P0300 – Random Cylinder Misfire (Cylinder Not Specified) (Toyota)

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By Reinier (Contact Me)
Last Updated 2023-11-21
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P0300 Random Cylinder Misfire (Cylinder Not Specified)
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Spark plug(s), HT lead(s), injector(s), ignition coil(s), low compression, wiring

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Table of Contents

  1. What Does Code P0300 Mean?
  2. Where is the P0300 sensor located?
  3. What are the common causes of code P0300?
  4. What are the symptoms of code P0300?
  5. Get Help with P0300

What Does Code P0300 Mean?

 

Special note on trouble code P0300 and Toyota 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 Toyota 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. While 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 directly, 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 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 Toyota vehicles?

When diagnosing non-specific misfire codes on any vehicle, it helps to remember that as far as misfire detection systems are concerned, the distinction between specific and non-specific causes are not always clear cut. This is aggravated by the fact that closely related codes known as “cylinder power contribution” codes may also be present. In some applications, cylinder power contribution trouble codes can be seen as “precursors” to fully-fledged misfire codes, because the effects of these codes are typically not as pronounced as the effects of proper misfire codes are.

Nonetheless, while some monitored parameters, such as ignition spark and fuel delivery failures can cause the non-specific, P0300 code to set, this is rare. In by far the vast majority of instances of code P0300 on Toyota applications, the most common causes could include the following-

  • Note that since almost all modern Toyota gasoline engines are extremely sensitive to not only unsuitable or incorrect spark plugs, but also to correct/suitable sparkplugs that are not gapped correctly. While this will typically cause ignition coil failures as a secondary effect, the immediate effect of incorrect spark plug gaps is reduced ignition coil performance under heavy engine loads, which can, and does, cause intermittent and/or unidentifiable misfires on one or more cylinders. Be aware that all modern Toyota engines require platinum (or similar) sparkplugs that are not adjustable, and that typically have service lives of about 100 000 miles. Therefore, the gaps on these sparkplugs are pre-set, and should NOT be adjusted
  • Failed, clogged, or defective PCV (Positive Crankcase Ventilation) valves. These valves are service items on most Toyota applications, and should therefore be replaced in accordance with the vehicle’s service schedule.
  • On older Toyota engines, it is common to find injector seals that have become hard and brittle, thus causing misfires. Note though that while this condition will typically set dedicated cylinder misfire codes, it can also cause unidentifiable misfires, so be sure to check the condition of injector seals when diagnosing this code on older Toyota engines

Where is the P0300 sensor located?

This image shows a platinum sparkplug that is not gapped correctly. While the needle-pointed electrode on this sparkplug will produce a spark even if the gap is set to three or four times what it should be (as shown here), the resultant weak spark will almost certainly not ignite the air/fuel mixture as well as it should. Thus, the cylinder(s) with such a sparkplug will misfire, and if all the spark plugs are incorrectly gapped, random, unidentifiable misfires will occur, and particularly so under heavy engine loads.

What are the common causes of code P0300?

Note that while it is rare for code P0300 to be accompanied by other, specific misfire codes, it does happen, but in cases where P0300 appears in isolation, the most common causes are listed below in their order of prevalence, with the most prevalent causes listed at the top of the list-

  • Worn or unsuitable spark plugs, and particularly when the load on the engine increases suddenly. Note that the condition of spark plugs is not monitored directly, and as a result, worn spark plugs could set a non-specific misfire code (P0300) until the worn spark plugs cause other components, such as ignition coils to fail. When this happens, the PCM will set an ignition system code and likely a specific misfire code as well
  • One or more clogged or defective fuel injectors
  • Insufficient fuel pressure, but note that this condition will typically set one or more dedicated fuel pressure codes as well
  • Defective crankshaft position sensor/and or reluctor ring, but note that in the case of defective sensors, it is almost certain that a dedicated sensor code will be present as well
  • In some cases, a damaged harmonic balancer can induce crankshaft speed fluctuations that are severe enough to set a non-specific misfire code
  • Note that while engine vacuum leaks through intake manifold gaskets and ruptured/dislodged vacuum lines could cause code P0300 to set on Toyota vehicles, this is rare because these kinds of issues typically do not affect random cylinders
  • Note that while camshaft/crankshaft correlation issues can sometimes cause code P0300 on Toyota applications, miscorrelation issues will typically be indicated by one or more dedicated trouble codes in addition to code P0300
  • Poor quality fuel, unsuitable fuel, or fuel that is contaminated with water
  • 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

NOTE: Low cylinder compression on one or more (or all) cylinders will typically produce specific misfire codes that relate directly to the affected cylinder(s).

What are the symptoms of code P0300?

Common symptoms of code P0300 on Toyota applications are much the same across all Toyota applications, but note that depending on the nature of the problem, one or more symptoms listed here may vary in intensity between applications-

  • Stored trouble code and illuminated warning light
  • Multiple additional trouble codes may be present along with P0300, in which case P0300 is likely to be the result of the additional codes as opposed to P0300 being the cause of the additional codes
  • Depending on the nature and severity of the problem, a hard starting or even a no-start condition may be present
  • Varying degrees of power loss may be present
  • The engine may run roughly at some speeds, or under heavy load(s)
  • Fuel consumption may increase
  • Idling may be rough, or the engine may not idle at all

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