P0301 – Cylinder #1 Misfire (Toyota)

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By Reinier (Contact Me)
Last Updated 2023-11-21
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P0301 Cylinder #1 Misfire
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Engine mechanical fault, wiring, ignition/fuel system, injector, ECT/MAF sensor, ECM

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Table of Contents

  1. What Does Code P0301 Mean?
  2. Where is the P0301 sensor located?
  3. What are the common causes of code P0301?
  4. What are the symptoms of code P0301?
  5. Get Help with P0301

What Does Code P0301 Mean?

Special note on trouble code P0301 and Toyota vehicles: While DTC P0310 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 P0301 as it applies specifically to Toyota vehicles.

OBD II fault code P0301 is a generic trouble code that is defined as “Cylinder 1 – misfire detected”         and is set when the PCM (Powertrain Control Module) detects a misfire on cylinder #1 via the built-in misfire detection system.

While what follows is a somewhat simplified explanation of how misfire detection systems work, modern misfire detection systems typically monitor the rotational speed of the crankshaft to detect slight variations in the crankshaft’s rate of rotation. On engines in which no misfires are present, the crankshaft rotates at a steady speed, and this is monitored continuously by the crankshaft position sensor and a rotating reluctor wheel. In practice, each tooth on the reluctor wheel creates an electrical signal when it passes in front of the sensor, and the misfire detection logic uses the time intervals between each signal to detect misfires.

However, when misfires occur (for whatever reason) the contribution that an affected cylinder makes to the crankshaft’s rotational speed is less than the contribution a non-misfiring cylinder makes. As a result, the reduced power contribution from the misfiring cylinder causes a momentary, but measurable reduction in the crankshaft’s speed, and the misfire detection system interprets these cyclical reductions in speed as misfires.

It should be noted though that all modern crankshafts flex and deform under normal operating conditions. Under some operating conditions, these deformations can cause cyclical variations in a crankshaft’s rotational speed. Therefore, to prevent normal torsional effects from being recorded as misfires, these deformations are typically absorbed or damped out by a harmonic balancer mounted on the front end of the crankshaft.

Thus, to prevent false-positive misfire detections, all misfire detection systems are calibrated to record variations in the crankshaft’s speed that exceed either a specified percentage of the crankshafts’ current speed, or, when a specified number of speed variations occur within a specified number of engine revolutions. These parameters vary between both manufacturers and engine designs of the same vehicle brand, but regardless of the calibration system in use on any given Toyota engine, the primary purpose of misfire detection systems on all engines is to detect misfires to prevent increased exhaust emissions on the one hand and to protect the engine and some exhaust components against possible damage, on the other.

It should be noted, though, that misfire trouble codes come in two “flavors”. The first is codes that identify a particular misfiring cylinder, such as code P0301, which identifies a misfire on cylinder #1. The other flavor is non-specific codes, such as P0300 – “Random/multiple cylinders – “Misfire detected”, which indicates a misfire that the PCM cannot relate to one or more specific cylinders or causes.

From a diagnostic perspective, the difference between the two trouble code “flavors” is significant because the possible causes of specific misfire codes are typically fewer and more clearly defined than the possible causes of non-specific misfire codes, which brings us to-

What causes code P0301 on Toyota vehicles?

Unlike many other generic trouble codes, there are no common causes of this code that appear more often on Toyota applications than on any other vehicle brand. Under real-world operating conditions, the causes that are likely to set code P0301 on Toyota applications are much the same across all vehicle brands and models.

However, some Toyota (Avalon, Camry, Sienna, and Solara) vehicles fitted with the V6 1MZ-FE engine could suffer ignition coil failures that may produce trouble codes P0300, P0301, P0302, P0303, P0304, P0305, and P0306. Refer to TSB (Technical Service Bulletin) EG008-04 dated 10/5/2017 for more details.

Nonetheless, with a few exceptions, the most likely causes of code P0301 on Toyota applications involve failures, defects, and malfunctions in systems and/or components that are not monitored directly by the PCM or any other control module(s), as is the case with all other vehicle brands. See the section on Common Causes for more details.

Where is the P0301 sensor located?

This image shows a COP (Coil over Plug) ignition coil used on a 1MZ-FE engine. Note that while ignition coil failures on these engines is a known problem, the actual cause(s) of the failures is/are not known with any degree of certainty. Therefore, to limit or reduce the chances of recurring failures somewhat, it is recommended that failed coils on 1MZ-FE engines be replaced by OEM parts.

What are the common causes of code P0301?

The most common causes of code P0301 are much the same across all Toyota applications and could include one or more of the following-

  • Failed, defective, unsuitable, or excessively worn sparkplug on cylinder #1
  • Failed, defective, unsuitable, or damaged ignition coil (and plug wire, if fitted)on cylinder #1
  • Failed, failing, defective, or clogged fuel injector on cylinder #1
  • Mechanical failure or damage, such as a broken valve spring, a broken rocker arm, a failed hydraulic valve lifter, or burnt/broken engine valve(s) causing a partial or complete loss of cylinder compression on cylinder #1
  • Failed or corrupted ignition and/or injector driver circuit serving cylinder #1 (in the PCM)
  • Burnt, shorted, disconnected, or corroded wiring and/or connectors in the wiring serving the ignition and/or fuel delivery systems of cylinder #1
  • Failed or failing PCM, but note that unlike most other trouble codes, failed or corrupted driver circuits in the PCM are a distinct possibility in all instances of specific misfire codes on Toyota applications

NOTE: While issues like leaking intake manifold gaskets/seals, damaged/leaking cylinder head gaskets, engine vacuum leaks, and failures or defects in variable valve/camshaft timing systems can sometimes cause a single cylinder-specific misfire code to set, these kinds of issues typically affect more than one cylinder simultaneously. 

What are the symptoms of code P0301?

The most common symptoms of code P0301 on Toyota vehicles are much the same on all Toyota applications and could include one or more of the following, but note that depending on the nature of the problem, the severity of one or more symptoms listed here could vary substantially between Toyota applications-

  • Stored trouble code and illuminated warning light
  • The engine may appear to “shake” or “vibrate” excessively as the result of an imbalance in the power input of the affected cylinder
  • Idling will be rough
  • The engine may exhibit a dramatic degree of power loss
  • Fuel consumption may increase
  • In some cases, and depending on the nature of the problem, mechanical “tapping” or “knocking” noises may present that typically vary with engine speed
  • The fuel injector on cylinder #1 will typically be disabled by the PCM or fuel control module to protect the catalytic converter(s) against overheating

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