|Trouble Code||Fault Location||Probable Cause|
|P0314||Single cylinder misfire -cylinder not specified||Engine mechanical fault, wiring, ignition/fuel system, injector|
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What Does Code P0314 Mean?
OBD II fault code P0314 is a generic code that is defined as “Single cylinder misfire -cylinder not specified”, and is set when the PCM (Powertrain Control Module) detects a misfire on a single cylinder that cannot be identified. Note that within the context of code P0314, the misfire can occur sporadically, intermittently, or, it can occur continuously. Depending on the severity of the misfire, the PCM may either illuminate a warning light permanently, or cause a warning light to flash whenever the misfire occurs.
While modern automotive electronics have reached a high level of sophistication, and is generally capable of controlling/managing all aspects of an engines’ operation, and even have some “fuzzy logic” capabilities in the sense that PCM programming can adapt autonomously to accommodate different driving styles/conditions, it must be understood that modern engine and fuel management systems are also severely limited in some areas.
For example, all of the advanced capabilities of an engine/fuel management system depend on the engine being in good, if not perfect running condition if these management systems are to work as intended, but as with so many other things concerning automotive diagnostics, the devil lives in the details. Let us briefly look at the details as they pertain to code P0314-
The principal reason why modern engine and fuel management systems are as affective as they are is the fact that both the ignition and fuel delivery systems have self-diagnostic capabilities that use dedicated feedback circuits to confirm whether or not an event had actually taken place. For instance, the ignition system includes circuits that confirm that a spark had been delivered to each spark plug at the proper time, while the fuel injection system includes dedicated circuits to confirm that the injectors had actually injected fuel into each cylinder, and that the injectors had done so at the correct moment.
If any of these circuits fail, or if something else happens that prevents sparks and fuel being delivered at the appropriate time, the PCM is made aware of that fact through dedicated feedback circuits and input data obtained from a variety of sensors. In this manner, the PCM is able to recognize that one or more cylinders are experiencing misfires, and it will set specific misfire codes that identify all the affected cylinders as a result.
However, the PCM is not able to monitor the mechanical condition of the engine directly; it can only infer that the engine is in good condition based on input data from sensors. These sensors include, but are not limited to, oxygen sensors, air/fuel ratio sensors, exhaust gas temperature sensors, NOx sensors, various engine and coolant/fluid temperature sensors, and others to a greater or lesser extent, depending on the application. If the engine is in good mechanical condition and there are no fault codes present, all the input data the PCM (and other control modules) receive from all sensor will fall into acceptable and/or allowable ranges.
As stated elsewhere, the PCM requires that the engine be in a good running condition for it to be able to manage the operation of the engine effectively, since it cannot control what it cannot monitor directly. For instance, the PCM cannot monitor the compression in each cylinder directly, nor can it monitor the actual combustion temperature in each cylinder. Thus, should the PCM recognize, say, a sudden slight decrease in the exhaust temperature, or a sudden increase in the temperature of the catalytic converter(s), it will check for failures in all the relevant implicated sensors or circuits that could conceivably have caused the change in the exhaust gas or catalytic converter temperature.
NOTE: A decrease in exhaust gas temperature will result from one cylinder misfiring because the air/fuel mixture is not combusted, or combusted completely in that cylinder, thereby causing an overall drop in the exhaust gas temperature. An increase in the temperature of the catalytic will result because uncombusted, or partially combusted hydrocarbons from the misfiring cylinder are allowed to enter the catalytic converter. Thus, to prevent damage to the catalytic converter, the PCM will disable the fuel injector on the misfiring cylinder, which may or may not set a code that identifies the affected cylinder.
If no failures are found, the PCM it will conclude that one cylinder is misfiring, but given the fact that no faults are present in systems/circuits that the PCM can monitor, it will not be able to identify the affected cylinder because some factors that influence the combustion process directly, cannot be monitored. One such factor could be the partial or complete loss compression pressure in one or more cylinders due to mechanical issues, but there are many other possibilities.
While the above example is a gross over simplification of the science of misfire detection on modern engines, it will suffice to demonstrate that on board diagnostic systems generally cannot identify the causes of misfires if the possible cause(s) do not involve systems and circuits that cannot be monitored or controlled by the PCM (or other control modules) directly. Therefore, the possible causes of code P0314 -“Single cylinder misfire -cylinder not specified” will almost invariably involve mechanical issues or failures in the engine that directly affect and/or influence the combustion process in the affected cylinder.
NOTE: At the risk of over stating the case, non-professional mechanics should take note that if the cause of a misfire is located in a system, circuit, part, or component that the PCM or other control module(s) can monitor, control, or regulate directly, the PCM (or other control module) will set a code that identifies the misfiring cylinder. Conversely, if the misfire is caused by a failure, malfunction, or defect in an engine part, component, or system that the PCM or other control module(s) cannot monitor, control, or regulate directly, the misfiring cylinder cannot be identified, and code P0314 will be set as a result.
WARNING: As with all rules, there are exceptions to the above: in some cases where purely mechanical failures cause misfires on several, or all cylinders, codes that identify the affected cylinders may be present. However, whether or not identifying codes are present or not largely depends on the application, as well as the nature and extent of the mechanical failure or defect.
Where is the P0314 sensor located?
The image above shows an example of the types of mechanical issues that can cause a misfire on a single, unspecified cylinder. In this case, one rocker arm on a Corvette engine is mislocated due to a failure of the valve spring, which damaged the valve, which in turn, caused a loss of compression on the affected cylinder. Since neither the ignition, nor the fuel injection systems were affected in this particular case, the misfiring cylinder could not be identified with a dedicated misfire code.
Note however, that this is just one example and the actual location of the issue that had caused code P0314 to set depends entirely on the nature of the problem. Be aware though that in cases where the problem is not immediately apparent, such as in the example above, it may sometimes be necessary to remove and disassemble the engine to locate the root cause of the problem.
What are the common causes of code P0314 ?
In cases where no codes are present that identify the misfiring cylinder, it can often be difficult to identify the root cause of code P0314, and especially if there are no mechanical noises like thuds, or knocks present, which is often an invaluable diagnostic aid. Nonetheless, in the absence of mechanical noises, some possible causes of code P0314 could include the following-
- Loss of compression due to-
- damaged valves
- dislodged valve seats
- broken or damaged valve springs
- broken or perforated pistons
- severe mechanical damage to the piston and/or cylinder wall caused by one or more broken piston rings
- broken camshafts caused by poorly executed cylinder head repairs
- leaking or damaged cylinder head gasket(s)- particularly if a leak path opens up between a cylinder and an oil passage, in which case the engine will very likely not overheat
- Failed or failing PCM or other control module. Note however that these are rare events, and the fault must therefore be sought elsewhere before any control module is replaced