|Trouble Code||Fault Location||Probable Cause|
|P0300||Random/multiple cylinder(s) -misfire detected||Spark plug(s), HT lead(s), injector(s), ignition coil(s), low compression, wiring|
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What Does Code P0300 Mean?
Special note on trouble code P0300 and Mitsubishi vehicles: While DTC P0300 is a generic code that affects all OBD II compliant vehicles, the most common causes of this code sometimes vary between most vehicle makes. This article will therefore deal with code P0300 as it applies specifically to Mitsubishi vehicles.
OBD II fault code P0300 is a generic trouble code that is defined as “Random/multiple cylinders – misfire detected” and is set when the PCM (Powertrain Control Module) detects misfires on random or multiple cylinders. Note that while the PCM has detected random misfires, it can identify neither the root cause(s) of the misfire(s) nor the cylinders that are misfiring via the built-in misfire detection system. Therefore, the PCM will set this non-specific misfire code.
Modern misfire detection systems typically use signals from the crankshaft position sensor to monitor the rotational speed of the crankshaft. When no misfires are present, the crankshaft rotates at a constant speed, but even slight misfires can cause slight variations in the rate at which the crankshaft rotates.
In practice, most advanced misfire detection systems can detect variations in the crankshafts’ speed as small as 2 percent, even at high engine speeds. Since the duration of such fluctuations is typically measured in microseconds, the misfire detection system uses both the fluctuation and the pattern in which they repeat to confirm the misfire.
Moreover, since the input data is derived from the crankshaft position sensor and its associated reluctor ring that is indexed or referenced to cylinder #1, the misfire detection system can usually identify misfiring cylinder(s). The system does this based on the pattern of crankshaft speed variations, coupled with input data on whether or not an ignition spark had been delivered (to misfiring cylinders) and whether or not one or more fuel injectors had been disabled because ignition sparks had not been delivered.
At this point, it is helpful to remember that the primary function of an OBD II system is not to detect faults: its primary function is to manage exhaust emissions. Therefore, an OBD II system will only recognize faults that have the potential to increase emissions beyond maximum allowable thresholds for that particular engine.
In the real world, however, all misfires have at least some potential to increase exhaust emissions, regardless of their cause(s). However, since OBD II systems can’t monitor some parameters such as the mechanical condition of the engine, the operation of the valve train on applications that are not fitted with variable valve timing, or loss of cylinder compression, OBD II systems are enabled to detect misfires they cannot relate to any specific causes.
Thus, while the PCM’s misfire detection system may detect misfires under such conditions, it can identify neither the cause nor the misfiring cylinder(s) because parameters that are monitored (ignition spark and fuel delivery) may be working within acceptable parameters.
Therefore, since even unidentified misfires have the potential to affect emissions negatively, the PCM will set code P0300 and illuminate a warning light when it specifically detects misfires it cannot identify, or relate to one or more specific cylinders.
NOTE: It is perhaps worth noting that in cases where the PCM can identify the misfiring cylinder(s), it will set codes such as P0301, P0302, P0303, P0304, etc., up to P0312, where the last digit(s) in the code(s) correspond(s) to the misfiring cylinder(s), which brings us to-
What causes code P0300 on Mitsubishi vehicles?
Unlike on many other vehicle brands, some very specific enabling conditions must be met before code P0300 will be set and stored as an active code, including the following-
- Multiple misfires must occur within 200 engine revolutions
- Misfires must have been detected in at least 2% of the immediately previous 1000 engine revolutions
- The temperature of the catalytic converter must be at, or above 18320F
See the Common Causes section for more details on the most common cause(s) of this code on Mitsubishi applications.
Where is the P0300 sensor located?
This image shows the location (circled) and the appearance of the EGR valve on 94-97 Mitsubishi 3000 engines. Note that while the EGR valve is easily accessible on this engine, it may be necessary to remove and/or disassemble some engine components to gain access to the EGR valve on later (or different) models.
Note that EGR valves are application-specific, and are therefore not interchangeable between Mitsubishi models- even if any given EGR valve fits on different models that are fitted with the same engine.
What are the common causes of code P0300?
Unlike most other vehicle brands, Mitsubishi engines are extremely sensitive to air/fuel mixture issues, but other common causes are listed below in their order of prevalence, with causes with the highest prevalence listed at the top of the list-
- Excessively lean air/fuel mixtures that cause misfires to “jump around” between cylinders, but note that lean conditions will typically also be indicated by dedicated codes
- Worn or unsuitable spark plugs
- Failed, defective, or malfunctioning oxygen sensors (Both upstream and downstream)
- Engine vacuum leaks through ruptured/dislodged hoses and/or damaged intake system gaskets/seals
- EGR system defects, failures, or malfunctions, but note that EGR issues will typically be indicated by one or more dedicated EGR codes along with P0300
- Unsuitable and/or contaminated fuel
- Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors in the fuel and/or ignition systems
- On modified high-performance models, reducing the mass of the flywheel may produce code P0300 because the lighter flywheel cannot absorb or damp out torsional vibrations at the rear end of the crankshaft
- Failed or failing PCM, but note that since this is a rare event, the fault must be sought elsewhere before any control module is reprogrammed or replaced
What are the symptoms of code P0300?
The common symptoms of code P0300 are much the same across all Mitsubishi applications, but note that depending on the nature of the problem, the severity of one or more symptoms listed here may vary substantially between Mitsubishi models-
- Stored trouble code and illuminated warning light
- Depending on the model and the nature of the problem, multiple additional codes may be present along with P0300
- A no-start or hard starting condition may be present
- The engine may stumble or hesitate upon acceleration
- The engine may run roughly at some engine speeds
- Varying degrees of power loss may be present
- Fuel consumption may increase substantially
- The engine may stall unexpectedly at low engine speeds
- Idling may be rough or erratic, and in some cases, the engine may not idle at all
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