensing elements to the required temperature, modern heated wide-band oxygen sensors contain heater elements that are controlled and monitored by the PCM via dedicated control and sensing circuits.
In terms of practicalities, this means that an oxygen sensor that is not at its required operating temperature cannot provide the PCM with input data. This is known as “open-loop” operation, and with unheated sensors, this period could be as long as several minutes, during which time the PCM used pre-programmed fuelling strategies that did not require input data from any engine sensors.
Heated oxygen sensors, on the other hand, typically reach their operating temperatures in less than about 30 seconds, meaning that they can begin to provide the PCM with input data on the composition of the exhaust stream, and by extension, on the quality of the air/fuel mixture, soon after engine start-up. As soon as the PCM begins to receive this data from the upstream oxygen sensor(s), the fuel management system enters a condition known as “closed-loop” operation, which is so-called because this data exchange is configured in a closed loop between the upstream oxygen sensor(s), the PCM, and the greater fuel control system.
However, to reduce exhaust emissions during cold start-ups, the PCM is programmed to expect input data from the upstream oxygen sensor within a predefined and strictly observed time limit. Thus, if any defect, failure, or malfunction occurs anywhere in the fuel control system that causes this time limit to be exceeded, the PCM will recognize that it cannot enter closed-loop operation at least once within the prescribed time limit, and it will set code P1128 and illuminate a warning light as a result.
Note, however, that on some applications, the PCM needs to detect the fault at least twice during two consecutive trips before it will set this code. Note also that when the PCM does not detect the fault in three consecutive trips, it will automatically clear this code and extinguish warning lights.
Where is the P1128 sensor located?
This image shows the location of the upstream oxygen sensors on a 2007 Jeep Liberty application. Note that while these sensors are easily accessible, it may be necessary to remove or disassemble the exhaust system and/or other unrelated components to gain access to the sensors for testing and or replacement on other applications.
Also, be sure to consult reliable service information for the affected application to identify the affected bank of cylinders correctly to avoid a misdiagnosis and the unnecessary replacement of parts.
What are the common causes of code P1128?
The most common causes of code P1128 are many and varied, and could include one or more of the following-
- Defective or failed upstream oxygen sensor (Most common)
- Contaminated oxygen sensor sensing element
- Abnormally high or low fuel pressure
- Damaged, burnt, shorted, disconnected, or corroded wiring and or connectors that affect the signal, control, feedback, or reference voltage circuits of the affected oxygen sensor, or any other implicated engine sensor
- Excessive oil consumption or other mechanical defects in the engine
- Defective, contaminated, or malfunctioning MAP and/or MAF sensors
- Defective or malfunctioning engine coolant temperature sensor
- Defective or malfunctioning intake air temperature sensor
- Failed or failing PCM or fuel control module, but note that since these are rare events, the fault must be sought elsewhere before any control module is replaced or reprogrammed
What are the symptoms of code P1128?
Common symptoms could include one or more of the following, but note that depending on the nature of the problem, the severity of one or more symptoms listed here could vary significantly between applications-
- Stored trouble code and illuminated warning light
- Depending on the nature of the problem, multiple additional codes may be present along with P1128
- Idling quality may be poor, or the idling speed may fluctuate wildly
- Fuel consumption may increase
- Varying degrees of power loss may be present
- The engine may stall unexpectedly or frequently at low engine speeds
- The engine may stumble or hesitate upon acceleration
- Spark plugs on the affected bank of cylinders may fail repeatedly if the problem is not resolved promptly
- Misfires may develop on the affected bank of cylinders if the problem is not resolved promptly
- Some readiness monitors may not initiate or run to completion
- The vehicle will fail a mandatory emissions test
Other Manufacturer Specific Definitions for P1128Front O2 Sensor Connectors Swapped (Ford)
Throttle actuator control (TAC) lock - malfunction (Toyota)
Long term fuel trim, system too lean (Volkswagen)
Manifold absolute pressure (MAP) sensor - pressure lower than expected (Acura)
Long term fuel trim, entire speed/load range, Bank 1 - system too lean (Audi)
Closed loop fuel control not achieved -bank 1 (Chrysler)
Heated oxygen sensor (H02S) 1, bank 1 signal error (Dodge)
MAP sensor – signal lower than expected (Honda)
Long term fuel trim – system too lean (Hyundai)
Throttle motor – short circuit (Infiniti)
Manifold absolute pressure (MAP) sensor - pressure lower than expected (Isuzu)
Closed loop fuel control not achieved -bank 1 (Jeep)
Heated oxygen sensor (H02S), upstream – transposed (Land Rover)
Throttle actuator control (TAC) lock- malfunction (Lexus)
Heated oxygen sensor (H02S) 1, bank 11 bank 2 -interchanged (Lincoln)
Heated oxygen sensor (H02S) -front – bank 1/bank 2 sensors transposed (Mazda)
Heated oxygen sensor (H02S) 1, bank 1/ bank 2 interchanged (Mercury)
Closed Loop Fueling Not Achieved Bank 1 (Mitsubishi)
Throttle valve position motor – short circuit (Nissan)
Manifold absolute pressure (MAP) sensor signallow (Saturn)
BAT Team Discussions for P1128
- 2000 VW Cabrio Code P1128
2000 VW Cabrio/100,000mi/4cyl I recieved a rejected inspection sticker because my car would not pass emissions. I went to autozone and had them check for codes and P1128 Fuel Air metering came up. I replaced the vacuum hoses I knew of as well as the rubber hose coming off of the valve cover, which...
- 2001 Wolfsburg Jetta Gas Motor Fuel Pressure Monitor Sensor
The code read when the fuel pressure monitor was replaced was PO172. The 2nd and 3rd time the check engine light came on was code P1128. Any help, anyone? Thanks, Nick...