|Trouble Code||Fault Location||Probable Cause|
|U1112||SCP (J1850) Invalid or Missing Data for Primary Id|
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What Does Code U1112 Mean?
OBD II fault code U1112 is a manufacturer-specific trouble code that is defined by carmakers Ford and Mazda as “SCP (J1850) Invalid or Missing Data for Primary Id”, and is set when the PCM (Powertrain Control Module) detects the failure of a control module to identify itself while transmitting data over a communications network. Note the following in the context of this code-
- “SCP” stands Standard Communications Protocol
- “Primary ID” refers to a control module that controls, monitors, or manages a clearly defined set of functions, but note that in the case of code U1112 on Ford and Mazda applications, that control module is not identified
- “J1850” is a shortened expression of SAE Standard J1850, which is the standard that describes a serial communications protocol used by Ford and Mazda since 1996
- This protocol uses a pulse width modulated signal to transmit data at average speeds of up to 41.6 kilobytes per second, although some versions of this protocol can transmit data at higher rates
- There are currently several hundred generic as well as manufacturer-specific trouble codes in common use that include the phrase “Invalid or Missing Data for Primary Id”. In some cases, the area of concern is indicated by information that may include the name of a specific control module and/or system, but in many more cases, no information identifies the area of concern. See the section on possible causes of code U1112 for more details
All modern vehicles use complex serial communications systems to distribute information about the operational status of dozens of systems and operating parameters between control modules. While there are significant design and technical differences between the various communications protocols and system topologies (system architectures) between vehicle manufacturers, all serial communications systems have the function of maintaining communications between control modules to ensure the safe and efficient operation of the vehicle.
One example of this need would be the distribution of information about the rates at which all the wheels rotate. In practice, this information is collected by wheel speed sensors that transmit the data to the ABS control module, which then relays it to the PCM and other control modules such as Traction-, and Stability Control Modules, since these modules use the ABS system to maintain and/or restore the vehicle’s stability.
In terms of practicalities though, each control module that shares information with other modules has a unique identifying code, which code is included in all data transmissions made by that control module. Moreover, all control modules are programmed with the identifying codes of all other control modules on a given network, and this information is compared during the transmission and reception of data. Thus, all control modules always “know” which control module transmitted which data, regardless of the system involved.
Moreover, all control modules that share a network also “know” what type of information all other control modules on that network are supposed to transmit. Put in another way, all control modules are programmed to transmit the operating parameters of the system, part, or component they control, manage, or monitor. These parameters are transmitted in strictly organized/structured messages, and all monitored parameters must be included in all data transmissions for the transmitted data to be valid.
Therefore, to maintain a serial communication system’s integrity, all receiving control modules that share that network perform “checksum” checks, in which all expected information must be included, along with the transmitting module’s unique identifying code. If all expected information is received, all receiving modules will acknowledge the data transmission to the transmitting module in a structured message that confirms that the received information agrees with the information that was expected.
However, in cases where a transmitting control module cannot transmit any particular operating parameter for whatever reason, it will resort to transmitting a default value for the missing parameter. All receiving control modules will interpret the default value as “Missing or invalid data”, and all control modules with the ability to perform diagnostics and set trouble codes will set a relevant trouble code as a result.
In some cases though, as in the case with code U1112 on Ford and Mazda applications, receiving modules are for one reason or another not able to identify the control module that had transmitted the invalid or missing data, which explains why no identifying information is included in the code definition. See the following section for details on how to treat this and other similar codes.
Where is the U1112 sensor located?
This diagram shows the standardized structure of all trouble codes, which structure is defined in an Act of Congress, known as the SAE Standard 2012: Diagnostic Trouble Code Definitions.
Using this standard, we can decode code U1112 as follows-
- “U” indicates a Communications or Network Code
- The second digit is “1”, which means that this code applies specifically to one or more vehicle makes, as opposed to being a “0”, in which case the code would have been generic, and would have had the same meaning/definition on all OBD II compliant vehicles
- The third digit is “1” also, which identifies the major system in which the fault, defect, or malfunction had occurred, and in this case, the third digit identifies the fuel delivery and air metering system
- The fourth and fifth digits, in this case, are “1” & “2” respectively, and this combination describes the approximate area of concern, as opposed to the actual fault
Note though that this method of decoding this kind of trouble code does NOT reveal the actual fault. All this method does is to reduce the number of possible areas of concern, and therefore, the symptoms should be used to narrow the search down to the actual circuits, components, and/or systems that are most likely to produce the observed symptoms, which in this particular case, are likely to include misfires.
What are the common causes of code U1112?
Typical causes of code U1112 could include one or more of the following-
- Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors in the affected communication system(s)
- Defective, faulty, or malfunctioning control module
- Failed or failing PCM, but note that this is unlikely to be the case if it is possible to extract a fault code
What are the symptoms of code U1112?
Since this code is related to the air and fuel metering system, the most likely symptoms could include one or more of the following-
- Stored trouble code and illuminated MIL (CHECK ENGINE) light
- Depending on the nature of the problem, multiple other codes relating misfires and/or ignition issues may be present along with U1112
- Engine may run roughly at all speeds, or only at some speeds
- Idling may be rough, or the idling speed may fluctuate
- Engine may stall unexpectedly at low engine speeds
- Varying degrees of power loss may be present at all, or only some engine speeds
- Fuel consumption may increase noticeably
- Exhaust emissions may increase measurably
- Depending on the nature of the problem, the engine may be hard to start, or a no-start condition may be present