|Code||Fault Location||Probable Cause|
|P1148|| P1148 – Closed Loop Control Function Bank 1 (Infiniti, Nissan) |
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Manufacturer Specific Definitions for P1148
Table of Contents
- What Does Code P1148 Mean?
- What are the common causes of code P1148?
- What are the symptoms of code P1148?
- How do you troubleshoot code P1148?
- Codes Related to P1148
- Get Help with P1148
What Does Code P1148 Mean?
OBD II fault code P1148 is a manufacturer specific code that is defined by car makers Nissan and Infinity as “Closed Loop Control Function Bank 1”, and on these applications, code P1148 is set when the PCM (Powertrain Control Module) detects a condition relating to an air/fuel ratio sensor that prevents it (the PCM) from entering closed loop operation. In this definition, “Bank 1” refers to the bank of cylinders that contains cylinder #1, and “closed loop” refers to a condition where the PCM uses input data from a sensor, in this case the #1 (upstream from the catalytic converter) air/fuel sensor, to make short-term fuel trim adjustments.
NOTE: Many, if not most Japanese-made vehicles use air/fuel ratio sensors, instead of conventional (and better-known) heated oxygen sensors to control fuel trims. Also, note that while air/fuel sensors and oxygen sensors perform the same function, there are significant differences between their operating principles. In practice, this means that while some malfunctions in oxygen sensors can be diagnosed with a digital multimeter, diagnosing the same malfunctions in air/fuel ratio sensors generally requires the use of an oscilloscope, due to the nature of the signals air/fuel sensors generate.
Like conventional oxygen sensors, air/fuel ratio sensors are fitted with heater elements, whose function it is to bring the sensor up to operating temperature, which is around 1 380 °F (750 °C). Note that the heater element is controlled via a dedicated control circuit, and that pulse modulation is used to control the sensors’ temperature very accurately, since the sensor’s operation depends on its operating temperature being within the range specified by the manufacturer.
In a fully functional system that employs air/fuel ratio sensors, regardless of whether the application is a Nissan or Infiniti product, the PCM depends on input data from the #1 sensor (located upstream of the catalytic converter) to regulate the air/fuel mixture in order to maintain an air/fuel ratio that is as close to the stoichiometric as possible. This value is also known as “Lambda”, which is expressed as “Lambda = 1” when the engine is running at the stoichiometric value for that engine. Note that for gasoline engines, this value represents an air/fuel mixture of 14.7 parts to one part of fuel.
Thus, assuming that the engine is in a mechanically sound condition, that there are no misfires present, that no unmetered air enters the engine, that the air/fuel sensor is at the correct operating temperature, and there are no exhaust leaks upstream of the #1 sensor, the PCM uses data from the #1 air/fuel ratio sensor in the form of a signal generated by the sensor that indicates the deviation from Lambda = 1, i.e., the ideal air fuel mixture.
Provided that all the above conditions are met, this deviation can indicate either a lean -, or rich running condition, depending on operating conditions such as engine load and speed. Once the PCM is able to use this information to correct the air/fuel mixture, the PCM is said to be in “closed loop” operation, and in this state, the PCM will make short-term fuel trim adjustments several times per second.
From the above it should be obvious that any problem, issue, malfunction, or failure in either the air/fuel ratio sensor or its control circuit that prevents the PCM from entering closed loop operation, translates into the fact that the PCM cannot control or regulate the air/fuel mixture effectively. When this happens, the PCM sets code P1148, and illuminates a warning light. Note that on Nissan/Infiniti applications, this code is set on the first fault cycle.
The image below shows one example of an air/fuel ratio sensor that is suitable for use on a late model Nissan Altima. Note that despite the fact that this sensor strongly resembles a conventional oxygen sensor the two types of sensor are NOT interchangeable on ANY application, regardless of the manufacturer.
What are the common causes of code P1148?
The possible causes of code P1148 are many and varied, and could include anything that can cause, or contribute to setting any of the following codes- P0036, P0037, P0038, P0042, P0043, P0044, P0050, P0051, P0052, P0056, P0057, P0058, P0062, P0063, and P0064, as well as codes P0130 through P0167, in addition to several other manufacturer specific P1XXX codes.
There are far too many possible causes of P1148 to list them all here, but as a general rule, problems such as damaged, burnt, shorted, disconnected, or corroded wiring and connectors form the bulk of possible causes. Other possibilities could include-
- Defective air/fuel ratio sensors
- Defective “tuning” resistors in the sensor connector
- Contamination of the sensor by oil, carbon, or silicone-based compounds in aftermarket anti-freeze and oil/fuel additives
- Exhaust leaks upstream of the #1 sensor. Note that only major leaks will cause, or contribute to code P1148 being set.
- Unauthorized or illegal modifications to any part of the exhaust system
- Failed or failing PCM. Note that this is a rare event, and the fault must be sought elsewhere before any controller is replaced
NOTE: To avoid confusion and to save time, always consult the manual for the application being worked on for details on the most likely causes of code P1148 on that particular application, as well as for details on the possible relationship between P1148 and any other code(s) that may be present along with P1148.
What are the symptoms of code P1148?
Most common symptoms of P1148 are much the same across all applications, and these could include the following-
- Reduced fuel economy
- Varying degrees of power loss
- Engine may idle roughly, or idling speed may fluctuate
- Idling speed may exceed the min/max acceptable speed
- In some cases, the engine may stumble or hesitate upon acceleration
- In some cases, it may be hard, or impossible to start the engine when it is hot, since the fuel management system often sets a rich-running condition by default if the PCM cannot enter closed loop operation.
How do you troubleshoot code P1148?
SPECIAL NOTES: Non-professional mechanics should take note that diagnosing faults in an air/fuel ratio sensor itself often requires the use of an oscilloscope. Moreover, diagnoses of this type can only be performed with the aid of relevant reference data in the form of sample waveforms, which may or may not be included in the repair manual for the application. Therefore, if the fault cannot be resolved by following the generic steps outlined below and the required diagnostic equipment and reference data are not available, the better option is to refer the vehicle to the dealer or other competent repair shop for professional diagnosis and repair. END OF SPECIAL NOTES.
Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.
NOTE: Although it does happen, it is rare for code P1148 to appear without other codes being present as well. Typical codes that relate to the sensors’ heater circuit could include P0036, P0037, P0038, P0042, P0043, P0044, P0050, P0051, P0052, P0056, P0057, P0058, P0062, P0063, and P0064, but depending on the application, there may be others. Other codes that relate to faults /failures/malfunctions in the sensor itself could include any code from P0130 through P0167, but again depending on the application, there may be manufacturer specific P1XXX codes present as well. In view of the above, it is critically important to note the order in which additional were stored, and to resolve these codes in this order since resolving preceding codes could resolve P1148 as well.
Apart from the codes listed in Step 1, it is important to check that there are no codes present that relate fuel pressure, misfires, unmetered air entering the engine, system voltage, and/or ignition system problems, since these codes can cause, or contribute to code P1148 being set.
If such codes are present, resolve all in the order in which they were stored before attempting a diagnosis and/or repair of P1148. Failure to do this will almost certainly result in misdiagnoses, confusion, and the unnecessary replacement of parts and components.
Assuming that all other codes had been resolved, or that none was present, refer to the manual to locate the #1 (upstream of the catalytic converter) air/fuel sensor, as well as all associated wiring. Also, determine the function, and color-coding of all related wiring to be sure that the correct circuits are tested.
Once all relevant components and wiring are identified, perform a thorough visual inspection of all wiring and connectors. Look for damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors. Make repairs as required if such damage is found.
Clear the code after repairs are complete, and operate the vehicle for at least one full drive cycle to see if the code returns.
If the wiring is free of damage but the code persists, prepare to perform reference voltage, resistance, continuity, and ground connectivity checks on all wiring. Be sure to disconnect the sensor from the PCM during this step to prevent damage to the controller.
Pay particular attention to the heater circuit during this step, and be sure to follow the directions provided in the manual EXACTLY to prevent getting false or misleading test results. Be aware that since any deviation from stated values in the heater circuit can cause, or contribute to P1148 being set, it is always preferable to replace wiring than to attempt repairs if deviations from stated values are found.
Also, be sure to perform the “wiggle” test on all connectors. The wiggle test involves wiggling, or twisting a connector while a current or resistance is being tested to see if the tested value changes. If it does change, replace the connector, or make suitable repairs to wiring terminations to ensure that proper electrical contact between all terminals/pins in the connector is restored.
Clear all codes after repairs are complete, and operate the vehicle for at least one full drive cycle to see if the code returns.
If all electrical values in the sensors’ control circuit check out, but the fault persists, refer to the manual for detailed information on the correct procedure(s) to follow to test the sensor itself. One such test would involve testing the resistance of the heater element, so be sure to consult the manual to determine which terminals on the sensor side of the connector to test. Replace the air/fuel ratio sensor if any resistance value does not agree with the value(s) stated in the manual.
One other test would be to check the resistance of the “tuning resistor” that is incorporated into the connector on most (not all) applications. The function of these resistors is to allow very precise control of the sensors’ temperature, so be sure to check that the resistance of the resistor falls exactly within the range specified by the manufacturer. Replace the resistor if the obtained reading does not fall within the range specified by the manufacturer.
Clear all codes after repairs are complete, and operate the vehicle for at least one full drive cycle to see if the code returns.
If the fault persists, but all electrical values agree with the manufacturers’ specifications, remove the sensor from the exhaust system to inspect it for signs of damage, or the presence of harmful deposits that could influence the sensors’ operation.
Bear in mind that oil or carbon contamination is relatively rare on Japanese applications, so if the engine is not consuming oil at an elevated rate, but the sensor appears to be contaminated, consider the possibility that coolant in the engine oil, or that aftermarket fuel/oil additives might be the cause.
Many aftermarket additives contain silicone-based compounds, as do some brands of anti-freeze, and these compounds are known to damage both air/fuel ratio sensors and conventional oxygen sensors. The only remedy for this condition is to replace the air/fuel ratio sensor, and to cease the use of all and any aftermarket additives.
If there is no contamination of the sensor, and all electrical values check out, consider the possibility that the sensor has become “lazy” due to long use. Air/fuel ratio sensors have a finite service life, usually around 100 000 miles or so, which means that the problem could merely be a sensor that has reached the end of its useful life.
However, in these cases, code P1148 will almost certainly be accompanied by a sensor “range”, or “performance” related code, which is why it is important to investigate the relationship between P1148 and any other accompanying codes as described in Step 1.
If a range or performance related code was indeed present, replacing the sensor with an OEM part will almost certainly resolve the problem. Make very sure also that the tuning resistor in the replacement sensor has EXACTLY the same resistance as the old one to prevent a recurrence of the problem.
If the sensor is replaced, be sure to secure all wiring away from hot exhaust components, and that all connectors are fully engaged and locked. Clear all codes after repairs/replacements are complete, and complete at least one complete drive cycle with a scanner connected to monitor the operation of the affected air/fuel sensor in real time.
In the unlikely event that the code does return, it is necessary to test the operation of the sensor in situ, which requires the use of an oscilloscope. However, even if an oscilloscope and reference data are available, interpreting the waveforms generated by air/fuel ratio sensors requires a degree of specialized knowledge, so the wiser option at this point would be to refer the vehicle for professional diagnosis and repair.
Codes Related to P1148
There are no known Nissan and Infiniti specific codes that are directly related to P1148 on these applications. However, there are many generic codes (refer to Step 1) that can, and do, cause or contribute to code P1148 being set on Nissan and Infinity applications.
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